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Merge branch 'master' of https://github.com/rust-lang/rust into readdir

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This commit is contained in:
Raph Levien 2017-02-22 09:30:46 -08:00
commit b3ee2490c2
196 changed files with 7057 additions and 5348 deletions
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src/Cargo.lock generated
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5
src/bootstrap/bin/rustc.rs
View file

@ -205,6 +205,11 @@ fn main() {
}
}
}
if target.contains("pc-windows-msvc") {
cmd.arg("-Z").arg("unstable-options");
cmd.arg("-C").arg("target-feature=+crt-static");
}
}
if verbose > 1 {

56
src/bootstrap/bootstrap.py
View file

@ -173,6 +173,8 @@ class RustBuild(object):
if not os.path.exists(tarball):
get("{}/{}".format(url, filename), tarball, verbose=self.verbose)
unpack(tarball, self.bin_root(), match="rustc", verbose=self.verbose)
self.fix_executable(self.bin_root() + "/bin/rustc")
self.fix_executable(self.bin_root() + "/bin/rustdoc")
with open(self.rustc_stamp(), 'w') as f:
f.write(self.stage0_rustc_date())
@ -185,9 +187,63 @@ class RustBuild(object):
if not os.path.exists(tarball):
get("{}/{}".format(url, filename), tarball, verbose=self.verbose)
unpack(tarball, self.bin_root(), match="cargo", verbose=self.verbose)
self.fix_executable(self.bin_root() + "/bin/cargo")
with open(self.cargo_stamp(), 'w') as f:
f.write(self.stage0_cargo_rev())
def fix_executable(self, fname):
# If we're on NixOS we need to change the path to the dynamic loader
default_encoding = sys.getdefaultencoding()
try:
ostype = subprocess.check_output(['uname', '-s']).strip().decode(default_encoding)
except (subprocess.CalledProcessError, WindowsError):
return
if ostype != "Linux":
return
if not os.path.exists("/etc/NIXOS"):
return
if os.path.exists("/lib"):
return
# At this point we're pretty sure the user is running NixOS
print("info: you seem to be running NixOS. Attempting to patch " + fname)
try:
interpreter = subprocess.check_output(["patchelf", "--print-interpreter", fname])
interpreter = interpreter.strip().decode(default_encoding)
except subprocess.CalledProcessError as e:
print("warning: failed to call patchelf: %s" % e)
return
loader = interpreter.split("/")[-1]
try:
ldd_output = subprocess.check_output(['ldd', '/run/current-system/sw/bin/sh'])
ldd_output = ldd_output.strip().decode(default_encoding)
except subprocess.CalledProcessError as e:
print("warning: unable to call ldd: %s" % e)
return
for line in ldd_output.splitlines():
libname = line.split()[0]
if libname.endswith(loader):
loader_path = libname[:len(libname) - len(loader)]
break
else:
print("warning: unable to find the path to the dynamic linker")
return
correct_interpreter = loader_path + loader
try:
subprocess.check_output(["patchelf", "--set-interpreter", correct_interpreter, fname])
except subprocess.CalledProcessError as e:
print("warning: failed to call patchelf: %s" % e)
return
def stage0_cargo_rev(self):
return self._cargo_rev

4
src/bootstrap/doc.rs
View file

@ -115,10 +115,6 @@ pub fn standalone(build: &Build, target: &str) {
.arg("-o").arg(&out)
.arg(&path);
if filename == "reference.md" {
cmd.arg("--html-in-header").arg(&full_toc);
}
if filename == "not_found.md" {
cmd.arg("--markdown-no-toc")
.arg("--markdown-css")

6
src/bootstrap/native.rs
View file

@ -99,6 +99,12 @@ pub fn llvm(build: &Build, target: &str) {
.define("LLVM_TARGET_ARCH", target.split('-').next().unwrap())
.define("LLVM_DEFAULT_TARGET_TRIPLE", target);
if target.contains("msvc") {
cfg.define("LLVM_USE_CRT_DEBUG", "MT");
cfg.define("LLVM_USE_CRT_RELEASE", "MT");
cfg.define("LLVM_USE_CRT_RELWITHDEBINFO", "MT");
}
if target.starts_with("i686") {
cfg.define("LLVM_BUILD_32_BITS", "ON");
}

9
src/bootstrap/step.rs
View file

@ -568,6 +568,15 @@ pub fn build_rules<'a>(build: &'a Build) -> Rules {
})
.default(build.config.docs)
.run(move |s| doc::rustbook(build, s.target, "nomicon"));
rules.doc("doc-reference", "src/doc/reference")
.dep(move |s| {
s.name("tool-rustbook")
.host(&build.config.build)
.target(&build.config.build)
.stage(0)
})
.default(build.config.docs)
.run(move |s| doc::rustbook(build, s.target, "reference"));
rules.doc("doc-standalone", "src/doc")
.dep(move |s| {
s.name("rustc")

2
src/doc/book/src/attributes.md
View file

@ -67,4 +67,4 @@ Rust attributes are used for a number of different things. There is a full list
of attributes [in the reference][reference]. Currently, you are not allowed to
create your own attributes, the Rust compiler defines them.
[reference]: ../reference.html#attributes
[reference]: ../reference/attributes.html

2
src/doc/book/src/casting-between-types.md
View file

@ -8,7 +8,7 @@ most dangerous features of Rust!
# Coercion
Coercion between types is implicit and has no syntax of its own, but can
be spelled out with [`as`](#Explicit%20coercions).
be spelled out with [`as`](#explicit-coercions).
Coercion occurs in `let`, `const`, and `static` statements; in
function call arguments; in field values in struct initialization; and in a

2
src/doc/book/src/closures.md
View file

@ -463,7 +463,7 @@ fn factory() -> &(Fn(i32) -> i32) {
Right. Because we have a reference, we need to give it a lifetime. But
our `factory()` function takes no arguments, so
[elision](lifetimes.html#Lifetime%20Elision) doesn’t kick in here. Then what
[elision](lifetimes.html#lifetime-elision) doesn’t kick in here. Then what
choices do we have? Try `'static`:
```rust,ignore

15
src/doc/book/src/compiler-plugins.md
View file

@ -119,7 +119,7 @@ The advantages over a simple `fn(&str) -> u32` are:
a way to define new literal syntax for any data type.
In addition to procedural macros, you can define new
[`derive`](../reference.html#derive)-like attributes and other kinds of
[`derive`](../reference/attributes.html#derive)-like attributes and other kinds of
extensions. See `Registry::register_syntax_extension` and the `SyntaxExtension`
enum. For a more involved macro example, see
[`regex_macros`](https://github.com/rust-lang/regex/blob/master/regex_macros/src/lib.rs).
@ -127,7 +127,7 @@ enum. For a more involved macro example, see
## Tips and tricks
Some of the [macro debugging tips](macros.html#Debugging%20macro%20code) are applicable.
Some of the [macro debugging tips](macros.html#debugging-macro-code) are applicable.
You can use `syntax::parse` to turn token trees into
higher-level syntax elements like expressions:
@ -165,8 +165,8 @@ quasiquote as an ordinary plugin library.
# Lint plugins
Plugins can extend [Rust's lint
infrastructure](../reference.html#lint-check-attributes) with additional checks for
code style, safety, etc. Now let's write a plugin
infrastructure](../reference/attributes.html#lint-check-attributes) with
additional checks for code style, safety, etc. Now let's write a plugin
[`lint_plugin_test.rs`](https://github.com/rust-lang/rust/blob/master/src/test/run-pass-fulldeps/auxiliary/lint_plugin_test.rs)
that warns about any item named `lintme`.
@ -244,9 +244,10 @@ mostly use the same infrastructure as lint plugins, and provide examples of how
to access type information.
Lints defined by plugins are controlled by the usual [attributes and compiler
flags](../reference.html#lint-check-attributes), e.g. `#[allow(test_lint)]` or
`-A test-lint`. These identifiers are derived from the first argument to
`declare_lint!`, with appropriate case and punctuation conversion.
flags](../reference/attributes.html#lint-check-attributes), e.g.
`#[allow(test_lint)]` or `-A test-lint`. These identifiers are derived from the
first argument to `declare_lint!`, with appropriate case and punctuation
conversion.
You can run `rustc -W help foo.rs` to see a list of lints known to `rustc`,
including those provided by plugins loaded by `foo.rs`.

4
src/doc/book/src/concurrency.md
View file

@ -55,7 +55,7 @@ For sharing references across threads, Rust provides a wrapper type called
`Arc<T>`. `Arc<T>` implements `Send` and `Sync` if and only if `T` implements
both `Send` and `Sync`. For example, an object of type `Arc<RefCell<U>>` cannot
be transferred across threads because
[`RefCell`](choosing-your-guarantees.html#RefCell%3CT%3E) does not implement
[`RefCell`](choosing-your-guarantees.html#refcellt) does not implement
`Sync`, consequently `Arc<RefCell<U>>` would not implement `Send`.
These two traits allow you to use the type system to make strong guarantees
@ -126,7 +126,7 @@ closure only captures a _reference to `x`_. This is a problem, because the
thread may outlive the scope of `x`, leading to a dangling pointer.
To fix this, we use a `move` closure as mentioned in the error message. `move`
closures are explained in depth [here](closures.html#move%20closures); basically
closures are explained in depth [here](closures.html#move-closures); basically
they move variables from their environment into themselves.
```rust

72
src/doc/book/src/error-handling.md
View file

@ -21,35 +21,35 @@ sum types and combinators, and try to motivate the way Rust does error handling
incrementally. As such, programmers with experience in other expressive type
systems may want to jump around.
* [The Basics](#The%20Basics)
* [Unwrapping explained](#Unwrapping%20explained)
* [The `Option` type](#The%20Option%20type)
* [Composing `Option<T>` values](#Composing%20Option%3CT%3E%20values)
* [The `Result` type](#The%20Result%20type)
* [Parsing integers](#Parsing%20integers)
* [The `Result` type alias idiom](#The%20Result%20type%20alias%20idiom)
* [A brief interlude: unwrapping isn't evil](#A%20brief%20interlude:%20unwrapping%20isnt%20evil)
* [Working with multiple error types](#Working%20with%20multiple%20error%20types)
* [Composing `Option` and `Result`](#Composing%20Option%20and%20Result)
* [The limits of combinators](#The%20limits%20of%20combinators)
* [Early returns](#Early%20returns)
* [The `try!` macro](#The%20try%20macro)
* [Defining your own error type](#Defining%20your%20own%20error%20type)
* [Standard library traits used for error handling](#Standard%20library%20traits%20used%20for%20error%20handling)
* [The `Error` trait](#The%20Error%20trait)
* [The `From` trait](#The%20From%20trait)
* [The real `try!` macro](#The%20real%20try%20macro)
* [Composing custom error types](#Composing%20custom%20error%20types)
* [Advice for library writers](#Advice%20for%20library%20writers)
* [Case study: A program to read population data](#Case%20study:%20A%20program%20to%20read%20population%20data)
* [Initial setup](#Initial%20setup)
* [Argument parsing](#Argument%20parsing)
* [Writing the logic](#Writing%20the%20logic)
* [Error handling with `Box<Error>`](#Error%20handling%20with%20Box%3CError%3E)
* [Reading from stdin](#Reading%20from%20stdin)
* [Error handling with a custom type](#Error%20handling%20with%20a%20custom%20type)
* [Adding functionality](#Adding%20functionality)
* [The short story](#The%20short%20story)
* [The Basics](#the-basics)
* [Unwrapping explained](#unwrapping-explained)
* [The `Option` type](#the-option-type)
* [Composing `Option<T>` values](#composing-optiont-values)
* [The `Result` type](#the-result-type)
* [Parsing integers](#parsing-integers)
* [The `Result` type alias idiom](#the-result-type-alias-idiom)
* [A brief interlude: unwrapping isn't evil](#a-brief-interlude-unwrapping-isnt-evil)
* [Working with multiple error types](#working-with-multiple-error-types)
* [Composing `Option` and `Result`](#composing-option-and-result)
* [The limits of combinators](#the-limits-of-combinators)
* [Early returns](#early-returns)
* [The `try!` macro](#the-try-macro)
* [Defining your own error type](#defining-your-own-error-type)
* [Standard library traits used for error handling](#standard-library-traits-used-for-error-handling)
* [The `Error` trait](#the-error-trait)
* [The `From` trait](#the-from-trait)
* [The real `try!` macro](#the-real-try-macro)
* [Composing custom error types](#composing-custom-error-types)
* [Advice for library writers](#advice-for-library-writers)
* [Case study: A program to read population data](#case-study-a-program-to-read-population-data)
* [Initial setup](#initial-setup)
* [Argument parsing](#argument-parsing)
* [Writing the logic](#writing-the-logic)
* [Error handling with `Box<Error>`](#error-handling-with-boxerror)
* [Reading from stdin](#reading-from-stdin)
* [Error handling with a custom type](#error-handling-with-a-custom-type)
* [Adding functionality](#adding-functionality)
* [The short story](#the-short-story)
# The Basics
@ -796,7 +796,7 @@ because of the return types of
[`std::fs::File::open`](../std/fs/struct.File.html#method.open) and
[`std::io::Read::read_to_string`](../std/io/trait.Read.html#method.read_to_string).
(Note that they both use the [`Result` type alias
idiom](#The%20Result%20type%20alias%20idiom) described previously. If you
idiom](#the-result-type-alias-idiom) described previously. If you
click on the `Result` type, you'll [see the type
alias](../std/io/type.Result.html), and consequently, the underlying
`io::Error` type.) The third problem is described by the
@ -1120,7 +1120,7 @@ returns an `&Error`, which is itself a trait object. We'll revisit the
For now, it suffices to show an example implementing the `Error` trait. Let's
use the error type we defined in the
[previous section](#Defining%20your%20own%20error%20type):
[previous section](#defining-your-own-error-type):
```rust
use std::io;
@ -1493,19 +1493,19 @@ representation. But certainly, this will vary depending on use cases.
At a minimum, you should probably implement the
[`Error`](../std/error/trait.Error.html)
trait. This will give users of your library some minimum flexibility for
[composing errors](#The%20real%20try%20macro). Implementing the `Error` trait also
[composing errors](#the-real-try-macro). Implementing the `Error` trait also
means that users are guaranteed the ability to obtain a string representation
of an error (because it requires impls for both `fmt::Debug` and
`fmt::Display`).
Beyond that, it can also be useful to provide implementations of `From` on your
error types. This allows you (the library author) and your users to
[compose more detailed errors](#Composing%20custom%20error%20types). For example,
[compose more detailed errors](#composing-custom-error-types). For example,
[`csv::Error`](http://burntsushi.net/rustdoc/csv/enum.Error.html)
provides `From` impls for both `io::Error` and `byteorder::Error`.
Finally, depending on your tastes, you may also want to define a
[`Result` type alias](#The%20Result%20type%20alias%20idiom), particularly if your
[`Result` type alias](#the-result-type-alias-idiom), particularly if your
library defines a single error type. This is used in the standard library
for [`io::Result`](../std/io/type.Result.html)
and [`fmt::Result`](../std/fmt/type.Result.html).
@ -1538,7 +1538,7 @@ and [`rustc-serialize`](https://crates.io/crates/rustc-serialize) crates.
We're not going to spend a lot of time on setting up a project with
Cargo because it is already covered well in [the Cargo
section](getting-started.html#Hello%20Cargo) and [Cargo's documentation][14].
section](getting-started.html#hello-cargo) and [Cargo's documentation][14].
To get started from scratch, run `cargo new --bin city-pop` and make sure your
`Cargo.toml` looks something like this:
@ -1729,7 +1729,7 @@ error types and you don't need any `From` implementations. The downside is that
since `Box<Error>` is a trait object, it *erases the type*, which means the
compiler can no longer reason about its underlying type.
[Previously](#The%20limits%20of%20combinators) we started refactoring our code by
[Previously](#the-limits-of-combinators) we started refactoring our code by
changing the type of our function from `T` to `Result<T, OurErrorType>`. In
this case, `OurErrorType` is only `Box<Error>`. But what's `T`? And can we add
a return type to `main`?

2
src/doc/book/src/ffi.md
View file

@ -680,7 +680,7 @@ pub extern fn hello_rust() -> *const u8 {
The `extern` makes this function adhere to the C calling convention, as
discussed above in "[Foreign Calling
Conventions](ffi.html#Foreign%20calling%20conventions)". The `no_mangle`
Conventions](ffi.html#foreign-calling-conventions)". The `no_mangle`
attribute turns off Rust's name mangling, so that it is easier to link to.
# FFI and panics

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@ -140,5 +140,51 @@ container types like [`Vec<T>`][Vec]. On the other hand, often you want to
trade that flexibility for increased expressive power. Read about [trait
bounds][traits] to see why and how.
## Resolving ambiguities
Most of the time when generics are involved, the compiler can infer the
generic parameters automatically:
```rust
// v must be a Vec<T> but we don't know what T is yet
let mut v = Vec::new();
// v just got a bool value, so T must be bool!
v.push(true);
// Debug-print v
println!("{:?}", v);
```
Sometimes though, the compiler needs a little help. For example, had we
omitted the last line, we would get a compile error:
```rust,ignore
let v = Vec::new();
// ^^^^^^^^ cannot infer type for `T`
//
// note: type annotations or generic parameter binding required
println!("{:?}", v);
```
We can solve this using either a type annotation:
```rust
let v: Vec<bool> = Vec::new();
println!("{:?}", v);
```
or by binding the generic parameter `T` via the so-called
[‘turbofish’][turbofish] `::<>` syntax:
```rust
let v = Vec::<bool>::new();
println!("{:?}", v);
```
The second approach is useful in situations where we don’t want to bind the
result to a variable. It can also be used to bind generic parameters in
functions or methods. See [Iterators § Consumers](iterators.html#consumers)
for an example.
[traits]: traits.html
[Vec]: ../std/vec/struct.Vec.html
[turbofish]: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.collect

2
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@ -236,7 +236,7 @@ language]*, which means that most things are expressions, rather than
statements. The `;` indicates that this expression is over, and the next one is
ready to begin. Most lines of Rust code end with a `;`.
[expression-oriented language]: glossary.html#Expression-Oriented%20Language
[expression-oriented language]: glossary.html#expression-oriented-language
## Compiling and Running Are Separate Steps

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@ -56,7 +56,7 @@ They can be used to manage control flow in a modular fashion.
A type without a statically known size or alignment. ([more info][link])
[link]: ../nomicon/exotic-sizes.html#Dynamically%20Sized%20Types%20(DSTs)
[link]: ../nomicon/exotic-sizes.html#dynamically-sized-types-dsts
### Expression
@ -76,8 +76,8 @@ In an expression-oriented language, (nearly) every statement is an expression
and therefore returns a value. Consequently, these expression statements can
themselves form part of larger expressions.
[expression]: glossary.html#Expression
[statement]: glossary.html#Statement
[expression]: glossary.html#expression
[statement]: glossary.html#statement
### Statement

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@ -119,7 +119,7 @@ there are no arguments, and `{` starts the body of the function. Because
we didn’t include a return type, it’s assumed to be `()`, an empty
[tuple][tuples].
[tuples]: primitive-types.html#Tuples
[tuples]: primitive-types.html#tuples
```rust,ignore
println!("Guess the number!");
@ -727,7 +727,7 @@ thirty-two bit integer. Rust has [a number of built-in number types][number],
but we’ve chosen `u32`. It’s a good default choice for a small positive number.
[parse]: ../std/primitive.str.html#method.parse
[number]: primitive-types.html#Numeric%20types
[number]: primitive-types.html#numeric-types
Just like `read_line()`, our call to `parse()` could cause an error. What if
our string contained `A👍%`? There’d be no way to convert that to a number. As

8
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@ -135,10 +135,10 @@ Here's the version that does compile:
let one_to_one_hundred = (1..101).collect::<Vec<i32>>();
```
If you remember, the `::<>` syntax allows us to give a type hint,
and so we tell it that we want a vector of integers. You don't always
need to use the whole type, though. Using a `_` will let you provide
a partial hint:
If you remember, the [`::<>` syntax](generics.html#resolving-ambiguities)
allows us to give a type hint that tells the compiler we want a vector of
integers. You don't always need to use the whole type, though. Using a `_`
will let you provide a partial hint:
```rust
let one_to_one_hundred = (1..101).collect::<Vec<_>>();

2
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@ -139,7 +139,7 @@ associated with it, but the compiler lets you elide (i.e. omit, see
["Lifetime Elision"][lifetime-elision] below) them in common cases. Before we
get to that, though, let’s look at a short example with explicit lifetimes:
[lifetime-elision]: #Lifetime%20Elision
[lifetime-elision]: #lifetime-elision
```rust,ignore
fn bar<'a>(...)

10
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@ -101,7 +101,7 @@ trees, at compile time. The semicolon is optional on the last (here, only)
case. The "pattern" on the left-hand side of `=>` is known as a ‘matcher’.
These have [their own little grammar] within the language.
[their own little grammar]: ../reference.html#macros
[their own little grammar]: ../reference/macros.html
The matcher `$x:expr` will match any Rust expression, binding that syntax tree
to the ‘metavariable’ `$x`. The identifier `expr` is a ‘fragment specifier’;
@ -363,7 +363,7 @@ fn main() {
}
```
[items]: ../reference.html#items
[items]: ../reference/items.html
# Recursive macros
@ -430,7 +430,7 @@ Even when Rust code contains un-expanded macros, it can be parsed as a full
tools that process code. It also has a few consequences for the design of
Rust’s macro system.
[ast]: glossary.html#Abstract%20Syntax%20Tree
[ast]: glossary.html#abstract-syntax-tree
One consequence is that Rust must determine, when it parses a macro invocation,
whether the macro stands in for
@ -490,7 +490,7 @@ be forced to choose between parsing `$i` and parsing `$e`. Changing the
invocation syntax to put a distinctive token in front can solve the problem. In
this case, you can write `$(I $i:ident)* E $e:expr`.
[item]: ../reference.html#items
[item]: ../reference/items.html
# Scoping and macro import/export
@ -565,7 +565,7 @@ When this library is loaded with `#[macro_use] extern crate`, only `m2` will
be imported.
The Rust Reference has a [listing of macro-related
attributes](../reference.html#macro-related-attributes).
attributes](../reference/attributes.html#macro-related-attributes).
# The variable `$crate`

2
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@ -89,7 +89,7 @@ philosophy, memory safety, and the mechanism by which Rust guarantees it, the
> * exactly one mutable reference (`&mut T`).
[ownership]: ownership.html
[borrowing]: references-and-borrowing.html#Borrowing
[borrowing]: references-and-borrowing.html#borrowing
So, that’s the real definition of ‘immutability’: is this safe to have two
pointers to? In `Arc<T>`’s case, yes: the mutation is entirely contained inside

8
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@ -65,10 +65,10 @@ elements onto them.
Vectors have a [generic type][generics] `Vec<T>`, so in this example `v` will have type
`Vec<i32>`. We'll cover [generics] in detail in a later chapter.
[arrays]: primitive-types.html#Arrays
[arrays]: primitive-types.html#arrays
[vectors]: vectors.html
[heap]: the-stack-and-the-heap.html#The%20Heap
[stack]: the-stack-and-the-heap.html#The%20Stack
[heap]: the-stack-and-the-heap.html#the-heap
[stack]: the-stack-and-the-heap.html#the-stack
[bindings]: variable-bindings.html
[generics]: generics.html
@ -136,7 +136,7 @@ Rust allocates memory for an integer [i32] on the [stack][sh], copies the bit
pattern representing the value of 10 to the allocated memory and binds the
variable name x to this memory region for future reference.
[i32]: primitive-types.html#Numeric%20types
[i32]: primitive-types.html#numeric-types
Now consider the following code fragment:

2
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@ -232,7 +232,7 @@ soon.
You can assign one tuple into another, if they have the same contained types
and [arity]. Tuples have the same arity when they have the same length.
[arity]: glossary.html#Arity
[arity]: glossary.html#arity
```rust
let mut x = (1, 2); // x: (i32, i32)

2
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@ -169,7 +169,7 @@ So this is where quotes comes in. The `ast` argument is a struct that gives us
a representation of our type (which can be either a `struct` or an `enum`).
Check out the [docs](https://docs.rs/syn/0.10.5/syn/struct.MacroInput.html),
there is some useful information there. We are able to get the name of the
type using `ast.ident`. The `quote!` macro let's us write up the Rust code
type using `ast.ident`. The `quote!` macro lets us write up the Rust code
that we wish to return and convert it into `Tokens`. `quote!` let's us use some
really cool templating mechanics; we simply write `#name` and `quote!` will
replace it with the variable named `name`. You can even do some repetition

2
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@ -134,7 +134,7 @@ fn main() {
let age = 27;
let peter = Person { name, age };
// Print debug struct
// Debug-print struct
println!("{:?}", peter);
}
```

60
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@ -125,7 +125,7 @@
<!-- Generics -->
* `path<…>` (*e.g.* `Vec<u8>`): specifies parameters to generic type *in a type*. See [Generics].
* `path::<…>`, `method::<…>` (*e.g.* `"42".parse::<i32>()`): specifies parameters to generic type, function, or method *in an expression*.
* `path::<…>`, `method::<…>` (*e.g.* `"42".parse::<i32>()`): specifies parameters to generic type, function, or method *in an expression*. See [Generics § Resolving ambiguities](generics.html#resolving-ambiguities).
* `fn ident<…> …`: define generic function. See [Generics].
* `struct ident<…> …`: define generic structure. See [Generics].
* `enum ident<…> …`: define generic enumeration. See [Generics].
@ -196,18 +196,18 @@
[Associated Types]: associated-types.html
[Attributes]: attributes.html
[Casting Between Types (`as`)]: casting-between-types.html#as
[Closures (`move` closures)]: closures.html#move%20closures
[Closures (`move` closures)]: closures.html#move-closures
[Closures]: closures.html
[Comments]: comments.html
[Crates and Modules (Defining Modules)]: crates-and-modules.html#Defining%20modules
[Crates and Modules (Exporting a Public Interface)]: crates-and-modules.html#Exporting%20a%20public%20interface
[Crates and Modules (Importing External Crates)]: crates-and-modules.html#Importing%20external%20crates
[Crates and Modules (Importing Modules with `use`)]: crates-and-modules.html#Importing%20modules%20with%20use
[Crates and Modules (Re-exporting with `pub use`)]: crates-and-modules.html#Re-exporting%20with%20pub%20use
[Diverging Functions]: functions.html#Diverging%20functions
[Crates and Modules (Defining Modules)]: crates-and-modules.html#defining-modules
[Crates and Modules (Exporting a Public Interface)]: crates-and-modules.html#exporting-a-public-interface
[Crates and Modules (Importing External Crates)]: crates-and-modules.html#importing-external-crates
[Crates and Modules (Importing Modules with `use`)]: crates-and-modules.html#importing-modules-with-use
[Crates and Modules (Re-exporting with `pub use`)]: crates-and-modules.html#re-exporting-with-pub-use
[Diverging Functions]: functions.html#diverging-functions
[Enums]: enums.html
[Foreign Function Interface]: ffi.html
[Functions (Early Returns)]: functions.html#Early%20returns
[Functions (Early Returns)]: functions.html#early-returns
[Functions]: functions.html
[Generics]: generics.html
[Iterators]: iterators.html
@ -216,38 +216,38 @@
[Loops (`for`)]: loops.html#for
[Loops (`loop`)]: loops.html#loop
[Loops (`while`)]: loops.html#while
[Loops (Ending Iteration Early)]: loops.html#Ending%20iteration%20early
[Loops (Loops Labels)]: loops.html#Loop%20labels
[Loops (Ending Iteration Early)]: loops.html#ending-iteration-early
[Loops (Loops Labels)]: loops.html#loop-labels
[Macros]: macros.html
[Match]: match.html
[Method Syntax (Method Calls)]: method-syntax.html#Method%20calls
[Method Syntax (Method Calls)]: method-syntax.html#method-calls
[Method Syntax]: method-syntax.html
[Mutability]: mutability.html
[Operators and Overloading]: operators-and-overloading.html
[Patterns (`ref` and `ref mut`)]: patterns.html#ref%20and%20ref%20mut
[Patterns (Bindings)]: patterns.html#Bindings
[Patterns (Ignoring bindings)]: patterns.html#Ignoring%20bindings
[Patterns (Multiple patterns)]: patterns.html#Multiple%20patterns
[Patterns (Ranges)]: patterns.html#Ranges
[Patterns (`ref` and `ref mut`)]: patterns.html#ref-and-ref-mut
[Patterns (Bindings)]: patterns.html#bindings
[Patterns (Ignoring bindings)]: patterns.html#ignoring-bindings
[Patterns (Multiple patterns)]: patterns.html#multiple-patterns
[Patterns (Ranges)]: patterns.html#ranges
[Primitive Types (`char`)]: primitive-types.html#char
[Primitive Types (Arrays)]: primitive-types.html#Arrays
[Primitive Types (Booleans)]: primitive-types.html#Booleans
[Primitive Types (Tuple Indexing)]: primitive-types.html#Tuple%20indexing
[Primitive Types (Tuples)]: primitive-types.html#Tuples
[Primitive Types (Arrays)]: primitive-types.html#arrays
[Primitive Types (Booleans)]: primitive-types.html#booleans
[Primitive Types (Tuple Indexing)]: primitive-types.html#tuple-indexing
[Primitive Types (Tuples)]: primitive-types.html#tuples
[Raw Pointers]: raw-pointers.html
[Reference (Byte String Literals)]: ../reference.html#byte-string-literals
[Reference (Integer literals)]: ../reference.html#integer-literals
[Reference (Raw Byte String Literals)]: ../reference.html#raw-byte-string-literals
[Reference (Raw String Literals)]: ../reference.html#raw-string-literals
[Reference (Byte String Literals)]: ../reference/tokens.html/#byte-string-literals
[Reference (Integer literals)]: ../reference/tokens.html#integer-literals
[Reference (Raw Byte String Literals)]: ../reference/tokens.html#raw-byte-string-literals
[Reference (Raw String Literals)]: ../reference/tokens.html#raw-string-literals
[References and Borrowing]: references-and-borrowing.html
[Strings]: strings.html
[Structs (Update syntax)]: structs.html#Update%20syntax
[Structs (Update syntax)]: structs.html#update-syntax
[Structs]: structs.html
[Traits (`where` clause)]: traits.html#Where%20clause
[Traits (Multiple Trait Bounds)]: traits.html#Multiple%20trait%20bounds
[Traits (`where` clause)]: traits.html#where-clause
[Traits (Multiple Trait Bounds)]: traits.html#multiple-trait-bounds
[Traits]: traits.html
[Universal Function Call Syntax]: ufcs.html
[Universal Function Call Syntax (Angle-bracket Form)]: ufcs.html#Angle-bracket%20Form
[Universal Function Call Syntax (Angle-bracket Form)]: ufcs.html#angle-bracket-form
[Unsafe]: unsafe.html
[Unsized Types (`?Sized`)]: unsized-types.html#Sized
[Unsized Types (`?Sized`)]: unsized-types.html#sized
[Variable Bindings]: variable-bindings.html

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@ -81,7 +81,7 @@ Traits are useful because they allow a type to make certain promises about its
behavior. Generic functions can exploit this to constrain, or [bound][bounds], the types they
accept. Consider this function, which does not compile:
[bounds]: glossary.html#Bounds
[bounds]: glossary.html#bounds
```rust,ignore
fn print_area<T>(shape: T) {

2
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@ -55,7 +55,7 @@ if x == y {
This compiles without error. Values of a `Num` type are the same as a value of
type `i32`, in every way. You can use [tuple struct] to really get a new type.
[tuple struct]: structs.html#Tuple%20structs
[tuple struct]: structs.html#tuple-structs
You can also use type aliases with generics:

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@ -181,7 +181,7 @@ print.
# Scope and shadowing
Let’s get back to bindings. Variable bindings have a scope - they are
constrained to live in a block they were defined in. A block is a collection
constrained to live in the block they were defined in. A block is a collection
of statements enclosed by `{` and `}`. Function definitions are also blocks!
In the following example we define two variable bindings, `x` and `y`, which
live in different blocks. `x` can be accessed from inside the `fn main() {}`

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@ -151,6 +151,6 @@ API documentation][vec].
[vec]: ../std/vec/index.html
[box]: ../std/boxed/index.html
[generic]: generics.html
[panic]: concurrency.html#Panics
[panic]: concurrency.html#panics
[get]: ../std/vec/struct.Vec.html#method.get
[get_mut]: ../std/vec/struct.Vec.html#method.get_mut

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@ -187,7 +187,7 @@ literal : [ string_lit | char_lit | byte_string_lit | byte_lit | num_lit | bool_
The optional `lit_suffix` production is only used for certain numeric literals,
but is reserved for future extension. That is, the above gives the lexical
grammar, but a Rust parser will reject everything but the 12 special cases
mentioned in [Number literals](reference.html#number-literals) in the
mentioned in [Number literals](reference/tokens.html#number-literals) in the
reference.
#### Character and string literals

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@ -16,14 +16,6 @@ builds documentation for individual Rust packages.
Rust provides a standard library with a number of features; [we host its
documentation here][api].
## Reference Documentation
Rust does not yet have a formal specification, but we have [a reference document
][ref]. It is guaranteed to be accurate, but not complete. We now have a
policy that all new features must be included in the reference before
stabilization; however, we are still back-filling things that landed before
then. That work is being tracked [here][38643].
## Extended Error Documentation
Many of Rust's errors come with error codes, and you can request extended
@ -37,11 +29,17 @@ nicknamed 'The Rust Bookshelf.'
* [The Rust Programming Language][book] teaches you how to program in Rust.
* [The Rustonomicon][nomicon] is your guidebook to the dark arts of unsafe Rust.
* [The Reference][ref] is not a formal spec, but is more detailed and comprehensive than the book.
Another few words about the reference: it is guaranteed to be accurate, but not
complete. We now have a policy that all new features must be included in the
reference before stabilization; however, we are still back-filling things that
landed before then. That work is being tracked [here][38643].
[Rust Learning]: https://github.com/ctjhoa/rust-learning
[Docs.rs]: https://docs.rs/
[api]: std/index.html
[ref]: reference.html
[ref]: reference/index.html
[38643]: https://github.com/rust-lang/rust/issues/38643
[err]: error-index.html
[book]: book/index.html

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@ -79,17 +79,5 @@ if condition {
}
```
As of Rust 1.0, the drop flags are actually not-so-secretly stashed in a hidden
field of any type that implements Drop. Rust sets the drop flag by overwriting
the entire value with a particular bit pattern. This is pretty obviously Not
The Fastest and causes a bunch of trouble with optimizing code. It's legacy from
a time when you could do much more complex conditional initialization.
As such work is currently under way to move the flags out onto the stack frame
where they more reasonably belong. Unfortunately, this work will take some time
as it requires fairly substantial changes to the compiler.
Regardless, Rust programs don't need to worry about uninitialized values on
the stack for correctness. Although they might care for performance. Thankfully,
Rust makes it easy to take control here! Uninitialized values are there, and
you can work with them in Safe Rust, but you're never in danger.
The drop flags are tracked on the stack and no longer stashed in types that
implement drop.

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@ -78,4 +78,4 @@ TODO: other common problems? SEME regions stuff, mostly?
[ex2]: lifetimes.html#Example%3A%20aliasing%20a%20mutable%20reference
[ex2]: lifetimes.html#example-aliasing-a-mutable-reference

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@ -151,4 +151,4 @@ use fairly elaborate algorithms to cache bits throughout nested types with
special constrained representations. As such it is *especially* desirable that
we leave enum layout unspecified today.
[dst]: exotic-sizes.html#Dynamically%20Sized%20Types%20(DSTs)
[dst]: exotic-sizes.html#dynamically-sized-types-dsts

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@ -0,0 +1 @@
book

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@ -0,0 +1,58 @@
# The Rust Reference
[Introduction](introduction.md)
- [Notation](notation.md)
- [Unicode productions](unicode-productions.md)
- [String table productions](string-table-productions.md)
- [Lexical structure](lexical-structure.md)
- [Input format](input-format.md)
- [Identifiers](identifiers.md)
- [Comments](comments.md)
- [Whitespace](whitespace.md)
- [Tokens](tokens.md)
- [Paths](paths.md)
- [Macros](macros.md)
- [Macros By Example](macros-by-example.md)
- [Procedrual Macros](procedural-macros.md)
- [Crates and source files](crates-and-source-files.md)
- [Items and attributes](items-and-attributes.md)
- [Items](items.md)
- [Visibility and Privacy](visibility-and-privacy.md)
- [Attributes](attributes.md)
- [Statements and expressions](statements-and-expressions.md)
- [Statements](statements.md)
- [Expressions](expressions.md)
- [Type system](type-system.md)
- [Types](types.md)
- [Subtyping](subtyping.md)
- [Type coercions](type-coercions.md)
- [Special traits](special-traits.md)
- [The Copy trait](the-copy-trait.md)
- [The Sized trait](the-sized-trait.md)
- [The Drop trait](the-drop-trait.md)
- [The Deref trait](the-deref-trait.md)
- [The Send trait](the-send-trait.md)
- [The Sync trait](the-sync-trait.md)
- [Memory model](memory-model.md)
- [Memory allocation and lifetime](memory-allocation-and-lifetime.md)
- [Memory ownership](memory-ownership.md)
- [Variables](variables.md)
- [Linkage](linkage.md)
- [Unsafety](unsafety.md)
- [Unsafe functions](unsafe-functions.md)
- [Unsafe blocks](unsafe-blocks.md)
- [Behavior considered undefined](behavior-considered-undefined.md)
- [Behavior not considered unsafe](behavior-not-considered-unsafe.md)
[Appendix: Influences](influences.md)

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@ -0,0 +1,630 @@
# Attributes
Any item declaration may have an _attribute_ applied to it. Attributes in Rust
are modeled on Attributes in ECMA-335, with the syntax coming from ECMA-334
(C#). An attribute is a general, free-form metadatum that is interpreted
according to name, convention, and language and compiler version. Attributes
may appear as any of:
* A single identifier, the attribute name
* An identifier followed by the equals sign '=' and a literal, providing a
key/value pair
* An identifier followed by a parenthesized list of sub-attribute arguments
Attributes with a bang ("!") after the hash ("#") apply to the item that the
attribute is declared within. Attributes that do not have a bang after the hash
apply to the item that follows the attribute.
An example of attributes:
```{.rust}
// General metadata applied to the enclosing module or crate.
#![crate_type = "lib"]
// A function marked as a unit test
#[test]
fn test_foo() {
/* ... */
}
// A conditionally-compiled module
#[cfg(target_os="linux")]
mod bar {
/* ... */
}
// A lint attribute used to suppress a warning/error
#[allow(non_camel_case_types)]
type int8_t = i8;
```
> **Note:** At some point in the future, the compiler will distinguish between
> language-reserved and user-available attributes. Until then, there is
> effectively no difference between an attribute handled by a loadable syntax
> extension and the compiler.
## Crate-only attributes
- `crate_name` - specify the crate's crate name.
- `crate_type` - see [linkage](linkage.html).
- `feature` - see [compiler features](#compiler-features).
- `no_builtins` - disable optimizing certain code patterns to invocations of
library functions that are assumed to exist
- `no_main` - disable emitting the `main` symbol. Useful when some other
object being linked to defines `main`.
- `no_start` - disable linking to the `native` crate, which specifies the
"start" language item.
- `no_std` - disable linking to the `std` crate.
- `plugin` - load a list of named crates as compiler plugins, e.g.
`#![plugin(foo, bar)]`. Optional arguments for each plugin,
i.e. `#![plugin(foo(... args ...))]`, are provided to the plugin's
registrar function. The `plugin` feature gate is required to use
this attribute.
- `recursion_limit` - Sets the maximum depth for potentially
infinitely-recursive compile-time operations like
auto-dereference or macro expansion. The default is
`#![recursion_limit="64"]`.
### Module-only attributes
- `no_implicit_prelude` - disable injecting `use std::prelude::*` in this
module.
- `path` - specifies the file to load the module from. `#[path="foo.rs"] mod
bar;` is equivalent to `mod bar { /* contents of foo.rs */ }`. The path is
taken relative to the directory that the current module is in.
## Function-only attributes
- `main` - indicates that this function should be passed to the entry point,
rather than the function in the crate root named `main`.
- `plugin_registrar` - mark this function as the registration point for
[compiler plugins][plugin], such as loadable syntax extensions.
- `start` - indicates that this function should be used as the entry point,
overriding the "start" language item. See the "start" [language
item](#language-items) for more details.
- `test` - indicates that this function is a test function, to only be compiled
in case of `--test`.
- `should_panic` - indicates that this test function should panic, inverting the success condition.
- `cold` - The function is unlikely to be executed, so optimize it (and calls
to it) differently.
- `naked` - The function utilizes a custom ABI or custom inline ASM that requires
epilogue and prologue to be skipped.
## Static-only attributes
- `thread_local` - on a `static mut`, this signals that the value of this
static may change depending on the current thread. The exact consequences of
this are implementation-defined.
## FFI attributes
On an `extern` block, the following attributes are interpreted:
- `link_args` - specify arguments to the linker, rather than just the library
name and type. This is feature gated and the exact behavior is
implementation-defined (due to variety of linker invocation syntax).
- `link` - indicate that a native library should be linked to for the
declarations in this block to be linked correctly. `link` supports an optional
`kind` key with three possible values: `dylib`, `static`, and `framework`. See
[external blocks](items.html#external-blocks) for more about external blocks. Two
examples: `#[link(name = "readline")]` and
`#[link(name = "CoreFoundation", kind = "framework")]`.
- `linked_from` - indicates what native library this block of FFI items is
coming from. This attribute is of the form `#[linked_from = "foo"]` where
`foo` is the name of a library in either `#[link]` or a `-l` flag. This
attribute is currently required to export symbols from a Rust dynamic library
on Windows, and it is feature gated behind the `linked_from` feature.
On declarations inside an `extern` block, the following attributes are
interpreted:
- `link_name` - the name of the symbol that this function or static should be
imported as.
- `linkage` - on a static, this specifies the [linkage
type](http://llvm.org/docs/LangRef.html#linkage-types).
On `enum`s:
- `repr` - on C-like enums, this sets the underlying type used for
representation. Takes one argument, which is the primitive
type this enum should be represented for, or `C`, which specifies that it
should be the default `enum` size of the C ABI for that platform. Note that
enum representation in C is undefined, and this may be incorrect when the C
code is compiled with certain flags.
On `struct`s:
- `repr` - specifies the representation to use for this struct. Takes a list
of options. The currently accepted ones are `C` and `packed`, which may be
combined. `C` will use a C ABI compatible struct layout, and `packed` will
remove any padding between fields (note that this is very fragile and may
break platforms which require aligned access).
## Macro-related attributes
- `macro_use` on a `mod` — macros defined in this module will be visible in the
module's parent, after this module has been included.
- `macro_use` on an `extern crate` — load macros from this crate. An optional
list of names `#[macro_use(foo, bar)]` restricts the import to just those
macros named. The `extern crate` must appear at the crate root, not inside
`mod`, which ensures proper function of the [`$crate` macro
variable](../book/macros.html#the-variable-crate).
- `macro_reexport` on an `extern crate` — re-export the named macros.
- `macro_export` - export a macro for cross-crate usage.
- `no_link` on an `extern crate` — even if we load this crate for macros, don't
link it into the output.
See the [macros section of the
book](../book/macros.html#scoping-and-macro-importexport) for more information on
macro scope.
## Miscellaneous attributes
- `deprecated` - mark the item as deprecated; the full attribute is
`#[deprecated(since = "crate version", note = "...")`, where both arguments
are optional.
- `export_name` - on statics and functions, this determines the name of the
exported symbol.
- `link_section` - on statics and functions, this specifies the section of the
object file that this item's contents will be placed into.
- `no_mangle` - on any item, do not apply the standard name mangling. Set the
symbol for this item to its identifier.
- `simd` - on certain tuple structs, derive the arithmetic operators, which
lower to the target's SIMD instructions, if any; the `simd` feature gate
is necessary to use this attribute.
- `unsafe_destructor_blind_to_params` - on `Drop::drop` method, asserts that the
destructor code (and all potential specializations of that code) will
never attempt to read from nor write to any references with lifetimes
that come in via generic parameters. This is a constraint we cannot
currently express via the type system, and therefore we rely on the
programmer to assert that it holds. Adding this to a Drop impl causes
the associated destructor to be considered "uninteresting" by the
Drop-Check rule, and thus it can help sidestep data ordering
constraints that would otherwise be introduced by the Drop-Check
rule. Such sidestepping of the constraints, if done incorrectly, can
lead to undefined behavior (in the form of reading or writing to data
outside of its dynamic extent), and thus this attribute has the word
"unsafe" in its name. To use this, the
`unsafe_destructor_blind_to_params` feature gate must be enabled.
- `doc` - Doc comments such as `/// foo` are equivalent to `#[doc = "foo"]`.
- `rustc_on_unimplemented` - Write a custom note to be shown along with the error
when the trait is found to be unimplemented on a type.
You may use format arguments like `{T}`, `{A}` to correspond to the
types at the point of use corresponding to the type parameters of the
trait of the same name. `{Self}` will be replaced with the type that is supposed
to implement the trait but doesn't. To use this, the `on_unimplemented` feature gate
must be enabled.
- `must_use` - on structs and enums, will warn if a value of this type isn't used or
assigned to a variable. You may also include an optional message by using
`#[must_use = "message"]` which will be given alongside the warning.
### Conditional compilation
Sometimes one wants to have different compiler outputs from the same code,
depending on build target, such as targeted operating system, or to enable
release builds.
Configuration options are boolean (on or off) and are named either with a
single identifier (e.g. `foo`) or an identifier and a string (e.g. `foo = "bar"`;
the quotes are required and spaces around the `=` are unimportant). Note that
similarly-named options, such as `foo`, `foo="bar"` and `foo="baz"` may each be set
or unset independently.
Configuration options are either provided by the compiler or passed in on the
command line using `--cfg` (e.g. `rustc main.rs --cfg foo --cfg 'bar="baz"'`).
Rust code then checks for their presence using the `#[cfg(...)]` attribute:
```
// The function is only included in the build when compiling for OSX
#[cfg(target_os = "macos")]
fn macos_only() {
// ...
}
// This function is only included when either foo or bar is defined
#[cfg(any(foo, bar))]
fn needs_foo_or_bar() {
// ...
}
// This function is only included when compiling for a unixish OS with a 32-bit
// architecture
#[cfg(all(unix, target_pointer_width = "32"))]
fn on_32bit_unix() {
// ...
}
// This function is only included when foo is not defined
#[cfg(not(foo))]
fn needs_not_foo() {
// ...
}
```
This illustrates some conditional compilation can be achieved using the
`#[cfg(...)]` attribute. `any`, `all` and `not` can be used to assemble
arbitrarily complex configurations through nesting.
The following configurations must be defined by the implementation:
* `target_arch = "..."` - Target CPU architecture, such as `"x86"`,
`"x86_64"` `"mips"`, `"powerpc"`, `"powerpc64"`, `"arm"`, or
`"aarch64"`. This value is closely related to the first element of
the platform target triple, though it is not identical.
* `target_os = "..."` - Operating system of the target, examples
include `"windows"`, `"macos"`, `"ios"`, `"linux"`, `"android"`,
`"freebsd"`, `"dragonfly"`, `"bitrig"` , `"openbsd"` or
`"netbsd"`. This value is closely related to the second and third
element of the platform target triple, though it is not identical.
* `target_family = "..."` - Operating system family of the target, e. g.
`"unix"` or `"windows"`. The value of this configuration option is defined
as a configuration itself, like `unix` or `windows`.
* `unix` - See `target_family`.
* `windows` - See `target_family`.
* `target_env = ".."` - Further disambiguates the target platform with
information about the ABI/libc. Presently this value is either
`"gnu"`, `"msvc"`, `"musl"`, or the empty string. For historical
reasons this value has only been defined as non-empty when needed
for disambiguation. Thus on many GNU platforms this value will be
empty. This value is closely related to the fourth element of the
platform target triple, though it is not identical. For example,
embedded ABIs such as `gnueabihf` will simply define `target_env` as
`"gnu"`.
* `target_endian = "..."` - Endianness of the target CPU, either `"little"` or
`"big"`.
* `target_pointer_width = "..."` - Target pointer width in bits. This is set
to `"32"` for targets with 32-bit pointers, and likewise set to `"64"` for
64-bit pointers.
* `target_has_atomic = "..."` - Set of integer sizes on which the target can perform
atomic operations. Values are `"8"`, `"16"`, `"32"`, `"64"` and `"ptr"`.
* `target_vendor = "..."` - Vendor of the target, for example `apple`, `pc`, or
simply `"unknown"`.
* `test` - Enabled when compiling the test harness (using the `--test` flag).
* `debug_assertions` - Enabled by default when compiling without optimizations.
This can be used to enable extra debugging code in development but not in
production. For example, it controls the behavior of the standard library's
`debug_assert!` macro.
You can also set another attribute based on a `cfg` variable with `cfg_attr`:
```rust,ignore
#[cfg_attr(a, b)]
```
This is the same as `#[b]` if `a` is set by `cfg`, and nothing otherwise.
Lastly, configuration options can be used in expressions by invoking the `cfg!`
macro: `cfg!(a)` evaluates to `true` if `a` is set, and `false` otherwise.
### Lint check attributes
A lint check names a potentially undesirable coding pattern, such as
unreachable code or omitted documentation, for the static entity to which the
attribute applies.
For any lint check `C`:
* `allow(C)` overrides the check for `C` so that violations will go
unreported,
* `deny(C)` signals an error after encountering a violation of `C`,
* `forbid(C)` is the same as `deny(C)`, but also forbids changing the lint
level afterwards,
* `warn(C)` warns about violations of `C` but continues compilation.
The lint checks supported by the compiler can be found via `rustc -W help`,
along with their default settings. [Compiler
plugins](../book/compiler-plugins.html#lint-plugins) can provide additional
lint checks.
```{.ignore}
pub mod m1 {
// Missing documentation is ignored here
#[allow(missing_docs)]
pub fn undocumented_one() -> i32 { 1 }
// Missing documentation signals a warning here
#[warn(missing_docs)]
pub fn undocumented_too() -> i32 { 2 }
// Missing documentation signals an error here
#[deny(missing_docs)]
pub fn undocumented_end() -> i32 { 3 }
}
```
This example shows how one can use `allow` and `warn` to toggle a particular
check on and off:
```{.ignore}
#[warn(missing_docs)]
pub mod m2{
#[allow(missing_docs)]
pub mod nested {
// Missing documentation is ignored here
pub fn undocumented_one() -> i32 { 1 }
// Missing documentation signals a warning here,
// despite the allow above.
#[warn(missing_docs)]
pub fn undocumented_two() -> i32 { 2 }
}
// Missing documentation signals a warning here
pub fn undocumented_too() -> i32 { 3 }
}
```
This example shows how one can use `forbid` to disallow uses of `allow` for
that lint check:
```{.ignore}
#[forbid(missing_docs)]
pub mod m3 {
// Attempting to toggle warning signals an error here
#[allow(missing_docs)]
/// Returns 2.
pub fn undocumented_too() -> i32 { 2 }
}
```
### Language items
Some primitive Rust operations are defined in Rust code, rather than being
implemented directly in C or assembly language. The definitions of these
operations have to be easy for the compiler to find. The `lang` attribute
makes it possible to declare these operations. For example, the `str` module
in the Rust standard library defines the string equality function:
```{.ignore}
#[lang = "str_eq"]
pub fn eq_slice(a: &str, b: &str) -> bool {
// details elided
}
```
The name `str_eq` has a special meaning to the Rust compiler, and the presence
of this definition means that it will use this definition when generating calls
to the string equality function.
The set of language items is currently considered unstable. A complete
list of the built-in language items will be added in the future.
### Inline attributes
The inline attribute suggests that the compiler should place a copy of
the function or static in the caller, rather than generating code to
call the function or access the static where it is defined.
The compiler automatically inlines functions based on internal heuristics.
Incorrectly inlining functions can actually make the program slower, so it
should be used with care.
`#[inline]` and `#[inline(always)]` always cause the function to be serialized
into the crate metadata to allow cross-crate inlining.
There are three different types of inline attributes:
* `#[inline]` hints the compiler to perform an inline expansion.
* `#[inline(always)]` asks the compiler to always perform an inline expansion.
* `#[inline(never)]` asks the compiler to never perform an inline expansion.
### `derive`
The `derive` attribute allows certain traits to be automatically implemented
for data structures. For example, the following will create an `impl` for the
`PartialEq` and `Clone` traits for `Foo`, the type parameter `T` will be given
the `PartialEq` or `Clone` constraints for the appropriate `impl`:
```
#[derive(PartialEq, Clone)]
struct Foo<T> {
a: i32,
b: T,
}
```
The generated `impl` for `PartialEq` is equivalent to
```
# struct Foo<T> { a: i32, b: T }
impl<T: PartialEq> PartialEq for Foo<T> {
fn eq(&self, other: &Foo<T>) -> bool {
self.a == other.a && self.b == other.b
}
fn ne(&self, other: &Foo<T>) -> bool {
self.a != other.a || self.b != other.b
}
}
```
You can implement `derive` for your own type through [procedural
macros](procedural-macros.html).
### Compiler Features
Certain aspects of Rust may be implemented in the compiler, but they're not
necessarily ready for every-day use. These features are often of "prototype
quality" or "almost production ready", but may not be stable enough to be
considered a full-fledged language feature.
For this reason, Rust recognizes a special crate-level attribute of the form:
```{.ignore}
#![feature(feature1, feature2, feature3)]
```
This directive informs the compiler that the feature list: `feature1`,
`feature2`, and `feature3` should all be enabled. This is only recognized at a
crate-level, not at a module-level. Without this directive, all features are
considered off, and using the features will result in a compiler error.
The currently implemented features of the reference compiler are:
* `advanced_slice_patterns` - See the [match
expressions](expressions.html#match-expressions)
section for discussion; the exact semantics of
slice patterns are subject to change, so some types are still unstable.
* `slice_patterns` - OK, actually, slice patterns are just scary and
completely unstable.
* `asm` - The `asm!` macro provides a means for inline assembly. This is often
useful, but the exact syntax for this feature along with its
semantics are likely to change, so this macro usage must be opted
into.
* `associated_consts` - Allows constants to be defined in `impl` and `trait`
blocks, so that they can be associated with a type or
trait in a similar manner to methods and associated
types.
* `box_patterns` - Allows `box` patterns, the exact semantics of which
is subject to change.
* `box_syntax` - Allows use of `box` expressions, the exact semantics of which
is subject to change.
* `cfg_target_vendor` - Allows conditional compilation using the `target_vendor`
matcher which is subject to change.
* `cfg_target_has_atomic` - Allows conditional compilation using the `target_has_atomic`
matcher which is subject to change.
* `concat_idents` - Allows use of the `concat_idents` macro, which is in many
ways insufficient for concatenating identifiers, and may be
removed entirely for something more wholesome.
* `custom_attribute` - Allows the usage of attributes unknown to the compiler
so that new attributes can be added in a backwards compatible
manner (RFC 572).
* `custom_derive` - Allows the use of `#[derive(Foo,Bar)]` as sugar for
`#[derive_Foo] #[derive_Bar]`, which can be user-defined syntax
extensions.
* `inclusive_range_syntax` - Allows use of the `a...b` and `...b` syntax for inclusive ranges.
* `inclusive_range` - Allows use of the types that represent desugared inclusive ranges.
* `intrinsics` - Allows use of the "rust-intrinsics" ABI. Compiler intrinsics
are inherently unstable and no promise about them is made.
* `lang_items` - Allows use of the `#[lang]` attribute. Like `intrinsics`,
lang items are inherently unstable and no promise about them
is made.
* `link_args` - This attribute is used to specify custom flags to the linker,
but usage is strongly discouraged. The compiler's usage of the
system linker is not guaranteed to continue in the future, and
if the system linker is not used then specifying custom flags
doesn't have much meaning.
* `link_llvm_intrinsics` - Allows linking to LLVM intrinsics via
`#[link_name="llvm.*"]`.
* `linkage` - Allows use of the `linkage` attribute, which is not portable.
* `log_syntax` - Allows use of the `log_syntax` macro attribute, which is a
nasty hack that will certainly be removed.
* `main` - Allows use of the `#[main]` attribute, which changes the entry point
into a Rust program. This capability is subject to change.
* `macro_reexport` - Allows macros to be re-exported from one crate after being imported
from another. This feature was originally designed with the sole
use case of the Rust standard library in mind, and is subject to
change.
* `non_ascii_idents` - The compiler supports the use of non-ascii identifiers,
but the implementation is a little rough around the
edges, so this can be seen as an experimental feature
for now until the specification of identifiers is fully
fleshed out.
* `no_std` - Allows the `#![no_std]` crate attribute, which disables the implicit
`extern crate std`. This typically requires use of the unstable APIs
behind the libstd "facade", such as libcore and libcollections. It
may also cause problems when using syntax extensions, including
`#[derive]`.
* `on_unimplemented` - Allows the `#[rustc_on_unimplemented]` attribute, which allows
trait definitions to add specialized notes to error messages
when an implementation was expected but not found.
* `optin_builtin_traits` - Allows the definition of default and negative trait
implementations. Experimental.
* `plugin` - Usage of [compiler plugins][plugin] for custom lints or syntax extensions.
These depend on compiler internals and are subject to change.
* `plugin_registrar` - Indicates that a crate provides [compiler plugins][plugin].
* `quote` - Allows use of the `quote_*!` family of macros, which are
implemented very poorly and will likely change significantly
with a proper implementation.
* `rustc_attrs` - Gates internal `#[rustc_*]` attributes which may be
for internal use only or have meaning added to them in the future.
* `rustc_diagnostic_macros`- A mysterious feature, used in the implementation
of rustc, not meant for mortals.
* `simd` - Allows use of the `#[simd]` attribute, which is overly simple and
not the SIMD interface we want to expose in the long term.
* `simd_ffi` - Allows use of SIMD vectors in signatures for foreign functions.
The SIMD interface is subject to change.
* `start` - Allows use of the `#[start]` attribute, which changes the entry point
into a Rust program. This capability, especially the signature for the
annotated function, is subject to change.
* `thread_local` - The usage of the `#[thread_local]` attribute is experimental
and should be seen as unstable. This attribute is used to
declare a `static` as being unique per-thread leveraging
LLVM's implementation which works in concert with the kernel
loader and dynamic linker. This is not necessarily available
on all platforms, and usage of it is discouraged.
* `trace_macros` - Allows use of the `trace_macros` macro, which is a nasty
hack that will certainly be removed.
* `unboxed_closures` - Rust's new closure design, which is currently a work in
progress feature with many known bugs.
* `allow_internal_unstable` - Allows `macro_rules!` macros to be tagged with the
`#[allow_internal_unstable]` attribute, designed
to allow `std` macros to call
`#[unstable]`/feature-gated functionality
internally without imposing on callers
(i.e. making them behave like function calls in
terms of encapsulation).
* `default_type_parameter_fallback` - Allows type parameter defaults to
influence type inference.
* `stmt_expr_attributes` - Allows attributes on expressions.
* `type_ascription` - Allows type ascription expressions `expr: Type`.
* `abi_vectorcall` - Allows the usage of the vectorcall calling convention
(e.g. `extern "vectorcall" func fn_();`)
* `abi_sysv64` - Allows the usage of the system V AMD64 calling convention
(e.g. `extern "sysv64" func fn_();`)
If a feature is promoted to a language feature, then all existing programs will
start to receive compilation warnings about `#![feature]` directives which enabled
the new feature (because the directive is no longer necessary). However, if a
feature is decided to be removed from the language, errors will be issued (if
there isn't a parser error first). The directive in this case is no longer
necessary, and it's likely that existing code will break if the feature isn't
removed.
If an unknown feature is found in a directive, it results in a compiler error.
An unknown feature is one which has never been recognized by the compiler.

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## Behavior considered undefined
The following is a list of behavior which is forbidden in all Rust code,
including within `unsafe` blocks and `unsafe` functions. Type checking provides
the guarantee that these issues are never caused by safe code.
* Data races
* Dereferencing a null/dangling raw pointer
* Reads of [undef](http://llvm.org/docs/LangRef.html#undefined-values)
(uninitialized) memory
* Breaking the [pointer aliasing
rules](http://llvm.org/docs/LangRef.html#pointer-aliasing-rules)
with raw pointers (a subset of the rules used by C)
* `&mut T` and `&T` follow LLVM’s scoped [noalias] model, except if the `&T`
contains an `UnsafeCell<U>`. Unsafe code must not violate these aliasing
guarantees.
* Mutating non-mutable data (that is, data reached through a shared reference or
data owned by a `let` binding), unless that data is contained within an `UnsafeCell<U>`.
* Invoking undefined behavior via compiler intrinsics:
* Indexing outside of the bounds of an object with `std::ptr::offset`
(`offset` intrinsic), with
the exception of one byte past the end which is permitted.
* Using `std::ptr::copy_nonoverlapping_memory` (`memcpy32`/`memcpy64`
intrinsics) on overlapping buffers
* Invalid values in primitive types, even in private fields/locals:
* Dangling/null references or boxes
* A value other than `false` (0) or `true` (1) in a `bool`
* A discriminant in an `enum` not included in the type definition
* A value in a `char` which is a surrogate or above `char::MAX`
* Non-UTF-8 byte sequences in a `str`
* Unwinding into Rust from foreign code or unwinding from Rust into foreign
code. Rust's failure system is not compatible with exception handling in
other languages. Unwinding must be caught and handled at FFI boundaries.
[noalias]: http://llvm.org/docs/LangRef.html#noalias

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## Behavior not considered unsafe
This is a list of behavior not considered *unsafe* in Rust terms, but that may
be undesired.
* Deadlocks
* Leaks of memory and other resources
* Exiting without calling destructors
* Integer overflow
- Overflow is considered "unexpected" behavior and is always user-error,
unless the `wrapping` primitives are used. In non-optimized builds, the compiler
will insert debug checks that panic on overflow, but in optimized builds overflow
instead results in wrapped values. See [RFC 560] for the rationale and more details.
[RFC 560]: https://github.com/rust-lang/rfcs/blob/master/text/0560-integer-overflow.md

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# Comments
Comments in Rust code follow the general C++ style of line (`//`) and
block (`/* ... */`) comment forms. Nested block comments are supported.
Line comments beginning with exactly _three_ slashes (`///`), and block
comments (`/** ... */`), are interpreted as a special syntax for `doc`
[attributes]. That is, they are equivalent to writing
`#[doc="..."]` around the body of the comment, i.e., `/// Foo` turns into
`#[doc="Foo"]`.
Line comments beginning with `//!` and block comments `/*! ... */` are
doc comments that apply to the parent of the comment, rather than the item
that follows. That is, they are equivalent to writing `#![doc="..."]` around
the body of the comment. `//!` comments are usually used to document
modules that occupy a source file.
Non-doc comments are interpreted as a form of whitespace.
[attributes]: attributes.html

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# Crates and source files
Although Rust, like any other language, can be implemented by an interpreter as
well as a compiler, the only existing implementation is a compiler,
and the language has
always been designed to be compiled. For these reasons, this section assumes a
compiler.
Rust's semantics obey a *phase distinction* between compile-time and
run-time.[^phase-distinction] Semantic rules that have a *static
interpretation* govern the success or failure of compilation, while
semantic rules
that have a *dynamic interpretation* govern the behavior of the program at
run-time.
The compilation model centers on artifacts called _crates_. Each compilation
processes a single crate in source form, and if successful, produces a single
crate in binary form: either an executable or some sort of
library.[^cratesourcefile]
A _crate_ is a unit of compilation and linking, as well as versioning,
distribution and runtime loading. A crate contains a _tree_ of nested
[module] scopes. The top level of this tree is a module that is
anonymous (from the point of view of paths within the module) and any item
within a crate has a canonical [module path] denoting its location
within the crate's module tree.
The Rust compiler is always invoked with a single source file as input, and
always produces a single output crate. The processing of that source file may
result in other source files being loaded as modules. Source files have the
extension `.rs`.
A Rust source file describes a module, the name and location of which &mdash;
in the module tree of the current crate &mdash; are defined from outside the
source file: either by an explicit `mod_item` in a referencing source file, or
by the name of the crate itself. Every source file is a module, but not every
module needs its own source file: [module definitions][module] can be nested
within one file.
Each source file contains a sequence of zero or more `item` definitions, and
may optionally begin with any number of [attributes]
that apply to the containing module, most of which influence the behavior of
the compiler. The anonymous crate module can have additional attributes that
apply to the crate as a whole.
```rust,no_run
// Specify the crate name.
#![crate_name = "projx"]
// Specify the type of output artifact.
#![crate_type = "lib"]
// Turn on a warning.
// This can be done in any module, not just the anonymous crate module.
#![warn(non_camel_case_types)]
```
A crate that contains a `main` function can be compiled to an executable. If a
`main` function is present, its return type must be `()`
("[unit]") and it must take no arguments.
[^phase-distinction]: This distinction would also exist in an interpreter.
Static checks like syntactic analysis, type checking, and lints should
happen before the program is executed regardless of when it is executed.
[^cratesourcefile]: A crate is somewhat analogous to an *assembly* in the
ECMA-335 CLI model, a *library* in the SML/NJ Compilation Manager, a *unit*
in the Owens and Flatt module system, or a *configuration* in Mesa.
[module]: items.html#modules
[module path]: paths.html
[attributes]: items-and-attributes.html
[unit]: types.html#tuple-types

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# Expressions
An expression may have two roles: it always produces a *value*, and it may have
*effects* (otherwise known as "side effects"). An expression *evaluates to* a
value, and has effects during *evaluation*. Many expressions contain
sub-expressions (operands). The meaning of each kind of expression dictates
several things:
* Whether or not to evaluate the sub-expressions when evaluating the expression
* The order in which to evaluate the sub-expressions
* How to combine the sub-expressions' values to obtain the value of the expression
In this way, the structure of expressions dictates the structure of execution.
Blocks are just another kind of expression, so blocks, statements, expressions,
and blocks again can recursively nest inside each other to an arbitrary depth.
### Lvalues, rvalues and temporaries
Expressions are divided into two main categories: _lvalues_ and _rvalues_.
Likewise within each expression, sub-expressions may occur in _lvalue context_
or _rvalue context_. The evaluation of an expression depends both on its own
category and the context it occurs within.
An lvalue is an expression that represents a memory location. These expressions
are [paths](#path-expressions) (which refer to local variables, function and
method arguments, or static variables), dereferences (`*expr`), [indexing
expressions](#index-expressions) (`expr[expr]`), and [field
references](#field-expressions) (`expr.f`). All other expressions are rvalues.
The left operand of an [assignment](#assignment-expressions) or
[compound-assignment](#compound-assignment-expressions) expression is
an lvalue context, as is the single operand of a unary
[borrow](#unary-operator-expressions). The discriminant or subject of
a [match expression](#match-expressions) may be an lvalue context, if
ref bindings are made, but is otherwise an rvalue context. All other
expression contexts are rvalue contexts.
When an lvalue is evaluated in an _lvalue context_, it denotes a memory
location; when evaluated in an _rvalue context_, it denotes the value held _in_
that memory location.
#### Temporary lifetimes
When an rvalue is used in an lvalue context, a temporary un-named
lvalue is created and used instead. The lifetime of temporary values
is typically the innermost enclosing statement; the tail expression of
a block is considered part of the statement that encloses the block.
When a temporary rvalue is being created that is assigned into a `let`
declaration, however, the temporary is created with the lifetime of
the enclosing block instead, as using the enclosing statement (the
`let` declaration) would be a guaranteed error (since a pointer to the
temporary would be stored into a variable, but the temporary would be
freed before the variable could be used). The compiler uses simple
syntactic rules to decide which values are being assigned into a `let`
binding, and therefore deserve a longer temporary lifetime.
Here are some examples:
- `let x = foo(&temp())`. The expression `temp()` is an rvalue. As it
is being borrowed, a temporary is created which will be freed after
the innermost enclosing statement (the `let` declaration, in this case).
- `let x = temp().foo()`. This is the same as the previous example,
except that the value of `temp()` is being borrowed via autoref on a
method-call. Here we are assuming that `foo()` is an `&self` method
defined in some trait, say `Foo`. In other words, the expression
`temp().foo()` is equivalent to `Foo::foo(&temp())`.
- `let x = &temp()`. Here, the same temporary is being assigned into
`x`, rather than being passed as a parameter, and hence the
temporary's lifetime is considered to be the enclosing block.
- `let x = SomeStruct { foo: &temp() }`. As in the previous case, the
temporary is assigned into a struct which is then assigned into a
binding, and hence it is given the lifetime of the enclosing block.
- `let x = [ &temp() ]`. As in the previous case, the
temporary is assigned into an array which is then assigned into a
binding, and hence it is given the lifetime of the enclosing block.
- `let ref x = temp()`. In this case, the temporary is created using a ref binding,
but the result is the same: the lifetime is extended to the enclosing block.
### Moved and copied types
When a [local variable](variables.html) is used as an
[rvalue](expressions.html#lvalues-rvalues-and-temporaries), the variable will
be copied if its type implements `Copy`. All others are moved.
## Literal expressions
A _literal expression_ consists of one of the [literal](tokens.html#literals) forms
described earlier. It directly describes a number, character, string, boolean
value, or the unit value.
```text
(); // unit type
"hello"; // string type
'5'; // character type
5; // integer type
```
## Path expressions
A [path](paths.html) used as an expression context denotes either a local
variable or an item. Path expressions are
[lvalues](expressions.html#lvalues-rvalues-and-temporaries).
## Tuple expressions
Tuples are written by enclosing zero or more comma-separated expressions in
parentheses. They are used to create [tuple-typed](types.html#tuple-types)
values.
```{.tuple}
(0.0, 4.5);
("a", 4usize, true);
```
You can disambiguate a single-element tuple from a value in parentheses with a
comma:
```
(0,); // single-element tuple
(0); // zero in parentheses
```
## Struct expressions
There are several forms of struct expressions. A _struct expression_
consists of the [path](paths.html) of a [struct item](items.html#structs), followed
by a brace-enclosed list of zero or more comma-separated name-value pairs,
providing the field values of a new instance of the struct. A field name can be
any identifier, and is separated from its value expression by a colon. The
location denoted by a struct field is mutable if and only if the enclosing
struct is mutable.
A _tuple struct expression_ consists of the [path](paths.html) of a [struct
item](items.html#structs), followed by a parenthesized list of one or more
comma-separated expressions (in other words, the path of a struct item followed
by a tuple expression). The struct item must be a tuple struct item.
A _unit-like struct expression_ consists only of the [path](paths.html) of a
[struct item](items.html#structs).
The following are examples of struct expressions:
```
# struct Point { x: f64, y: f64 }
# struct NothingInMe { }
# struct TuplePoint(f64, f64);
# mod game { pub struct User<'a> { pub name: &'a str, pub age: u32, pub score: usize } }
# struct Cookie; fn some_fn<T>(t: T) {}
Point {x: 10.0, y: 20.0};
NothingInMe {};
TuplePoint(10.0, 20.0);
let u = game::User {name: "Joe", age: 35, score: 100_000};
some_fn::<Cookie>(Cookie);
```
A struct expression forms a new value of the named struct type. Note
that for a given *unit-like* struct type, this will always be the same
value.
A struct expression can terminate with the syntax `..` followed by an
expression to denote a functional update. The expression following `..` (the
base) must have the same struct type as the new struct type being formed.
The entire expression denotes the result of constructing a new struct (with
the same type as the base expression) with the given values for the fields that
were explicitly specified and the values in the base expression for all other
fields.
```
# struct Point3d { x: i32, y: i32, z: i32 }
let base = Point3d {x: 1, y: 2, z: 3};
Point3d {y: 0, z: 10, .. base};
```
#### Struct field init shorthand
When initializing a data structure (struct, enum, union) with named fields,
it is allowed to write `fieldname` as a shorthand for `fieldname: fieldname`.
This allows a compact syntax with less duplication.
Example:
```
# struct Point3d { x: i32, y: i32, z: i32 }
# let x = 0;
# let y_value = 0;
# let z = 0;
Point3d { x: x, y: y_value, z: z };
Point3d { x, y: y_value, z };
```
## Block expressions
A _block expression_ is similar to a module in terms of the declarations that
are possible. Each block conceptually introduces a new namespace scope. Use
items can bring new names into scopes and declared items are in scope for only
the block itself.
A block will execute each statement sequentially, and then execute the
expression (if given). If the block ends in a statement, its value is `()`:
```
let x: () = { println!("Hello."); };
```
If it ends in an expression, its value and type are that of the expression:
```
let x: i32 = { println!("Hello."); 5 };
assert_eq!(5, x);
```
## Method-call expressions
A _method call_ consists of an expression followed by a single dot, an
identifier, and a parenthesized expression-list. Method calls are resolved to
methods on specific traits, either statically dispatching to a method if the
exact `self`-type of the left-hand-side is known, or dynamically dispatching if
the left-hand-side expression is an indirect [trait
object](types.html#trait-objects).
## Field expressions
A _field expression_ consists of an expression followed by a single dot and an
identifier, when not immediately followed by a parenthesized expression-list
(the latter is a [method call expression](#method-call-expressions)). A field
expression denotes a field of a [struct](types.html#struct-types).
```{.ignore .field}
mystruct.myfield;
foo().x;
(Struct {a: 10, b: 20}).a;
```
A field access is an [lvalue](expressions.html#lvalues-rvalues-and-temporaries)
referring to the value of that field. When the type providing the field
inherits mutability, it can be [assigned](#assignment-expressions) to.
Also, if the type of the expression to the left of the dot is a
pointer, it is automatically dereferenced as many times as necessary
to make the field access possible. In cases of ambiguity, we prefer
fewer autoderefs to more.
## Array expressions
An [array](types.html#array-and-slice-types) _expression_ is written by
enclosing zero or more comma-separated expressions of uniform type in square
brackets.
In the `[expr ';' expr]` form, the expression after the `';'` must be a
constant expression that can be evaluated at compile time, such as a
[literal](tokens.html#literals) or a [static item](items.html#static-items).
```
[1, 2, 3, 4];
["a", "b", "c", "d"];
[0; 128]; // array with 128 zeros
[0u8, 0u8, 0u8, 0u8];
```
## Index expressions
[Array](types.html#array-and-slice-types)-typed expressions can be indexed by
writing a square-bracket-enclosed expression (the index) after them. When the
array is mutable, the resulting
[lvalue](expressions.html#lvalues-rvalues-and-temporaries) can be assigned to.
Indices are zero-based, and may be of any integral type. Vector access is
bounds-checked at compile-time for constant arrays being accessed with a
constant index value. Otherwise a check will be performed at run-time that
will put the thread in a _panicked state_ if it fails.
```{should-fail}
([1, 2, 3, 4])[0];
let x = (["a", "b"])[10]; // compiler error: const index-expr is out of bounds
let n = 10;
let y = (["a", "b"])[n]; // panics
let arr = ["a", "b"];
arr[10]; // panics
```
Also, if the type of the expression to the left of the brackets is a
pointer, it is automatically dereferenced as many times as necessary
to make the indexing possible. In cases of ambiguity, we prefer fewer
autoderefs to more.
## Range expressions
The `..` operator will construct an object of one of the `std::ops::Range` variants.
```
1..2; // std::ops::Range
3..; // std::ops::RangeFrom
..4; // std::ops::RangeTo
..; // std::ops::RangeFull
```
The following expressions are equivalent.
```
let x = std::ops::Range {start: 0, end: 10};
let y = 0..10;
assert_eq!(x, y);
```
Similarly, the `...` operator will construct an object of one of the
`std::ops::RangeInclusive` variants.
```
# #![feature(inclusive_range_syntax)]
1...2; // std::ops::RangeInclusive
...4; // std::ops::RangeToInclusive
```
The following expressions are equivalent.
```
# #![feature(inclusive_range_syntax, inclusive_range)]
let x = std::ops::RangeInclusive::NonEmpty {start: 0, end: 10};
let y = 0...10;
assert_eq!(x, y);
```
## Unary operator expressions
Rust defines the following unary operators. With the exception of `?`, they are
all written as prefix operators, before the expression they apply to.
* `-`
: Negation. Signed integer types and floating-point types support negation. It
is an error to apply negation to unsigned types; for example, the compiler
rejects `-1u32`.
* `*`
: Dereference. When applied to a [pointer](types.html#pointer-types) it
denotes the pointed-to location. For pointers to mutable locations, the
resulting [lvalue](expressions.html#lvalues-rvalues-and-temporaries) can be
assigned to. On non-pointer types, it calls the `deref` method of the
`std::ops::Deref` trait, or the `deref_mut` method of the
`std::ops::DerefMut` trait (if implemented by the type and required for an
outer expression that will or could mutate the dereference), and produces
the result of dereferencing the `&` or `&mut` borrowed pointer returned
from the overload method.
* `!`
: Logical negation. On the boolean type, this flips between `true` and
`false`. On integer types, this inverts the individual bits in the
two's complement representation of the value.
* `&` and `&mut`
: Borrowing. When applied to an lvalue, these operators produce a
reference (pointer) to the lvalue. The lvalue is also placed into
a borrowed state for the duration of the reference. For a shared
borrow (`&`), this implies that the lvalue may not be mutated, but
it may be read or shared again. For a mutable borrow (`&mut`), the
lvalue may not be accessed in any way until the borrow expires.
If the `&` or `&mut` operators are applied to an rvalue, a
temporary value is created; the lifetime of this temporary value
is defined by [syntactic rules](#temporary-lifetimes).
* `?`
: Propagating errors if applied to `Err(_)` and unwrapping if
applied to `Ok(_)`. Only works on the `Result<T, E>` type,
and written in postfix notation.
## Binary operator expressions
Binary operators expressions are given in terms of [operator
precedence](#operator-precedence).
### Arithmetic operators
Binary arithmetic expressions are syntactic sugar for calls to built-in traits,
defined in the `std::ops` module of the `std` library. This means that
arithmetic operators can be overridden for user-defined types. The default
meaning of the operators on standard types is given here.
* `+`
: Addition and array/string concatenation.
Calls the `add` method on the `std::ops::Add` trait.
* `-`
: Subtraction.
Calls the `sub` method on the `std::ops::Sub` trait.
* `*`
: Multiplication.
Calls the `mul` method on the `std::ops::Mul` trait.
* `/`
: Quotient.
Calls the `div` method on the `std::ops::Div` trait.
* `%`
: Remainder.
Calls the `rem` method on the `std::ops::Rem` trait.
### Bitwise operators
Like the [arithmetic operators](#arithmetic-operators), bitwise operators are
syntactic sugar for calls to methods of built-in traits. This means that
bitwise operators can be overridden for user-defined types. The default
meaning of the operators on standard types is given here. Bitwise `&`, `|` and
`^` applied to boolean arguments are equivalent to logical `&&`, `||` and `!=`
evaluated in non-lazy fashion.
* `&`
: Bitwise AND.
Calls the `bitand` method of the `std::ops::BitAnd` trait.
* `|`
: Bitwise inclusive OR.
Calls the `bitor` method of the `std::ops::BitOr` trait.
* `^`
: Bitwise exclusive OR.
Calls the `bitxor` method of the `std::ops::BitXor` trait.
* `<<`
: Left shift.
Calls the `shl` method of the `std::ops::Shl` trait.
* `>>`
: Right shift (arithmetic).
Calls the `shr` method of the `std::ops::Shr` trait.
### Lazy boolean operators
The operators `||` and `&&` may be applied to operands of boolean type. The
`||` operator denotes logical 'or', and the `&&` operator denotes logical
'and'. They differ from `|` and `&` in that the right-hand operand is only
evaluated when the left-hand operand does not already determine the result of
the expression. That is, `||` only evaluates its right-hand operand when the
left-hand operand evaluates to `false`, and `&&` only when it evaluates to
`true`.
### Comparison operators
Comparison operators are, like the [arithmetic
operators](#arithmetic-operators), and [bitwise operators](#bitwise-operators),
syntactic sugar for calls to built-in traits. This means that comparison
operators can be overridden for user-defined types. The default meaning of the
operators on standard types is given here.
* `==`
: Equal to.
Calls the `eq` method on the `std::cmp::PartialEq` trait.
* `!=`
: Unequal to.
Calls the `ne` method on the `std::cmp::PartialEq` trait.
* `<`
: Less than.
Calls the `lt` method on the `std::cmp::PartialOrd` trait.
* `>`
: Greater than.
Calls the `gt` method on the `std::cmp::PartialOrd` trait.
* `<=`
: Less than or equal.
Calls the `le` method on the `std::cmp::PartialOrd` trait.
* `>=`
: Greater than or equal.
Calls the `ge` method on the `std::cmp::PartialOrd` trait.
### Type cast expressions
A type cast expression is denoted with the binary operator `as`.
Executing an `as` expression casts the value on the left-hand side to the type
on the right-hand side.
An example of an `as` expression:
```
# fn sum(values: &[f64]) -> f64 { 0.0 }
# fn len(values: &[f64]) -> i32 { 0 }
fn average(values: &[f64]) -> f64 {
let sum: f64 = sum(values);
let size: f64 = len(values) as f64;
sum / size
}
```
Some of the conversions which can be done through the `as` operator
can also be done implicitly at various points in the program, such as
argument passing and assignment to a `let` binding with an explicit
type. Implicit conversions are limited to "harmless" conversions that
do not lose information and which have minimal or no risk of
surprising side-effects on the dynamic execution semantics.
### Assignment expressions
An _assignment expression_ consists of an
[lvalue](expressions.html#lvalues-rvalues-and-temporaries) expression followed
by an equals sign (`=`) and an
[rvalue](expressions.html#lvalues-rvalues-and-temporaries) expression.
Evaluating an assignment expression [either copies or
moves](#moved-and-copied-types) its right-hand operand to its left-hand
operand.
```
# let mut x = 0;
# let y = 0;
x = y;
```
### Compound assignment expressions
The `+`, `-`, `*`, `/`, `%`, `&`, `|`, `^`, `<<`, and `>>` operators may be
composed with the `=` operator. The expression `lval OP= val` is equivalent to
`lval = lval OP val`. For example, `x = x + 1` may be written as `x += 1`.
Any such expression always has the [`unit`](types.html#tuple-types) type.
### Operator precedence
The precedence of Rust binary operators is ordered as follows, going from
strong to weak:
```{.text .precedence}
as :
* / %
+ -
<< >>
&
^
|
== != < > <= >=
&&
||
.. ...
<-
=
```
Operators at the same precedence level are evaluated left-to-right. [Unary
operators](#unary-operator-expressions) have the same precedence level and are
stronger than any of the binary operators.
## Grouped expressions
An expression enclosed in parentheses evaluates to the result of the enclosed
expression. Parentheses can be used to explicitly specify evaluation order
within an expression.
An example of a parenthesized expression:
```
let x: i32 = (2 + 3) * 4;
```
## Call expressions
A _call expression_ invokes a function, providing zero or more input variables
and an optional location to move the function's output into. If the function
eventually returns, then the expression completes.
Some examples of call expressions:
```
# fn add(x: i32, y: i32) -> i32 { 0 }
let x: i32 = add(1i32, 2i32);
let pi: Result<f32, _> = "3.14".parse();
```
## Lambda expressions
A _lambda expression_ (sometimes called an "anonymous function expression")
defines a function and denotes it as a value, in a single expression. A lambda
expression is a pipe-symbol-delimited (`|`) list of identifiers followed by an
expression.
A lambda expression denotes a function that maps a list of parameters
(`ident_list`) onto the expression that follows the `ident_list`. The
identifiers in the `ident_list` are the parameters to the function. These
parameters' types need not be specified, as the compiler infers them from
context.
Lambda expressions are most useful when passing functions as arguments to other
functions, as an abbreviation for defining and capturing a separate function.
Significantly, lambda expressions _capture their environment_, which regular
[function definitions](items.html#functions) do not. The exact type of capture
depends on the [function type](types.html#function-types) inferred for the
lambda expression. In the simplest and least-expensive form (analogous to a
```|| { }``` expression), the lambda expression captures its environment by
reference, effectively borrowing pointers to all outer variables mentioned
inside the function. Alternately, the compiler may infer that a lambda
expression should copy or move values (depending on their type) from the
environment into the lambda expression's captured environment. A lambda can be
forced to capture its environment by moving values by prefixing it with the
`move` keyword.
In this example, we define a function `ten_times` that takes a higher-order
function argument, and we then call it with a lambda expression as an argument,
followed by a lambda expression that moves values from its environment.
```
fn ten_times<F>(f: F) where F: Fn(i32) {
for index in 0..10 {
f(index);
}
}
ten_times(|j| println!("hello, {}", j));
let word = "konnichiwa".to_owned();
ten_times(move |j| println!("{}, {}", word, j));
```
## Infinite loops
A `loop` expression denotes an infinite loop.
A `loop` expression may optionally have a _label_. The label is written as
a lifetime preceding the loop expression, as in `'foo: loop{ }`. If a
label is present, then labeled `break` and `continue` expressions nested
within this loop may exit out of this loop or return control to its head.
See [break expressions](#break-expressions) and [continue
expressions](#continue-expressions).
## `break` expressions
A `break` expression has an optional _label_. If the label is absent, then
executing a `break` expression immediately terminates the innermost loop
enclosing it. It is only permitted in the body of a loop. If the label is
present, then `break 'foo` terminates the loop with label `'foo`, which need not
be the innermost label enclosing the `break` expression, but must enclose it.
## `continue` expressions
A `continue` expression has an optional _label_. If the label is absent, then
executing a `continue` expression immediately terminates the current iteration
of the innermost loop enclosing it, returning control to the loop *head*. In
the case of a `while` loop, the head is the conditional expression controlling
the loop. In the case of a `for` loop, the head is the call-expression
controlling the loop. If the label is present, then `continue 'foo` returns
control to the head of the loop with label `'foo`, which need not be the
innermost label enclosing the `continue` expression, but must enclose it.
A `continue` expression is only permitted in the body of a loop.
## `while` loops
A `while` loop begins by evaluating the boolean loop conditional expression.
If the loop conditional expression evaluates to `true`, the loop body block
executes and control returns to the loop conditional expression. If the loop
conditional expression evaluates to `false`, the `while` expression completes.
An example:
```
let mut i = 0;
while i < 10 {
println!("hello");
i = i + 1;
}
```
Like `loop` expressions, `while` loops can be controlled with `break` or
`continue`, and may optionally have a _label_. See [infinite
loops](#infinite-loops), [break expressions](#break-expressions), and
[continue expressions](#continue-expressions) for more information.
## `for` expressions
A `for` expression is a syntactic construct for looping over elements provided
by an implementation of `std::iter::IntoIterator`.
An example of a `for` loop over the contents of an array:
```
# type Foo = i32;
# fn bar(f: &Foo) { }
# let a = 0;
# let b = 0;
# let c = 0;
let v: &[Foo] = &[a, b, c];
for e in v {
bar(e);
}
```
An example of a for loop over a series of integers:
```
# fn bar(b:usize) { }
for i in 0..256 {
bar(i);
}
```
Like `loop` expressions, `for` loops can be controlled with `break` or
`continue`, and may optionally have a _label_. See [infinite
loops](#infinite-loops), [break expressions](#break-expressions), and
[continue expressions](#continue-expressions) for more information.
## `if` expressions
An `if` expression is a conditional branch in program control. The form of an
`if` expression is a condition expression, followed by a consequent block, any
number of `else if` conditions and blocks, and an optional trailing `else`
block. The condition expressions must have type `bool`. If a condition
expression evaluates to `true`, the consequent block is executed and any
subsequent `else if` or `else` block is skipped. If a condition expression
evaluates to `false`, the consequent block is skipped and any subsequent `else
if` condition is evaluated. If all `if` and `else if` conditions evaluate to
`false` then any `else` block is executed.
## `match` expressions
A `match` expression branches on a *pattern*. The exact form of matching that
occurs depends on the pattern. Patterns consist of some combination of
literals, destructured arrays or enum constructors, structs and tuples,
variable binding specifications, wildcards (`..`), and placeholders (`_`). A
`match` expression has a *head expression*, which is the value to compare to
the patterns. The type of the patterns must equal the type of the head
expression.
In a pattern whose head expression has an `enum` type, a placeholder (`_`)
stands for a *single* data field, whereas a wildcard `..` stands for *all* the
fields of a particular variant.
A `match` behaves differently depending on whether or not the head expression
is an [lvalue or an rvalue](expressions.html#lvalues-rvalues-and-temporaries).
If the head expression is an rvalue, it is first evaluated into a temporary
location, and the resulting value is sequentially compared to the patterns in
the arms until a match is found. The first arm with a matching pattern is
chosen as the branch target of the `match`, any variables bound by the pattern
are assigned to local variables in the arm's block, and control enters the
block.
When the head expression is an lvalue, the match does not allocate a temporary
location (however, a by-value binding may copy or move from the lvalue). When
possible, it is preferable to match on lvalues, as the lifetime of these
matches inherits the lifetime of the lvalue, rather than being restricted to
the inside of the match.
An example of a `match` expression:
```
let x = 1;
match x {
1 => println!("one"),
2 => println!("two"),
3 => println!("three"),
4 => println!("four"),
5 => println!("five"),
_ => println!("something else"),
}
```
Patterns that bind variables default to binding to a copy or move of the
matched value (depending on the matched value's type). This can be changed to
bind to a reference by using the `ref` keyword, or to a mutable reference using
`ref mut`.
Subpatterns can also be bound to variables by the use of the syntax `variable @
subpattern`. For example:
```
let x = 1;
match x {
e @ 1 ... 5 => println!("got a range element {}", e),
_ => println!("anything"),
}
```
Patterns can also dereference pointers by using the `&`, `&mut` and `box`
symbols, as appropriate. For example, these two matches on `x: &i32` are
equivalent:
```
# let x = &3;
let y = match *x { 0 => "zero", _ => "some" };
let z = match x { &0 => "zero", _ => "some" };
assert_eq!(y, z);
```
Multiple match patterns may be joined with the `|` operator. A range of values
may be specified with `...`. For example:
```
# let x = 2;
let message = match x {
0 | 1 => "not many",
2 ... 9 => "a few",
_ => "lots"
};
```
Range patterns only work on scalar types (like integers and characters; not
like arrays and structs, which have sub-components). A range pattern may not
be a sub-range of another range pattern inside the same `match`.
Finally, match patterns can accept *pattern guards* to further refine the
criteria for matching a case. Pattern guards appear after the pattern and
consist of a bool-typed expression following the `if` keyword. A pattern guard
may refer to the variables bound within the pattern they follow.
```
# let maybe_digit = Some(0);
# fn process_digit(i: i32) { }
# fn process_other(i: i32) { }
let message = match maybe_digit {
Some(x) if x < 10 => process_digit(x),
Some(x) => process_other(x),
None => panic!(),
};
```
## `if let` expressions
An `if let` expression is semantically identical to an `if` expression but in
place of a condition expression it expects a `let` statement with a refutable
pattern. If the value of the expression on the right hand side of the `let`
statement matches the pattern, the corresponding block will execute, otherwise
flow proceeds to the first `else` block that follows.
```
let dish = ("Ham", "Eggs");
// this body will be skipped because the pattern is refuted
if let ("Bacon", b) = dish {
println!("Bacon is served with {}", b);
}
// this body will execute
if let ("Ham", b) = dish {
println!("Ham is served with {}", b);
}
```
## `while let` loops
A `while let` loop is semantically identical to a `while` loop but in place of
a condition expression it expects `let` statement with a refutable pattern. If
the value of the expression on the right hand side of the `let` statement
matches the pattern, the loop body block executes and control returns to the
pattern matching statement. Otherwise, the while expression completes.
## `return` expressions
Return expressions are denoted with the keyword `return`. Evaluating a `return`
expression moves its argument into the designated output location for the
current function call, destroys the current function activation frame, and
transfers control to the caller frame.
An example of a `return` expression:
```
fn max(a: i32, b: i32) -> i32 {
if a > b {
return a;
}
return b;
}
```

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# Identifiers
An identifier is any nonempty Unicode[^non_ascii_idents] string of the following form:
Either
* The first character has property `XID_start`
* The remaining characters have property `XID_continue`
Or
* The first character is `_`
* The identifier is more than one character, `_` alone is not an identifier
* The remaining characters have property `XID_continue`
that does _not_ occur in the set of [keywords].
> **Note**: `XID_start` and `XID_continue` as character properties cover the
> character ranges used to form the more familiar C and Java language-family
> identifiers.
[keywords]: ../grammar.html#keywords
[^non_ascii_idents]: Non-ASCII characters in identifiers are currently feature
gated. This is expected to improve soon.

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# Influences
Rust is not a particularly original language, with design elements coming from
a wide range of sources. Some of these are listed below (including elements
that have since been removed):
* SML, OCaml: algebraic data types, pattern matching, type inference,
semicolon statement separation
* C++: references, RAII, smart pointers, move semantics, monomorphization,
memory model
* ML Kit, Cyclone: region based memory management
* Haskell (GHC): typeclasses, type families
* Newsqueak, Alef, Limbo: channels, concurrency
* Erlang: message passing, thread failure, <strike>linked thread failure</strike>,
<strike>lightweight concurrency</strike>
* Swift: optional bindings
* Scheme: hygienic macros
* C#: attributes
* Ruby: <strike>block syntax</strike>
* NIL, Hermes: <strike>typestate</strike>
* [Unicode Annex #31](http://www.unicode.org/reports/tr31/): identifier and
pattern syntax

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# Input format
Rust input is interpreted as a sequence of Unicode code points encoded in UTF-8.
Most Rust grammar rules are defined in terms of printable ASCII-range
code points, but a small number are defined in terms of Unicode properties or
explicit code point lists. [^inputformat]
[^inputformat]: Substitute definitions for the special Unicode productions are
provided to the grammar verifier, restricted to ASCII range, when verifying the
grammar in this document.

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# Introduction
This document is the primary reference for the Rust programming language. It
provides three kinds of material:
- Chapters that informally describe each language construct and their use.
- Chapters that informally describe the memory model, concurrency model,
runtime services, linkage model and debugging facilities.
- Appendix chapters providing rationale and references to languages that
influenced the design.
This document does not serve as an introduction to the language. Background
familiarity with the language is assumed. A separate [book] is available to
help acquire such background familiarity.
This document also does not serve as a reference to the [standard] library
included in the language distribution. Those libraries are documented
separately by extracting documentation attributes from their source code. Many
of the features that one might expect to be language features are library
features in Rust, so what you're looking for may be there, not here.
Finally, this document is not normative. It may include details that are
specific to `rustc` itself, and should not be taken as a specification for
the Rust language. We intend to produce such a document someday, but this
is what we have for now.
You may also be interested in the [grammar].
[book]: ../book/index.html
[standard]: ../std/index.html
[grammar]: ../grammar.html

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# Items and attributes
Crates contain [items], each of which may have some number of
[attributes] attached to it.
[items]: items.html
[attributes]: attributes.html

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# Lexical structure

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# Linkage
The Rust compiler supports various methods to link crates together both
statically and dynamically. This section will explore the various methods to
link Rust crates together, and more information about native libraries can be
found in the [FFI section of the book][ffi].
[ffi]: ../book/ffi.html
In one session of compilation, the compiler can generate multiple artifacts
through the usage of either command line flags or the `crate_type` attribute.
If one or more command line flags are specified, all `crate_type` attributes will
be ignored in favor of only building the artifacts specified by command line.
* `--crate-type=bin`, `#[crate_type = "bin"]` - A runnable executable will be
produced. This requires that there is a `main` function in the crate which
will be run when the program begins executing. This will link in all Rust and
native dependencies, producing a distributable binary.
* `--crate-type=lib`, `#[crate_type = "lib"]` - A Rust library will be produced.
This is an ambiguous concept as to what exactly is produced because a library
can manifest itself in several forms. The purpose of this generic `lib` option
is to generate the "compiler recommended" style of library. The output library
will always be usable by rustc, but the actual type of library may change from
time-to-time. The remaining output types are all different flavors of
libraries, and the `lib` type can be seen as an alias for one of them (but the
actual one is compiler-defined).
* `--crate-type=dylib`, `#[crate_type = "dylib"]` - A dynamic Rust library will
be produced. This is different from the `lib` output type in that this forces
dynamic library generation. The resulting dynamic library can be used as a
dependency for other libraries and/or executables. This output type will
create `*.so` files on linux, `*.dylib` files on osx, and `*.dll` files on
windows.
* `--crate-type=staticlib`, `#[crate_type = "staticlib"]` - A static system
library will be produced. This is different from other library outputs in that
the Rust compiler will never attempt to link to `staticlib` outputs. The
purpose of this output type is to create a static library containing all of
the local crate's code along with all upstream dependencies. The static
library is actually a `*.a` archive on linux and osx and a `*.lib` file on
windows. This format is recommended for use in situations such as linking
Rust code into an existing non-Rust application because it will not have
dynamic dependencies on other Rust code.
* `--crate-type=cdylib`, `#[crate_type = "cdylib"]` - A dynamic system
library will be produced. This is used when compiling Rust code as
a dynamic library to be loaded from another language. This output type will
create `*.so` files on Linux, `*.dylib` files on OSX, and `*.dll` files on
Windows.
* `--crate-type=rlib`, `#[crate_type = "rlib"]` - A "Rust library" file will be
produced. This is used as an intermediate artifact and can be thought of as a
"static Rust library". These `rlib` files, unlike `staticlib` files, are
interpreted by the Rust compiler in future linkage. This essentially means
that `rustc` will look for metadata in `rlib` files like it looks for metadata
in dynamic libraries. This form of output is used to produce statically linked
executables as well as `staticlib` outputs.
* `--crate-type=proc-macro`, `#[crate_type = "proc-macro"]` - The output
produced is not specified, but if a `-L` path is provided to it then the
compiler will recognize the output artifacts as a macro and it can be loaded
for a program. If a crate is compiled with the `proc-macro` crate type it
will forbid exporting any items in the crate other than those functions
tagged `#[proc_macro_derive]` and those functions must also be placed at the
crate root. Finally, the compiler will automatically set the
`cfg(proc_macro)` annotation whenever any crate type of a compilation is the
`proc-macro` crate type.
Note that these outputs are stackable in the sense that if multiple are
specified, then the compiler will produce each form of output at once without
having to recompile. However, this only applies for outputs specified by the
same method. If only `crate_type` attributes are specified, then they will all
be built, but if one or more `--crate-type` command line flags are specified,
then only those outputs will be built.
With all these different kinds of outputs, if crate A depends on crate B, then
the compiler could find B in various different forms throughout the system. The
only forms looked for by the compiler, however, are the `rlib` format and the
dynamic library format. With these two options for a dependent library, the
compiler must at some point make a choice between these two formats. With this
in mind, the compiler follows these rules when determining what format of
dependencies will be used:
1. If a static library is being produced, all upstream dependencies are
required to be available in `rlib` formats. This requirement stems from the
reason that a dynamic library cannot be converted into a static format.
Note that it is impossible to link in native dynamic dependencies to a static
library, and in this case warnings will be printed about all unlinked native
dynamic dependencies.
2. If an `rlib` file is being produced, then there are no restrictions on what
format the upstream dependencies are available in. It is simply required that
all upstream dependencies be available for reading metadata from.
The reason for this is that `rlib` files do not contain any of their upstream
dependencies. It wouldn't be very efficient for all `rlib` files to contain a
copy of `libstd.rlib`!
3. If an executable is being produced and the `-C prefer-dynamic` flag is not
specified, then dependencies are first attempted to be found in the `rlib`
format. If some dependencies are not available in an rlib format, then
dynamic linking is attempted (see below).
4. If a dynamic library or an executable that is being dynamically linked is
being produced, then the compiler will attempt to reconcile the available
dependencies in either the rlib or dylib format to create a final product.
A major goal of the compiler is to ensure that a library never appears more
than once in any artifact. For example, if dynamic libraries B and C were
each statically linked to library A, then a crate could not link to B and C
together because there would be two copies of A. The compiler allows mixing
the rlib and dylib formats, but this restriction must be satisfied.
The compiler currently implements no method of hinting what format a library
should be linked with. When dynamically linking, the compiler will attempt to
maximize dynamic dependencies while still allowing some dependencies to be
linked in via an rlib.
For most situations, having all libraries available as a dylib is recommended
if dynamically linking. For other situations, the compiler will emit a
warning if it is unable to determine which formats to link each library with.
In general, `--crate-type=bin` or `--crate-type=lib` should be sufficient for
all compilation needs, and the other options are just available if more
fine-grained control is desired over the output format of a Rust crate.

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# Macros By Example
`macro_rules` allows users to define syntax extension in a declarative way. We
call such extensions "macros by example" or simply "macros".
Currently, macros can expand to expressions, statements, items, or patterns.
(A `sep_token` is any token other than `*` and `+`. A `non_special_token` is
any token other than a delimiter or `$`.)
The macro expander looks up macro invocations by name, and tries each macro
rule in turn. It transcribes the first successful match. Matching and
transcription are closely related to each other, and we will describe them
together.
The macro expander matches and transcribes every token that does not begin with
a `$` literally, including delimiters. For parsing reasons, delimiters must be
balanced, but they are otherwise not special.
In the matcher, `$` _name_ `:` _designator_ matches the nonterminal in the Rust
syntax named by _designator_. Valid designators are:
* `item`: an [item]
* `block`: a [block]
* `stmt`: a [statement]
* `pat`: a [pattern]
* `expr`: an [expression]
* `ty`: a [type]
* `ident`: an [identifier]
* `path`: a [path]
* `tt`: a token tree (a single [token] by matching `()`, `[]`, or `{}`)
* `meta`: the contents of an [attribute]
[item]: items.html
[block]: expressions.html#block-expressions
[statement]: statements.html
[pattern]: expressions.html#match-expressions
[expression]: expressions.html
[type]: types.html
[identifier]: identifiers.html
[path]: paths.html
[token]: tokens.html
[attribute]: attributes.html
In the transcriber, the
designator is already known, and so only the name of a matched nonterminal comes
after the dollar sign.
In both the matcher and transcriber, the Kleene star-like operator indicates
repetition. The Kleene star operator consists of `$` and parentheses, optionally
followed by a separator token, followed by `*` or `+`. `*` means zero or more
repetitions, `+` means at least one repetition. The parentheses are not matched or
transcribed. On the matcher side, a name is bound to _all_ of the names it
matches, in a structure that mimics the structure of the repetition encountered
on a successful match. The job of the transcriber is to sort that structure
out.
The rules for transcription of these repetitions are called "Macro By Example".
Essentially, one "layer" of repetition is discharged at a time, and all of them
must be discharged by the time a name is transcribed. Therefore, `( $( $i:ident
),* ) => ( $i )` is an invalid macro, but `( $( $i:ident ),* ) => ( $( $i:ident
),* )` is acceptable (if trivial).
When Macro By Example encounters a repetition, it examines all of the `$`
_name_ s that occur in its body. At the "current layer", they all must repeat
the same number of times, so ` ( $( $i:ident ),* ; $( $j:ident ),* ) => ( $(
($i,$j) ),* )` is valid if given the argument `(a,b,c ; d,e,f)`, but not
`(a,b,c ; d,e)`. The repetition walks through the choices at that layer in
lockstep, so the former input transcribes to `(a,d), (b,e), (c,f)`.
Nested repetitions are allowed.
### Parsing limitations
The parser used by the macro system is reasonably powerful, but the parsing of
Rust syntax is restricted in two ways:
1. Macro definitions are required to include suitable separators after parsing
expressions and other bits of the Rust grammar. This implies that
a macro definition like `$i:expr [ , ]` is not legal, because `[` could be part
of an expression. A macro definition like `$i:expr,` or `$i:expr;` would be legal,
however, because `,` and `;` are legal separators. See [RFC 550] for more information.
2. The parser must have eliminated all ambiguity by the time it reaches a `$`
_name_ `:` _designator_. This requirement most often affects name-designator
pairs when they occur at the beginning of, or immediately after, a `$(...)*`;
requiring a distinctive token in front can solve the problem.
[RFC 550]: https://github.com/rust-lang/rfcs/blob/master/text/0550-macro-future-proofing.md

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# Macros
A number of minor features of Rust are not central enough to have their own
syntax, and yet are not implementable as functions. Instead, they are given
names, and invoked through a consistent syntax: `some_extension!(...)`.
Users of `rustc` can define new macros in two ways:
* [Macros] define new syntax in a higher-level,
declarative way.
* [Procedural Macros] can be used to implement custom derive.
And one unstable way: [compiler plugins].
[Macros]: ../book/macros.html
[Procedural Macros]: ../book/procedural-macros.html
[compiler plugins]: ../book/compiler-plugins.html

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# Memory allocation and lifetime
The _items_ of a program are those functions, modules and types that have their
value calculated at compile-time and stored uniquely in the memory image of the
rust process. Items are neither dynamically allocated nor freed.
The _heap_ is a general term that describes boxes. The lifetime of an
allocation in the heap depends on the lifetime of the box values pointing to
it. Since box values may themselves be passed in and out of frames, or stored
in the heap, heap allocations may outlive the frame they are allocated within.
An allocation in the heap is guaranteed to reside at a single location in the
heap for the whole lifetime of the allocation - it will never be relocated as
a result of moving a box value.

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# Memory model
A Rust program's memory consists of a static set of *items* and a *heap*.
Immutable portions of the heap may be safely shared between threads, mutable
portions may not be safely shared, but several mechanisms for effectively-safe
sharing of mutable values, built on unsafe code but enforcing a safe locking
discipline, exist in the standard library.
Allocations in the stack consist of *variables*, and allocations in the heap
consist of *boxes*.

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## Memory ownership
When a stack frame is exited, its local allocations are all released, and its
references to boxes are dropped.

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# Notation

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# Paths
A _path_ is a sequence of one or more path components _logically_ separated by
a namespace qualifier (`::`). If a path consists of only one component, it may
refer to either an [item] or a [variable] in a local control
scope. If a path has multiple components, it refers to an item.
[item]: items.html
[variable]: variables.html
Every item has a _canonical path_ within its crate, but the path naming an item
is only meaningful within a given crate. There is no global namespace across
crates; an item's canonical path merely identifies it within the crate.
Two examples of simple paths consisting of only identifier components:
```{.ignore}
x;
x::y::z;
```
Path components are usually [identifiers], but they may
also include angle-bracket-enclosed lists of type arguments. In
[expression] context, the type argument list is given
after a `::` namespace qualifier in order to disambiguate it from a
relational expression involving the less-than symbol (`<`). In type
expression context, the final namespace qualifier is omitted.
[identifiers]: identifiers.html
[expression]: expressions.html
Two examples of paths with type arguments:
```rust
# struct HashMap<K, V>(K,V);
# fn f() {
# fn id<T>(t: T) -> T { t }
type T = HashMap<i32,String>; // Type arguments used in a type expression
let x = id::<i32>(10); // Type arguments used in a call expression
# }
```
Paths can be denoted with various leading qualifiers to change the meaning of
how it is resolved:
* Paths starting with `::` are considered to be global paths where the
components of the path start being resolved from the crate root. Each
identifier in the path must resolve to an item.
```rust
mod a {
pub fn foo() {}
}
mod b {
pub fn foo() {
::a::foo(); // call a's foo function
}
}
# fn main() {}
```
* Paths starting with the keyword `super` begin resolution relative to the
parent module. Each further identifier must resolve to an item.
```rust
mod a {
pub fn foo() {}
}
mod b {
pub fn foo() {
super::a::foo(); // call a's foo function
}
}
# fn main() {}
```
* Paths starting with the keyword `self` begin resolution relative to the
current module. Each further identifier must resolve to an item.
```rust
fn foo() {}
fn bar() {
self::foo();
}
# fn main() {}
```
Additionally keyword `super` may be repeated several times after the first
`super` or `self` to refer to ancestor modules.
```rust
mod a {
fn foo() {}
mod b {
mod c {
fn foo() {
super::super::foo(); // call a's foo function
self::super::super::foo(); // call a's foo function
}
}
}
}
# fn main() {}
```

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## Procedural Macros
"Procedural macros" are the second way to implement a macro. For now, the only
thing they can be used for is to implement derive on your own types. See
[the book][procedural macros] for a tutorial.
[procedural macros]: ../book/procedural-macros.html
Procedural macros involve a few different parts of the language and its
standard libraries. First is the `proc_macro` crate, included with Rust,
that defines an interface for building a procedural macro. The
`#[proc_macro_derive(Foo)]` attribute is used to mark the deriving
function. This function must have the type signature:
```rust,ignore
use proc_macro::TokenStream;
#[proc_macro_derive(Hello)]
pub fn hello_world(input: TokenStream) -> TokenStream
```
Finally, procedural macros must be in their own crate, with the `proc-macro`
crate type.

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# Special traits
Several traits define special evaluation behavior.

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# Statements and expressions
Rust is _primarily_ an expression language. This means that most forms of
value-producing or effect-causing evaluation are directed by the uniform syntax
category of _expressions_. Each kind of expression can typically _nest_ within
each other kind of expression, and rules for evaluation of expressions involve
specifying both the value produced by the expression and the order in which its
sub-expressions are themselves evaluated.
In contrast, statements in Rust serve _mostly_ to contain and explicitly
sequence expression evaluation.

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# Statements
A _statement_ is a component of a block, which is in turn a component of an
outer [expression](expressions.html) or [function](items.html#functions).
Rust has two kinds of statement: [declaration
statements](#declaration-statements) and [expression
statements](#expression-statements).
## Declaration statements
A _declaration statement_ is one that introduces one or more *names* into the
enclosing statement block. The declared names may denote new variables or new
items.
### Item declarations
An _item declaration statement_ has a syntactic form identical to an
[item](items.html) declaration within a module. Declaring an item &mdash; a
function, enumeration, struct, type, static, trait, implementation or module
&mdash; locally within a statement block is simply a way of restricting its
scope to a narrow region containing all of its uses; it is otherwise identical
in meaning to declaring the item outside the statement block.
> **Note**: there is no implicit capture of the function's dynamic environment when
> declaring a function-local item.
### `let` statements
A _`let` statement_ introduces a new set of variables, given by a pattern. The
pattern may be followed by a type annotation, and/or an initializer expression.
When no type annotation is given, the compiler will infer the type, or signal
an error if insufficient type information is available for definite inference.
Any variables introduced by a variable declaration are visible from the point of
declaration until the end of the enclosing block scope.
## Expression statements
An _expression statement_ is one that evaluates an [expression](expressions.html)
and ignores its result. The type of an expression statement `e;` is always
`()`, regardless of the type of `e`. As a rule, an expression statement's
purpose is to trigger the effects of evaluating its expression.

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# String table productions
Some rules in the grammar &mdash; notably [unary
operators], [binary operators], and [keywords][keywords] &mdash; are
given in a simplified form: as a listing of a table of unquoted, printable
whitespace-separated strings. These cases form a subset of the rules regarding
the [token][tokens] rule, and are assumed to be the result of a
lexical-analysis phase feeding the parser, driven by a DFA, operating over the
disjunction of all such string table entries.
When such a string enclosed in double-quotes (`"`) occurs inside the grammar,
it is an implicit reference to a single member of such a string table
production. See [tokens] for more information.
[binary operators]: expressions.html#binary-operator-expressions
[keywords]: ../grammar.html#keywords
[tokens]: tokens.html
[unary operators]: expressions.html#unary-operator-expressions

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# Subtyping
Subtyping is implicit and can occur at any stage in type checking or
inference. Subtyping in Rust is very restricted and occurs only due to
variance with respect to lifetimes and between types with higher ranked
lifetimes. If we were to erase lifetimes from types, then the only subtyping
would be due to type equality.
Consider the following example: string literals always have `'static`
lifetime. Nevertheless, we can assign `s` to `t`:
```
fn bar<'a>() {
let s: &'static str = "hi";
let t: &'a str = s;
}
```
Since `'static` "lives longer" than `'a`, `&'static str` is a subtype of
`&'a str`.

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# The `Copy` trait
The `Copy` trait changes the semantics of a type implementing it. Values whose
type implements `Copy` are copied rather than moved upon assignment.

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# The `Deref` trait
The `Deref<Target = U>` trait allows a type to implicitly implement all the methods
of the type `U`. When attempting to resolve a method call, the compiler will search
the top-level type for the implementation of the called method. If no such method is
found, `.deref()` is called and the compiler continues to search for the method
implementation in the returned type `U`.

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# The `Drop` trait
The `Drop` trait provides a destructor, to be run whenever a value of this type
is to be destroyed.

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# The `Send` trait
The `Send` trait indicates that a value of this type is safe to send from one
thread to another.

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# The `Sized` trait
The `Sized` trait indicates that the size of this type is known at compile-time.

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# The `Sync` trait
The `Sync` trait indicates that a value of this type is safe to share between
multiple threads.

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html,
body {
font-family: "Open Sans", sans-serif;
color: #333;
}
.left {
float: left;
}
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float: right;
}
.hidden {
display: none;
}
h2,
h3 {
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justify-content: center;
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align-content: center;
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transition: color 0.5s;
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display: none;
}
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}
.theme-popup {
position: relative;
left: 10px;
z-index: 1000;
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border-radius: 4px;
font-size: 0.7em;
}
.theme-popup .theme {
margin: 0;
padding: 2px 10px;
line-height: 25px;
white-space: nowrap;
}
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border-top-right-radius: inherit;
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.theme-popup .theme:hover:last-child {
border-bottom-left-radius: inherit;
border-bottom-right-radius: inherit;
}
@media only screen and (max-width: 1250px) {
.nav-chapters {
display: none;
}
.mobile-nav-chapters {
font-size: 2.5em;
text-align: center;
text-decoration: none;
max-width: 150px;
min-width: 90px;
-webkit-box-pack: center;
-moz-box-pack: center;
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justify-content: center;
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-webkit-align-content: center;
align-content: center;
position: relative;
display: inline-block;
margin-bottom: 50px;
-webkit-border-radius: 5px;
border-radius: 5px;
}
.next {
float: right;
}
.previous {
float: left;
}
}
.light {
color: #333;
background-color: #fff;
/* Inline code */
}
.light .content .header:link,
.light .content .header:visited {
color: #333;
pointer: cursor;
}
.light .content .header:link:hover,
.light .content .header:visited:hover {
text-decoration: none;
}
.light .sidebar {
background-color: #fafafa;
color: #364149;
}
.light .chapter li {
color: #aaa;
}
.light .chapter li a {
color: #364149;
}
.light .chapter li .active,
.light .chapter li a:hover {
/* Animate color change */
color: #008cff;
}
.light .chapter .spacer {
background-color: #f4f4f4;
}
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.light .menu-bar:visited,
.light .nav-chapters,
.light .nav-chapters:visited,
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color: #ccc;
}
.light .menu-bar i:hover,
.light .nav-chapters:hover,
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color: #333;
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color: #364149;
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background-color: #fafafa;
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.light a:visited {
color: #4183c4;
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color: #333;
background: #fafafa;
border: 1px solid #ccc;
}
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background-color: #e6e6e6;
}
.light .theme-popup .default {
color: #ccc;
}
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margin: 20px 0;
padding: 0 20px;
color: #333;
background-color: #f2f7f9;
border-top: 0.1em solid #e1edf1;
border-bottom: 0.1em solid #e1edf1;
}
.light table td {
border-color: #f2f2f2;
}
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background: #f7f7f7;
}
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background: #ccc;
}
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border: none;
}
.light table thead tr {
border: 1px #ccc solid;
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display: inline-block;
vertical-align: middle;
padding: 0.1em 0.3em;
-webkit-border-radius: 3px;
border-radius: 3px;
}
.light pre {
position: relative;
}
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position: absolute;
right: 5px;
top: 5px;
color: #364149;
cursor: pointer;
}
.light pre > .buttons :hover {
color: #008cff;
}
.light pre > .buttons i {
margin-left: 8px;
}
.light pre > .result {
margin-top: 10px;
}
.coal {
color: #98a3ad;
background-color: #141617;
/* Inline code */
}
.coal .content .header:link,
.coal .content .header:visited {
color: #98a3ad;
pointer: cursor;
}
.coal .content .header:link:hover,
.coal .content .header:visited:hover {
text-decoration: none;
}
.coal .sidebar {
background-color: #292c2f;
color: #a1adb8;
}
.coal .chapter li {
color: #505254;
}
.coal .chapter li a {
color: #a1adb8;
}
.coal .chapter li .active,
.coal .chapter li a:hover {
/* Animate color change */
color: #3473ad;
}
.coal .chapter .spacer {
background-color: #393939;
}
.coal .menu-bar,
.coal .menu-bar:visited,
.coal .nav-chapters,
.coal .nav-chapters:visited,
.coal .mobile-nav-chapters,
.coal .mobile-nav-chapters:visited {
color: #43484d;
}
.coal .menu-bar i:hover,
.coal .nav-chapters:hover,
.coal .mobile-nav-chapters i:hover {
color: #b3c0cc;
}
.coal .mobile-nav-chapters i:hover {
color: #a1adb8;
}
.coal .mobile-nav-chapters {
background-color: #292c2f;
}
.coal .content a:link,
.coal a:visited {
color: #2b79a2;
}
.coal .theme-popup {
color: #98a3ad;
background: #141617;
border: 1px solid #43484d;
}
.coal .theme-popup .theme:hover {
background-color: #1f2124;
}
.coal .theme-popup .default {
color: #43484d;
}
.coal blockquote {
margin: 20px 0;
padding: 0 20px;
color: #98a3ad;
background-color: #242637;
border-top: 0.1em solid #2c2f44;
border-bottom: 0.1em solid #2c2f44;
}
.coal table td {
border-color: #1f2223;
}
.coal table tbody tr:nth-child(2n) {
background: #1b1d1e;
}
.coal table thead {
background: #3f4649;
}
.coal table thead td {
border: none;
}
.coal table thead tr {
border: 1px #3f4649 solid;
}
.coal :not(pre) > .hljs {
display: inline-block;
vertical-align: middle;
padding: 0.1em 0.3em;
-webkit-border-radius: 3px;
border-radius: 3px;
}
.coal pre {
position: relative;
}
.coal pre > .buttons {
position: absolute;
right: 5px;
top: 5px;
color: #a1adb8;
cursor: pointer;
}
.coal pre > .buttons :hover {
color: #3473ad;
}
.coal pre > .buttons i {
margin-left: 8px;
}
.coal pre > .result {
margin-top: 10px;
}
.navy {
color: #bcbdd0;
background-color: #161923;
/* Inline code */
}
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.navy .content .header:visited {
color: #bcbdd0;
pointer: cursor;
}
.navy .content .header:link:hover,
.navy .content .header:visited:hover {
text-decoration: none;
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background-color: #282d3f;
color: #c8c9db;
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color: #505274;
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color: #c8c9db;
}
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.navy .chapter li a:hover {
/* Animate color change */
color: #2b79a2;
}
.navy .chapter .spacer {
background-color: #2d334f;
}
.navy .menu-bar,
.navy .menu-bar:visited,
.navy .nav-chapters,
.navy .nav-chapters:visited,
.navy .mobile-nav-chapters,
.navy .mobile-nav-chapters:visited {
color: #737480;
}
.navy .menu-bar i:hover,
.navy .nav-chapters:hover,
.navy .mobile-nav-chapters i:hover {
color: #b7b9cc;
}
.navy .mobile-nav-chapters i:hover {
color: #c8c9db;
}
.navy .mobile-nav-chapters {
background-color: #282d3f;
}
.navy .content a:link,
.navy a:visited {
color: #2b79a2;
}
.navy .theme-popup {
color: #bcbdd0;
background: #161923;
border: 1px solid #737480;
}
.navy .theme-popup .theme:hover {
background-color: #282e40;
}
.navy .theme-popup .default {
color: #737480;
}
.navy blockquote {
margin: 20px 0;
padding: 0 20px;
color: #bcbdd0;
background-color: #262933;
border-top: 0.1em solid #2f333f;
border-bottom: 0.1em solid #2f333f;
}
.navy table td {
border-color: #1f2331;
}
.navy table tbody tr:nth-child(2n) {
background: #1b1f2b;
}
.navy table thead {
background: #39415b;
}
.navy table thead td {
border: none;
}
.navy table thead tr {
border: 1px #39415b solid;
}
.navy :not(pre) > .hljs {
display: inline-block;
vertical-align: middle;
padding: 0.1em 0.3em;
-webkit-border-radius: 3px;
border-radius: 3px;
}
.navy pre {
position: relative;
}
.navy pre > .buttons {
position: absolute;
right: 5px;
top: 5px;
color: #c8c9db;
cursor: pointer;
}
.navy pre > .buttons :hover {
color: #2b79a2;
}
.navy pre > .buttons i {
margin-left: 8px;
}
.navy pre > .result {
margin-top: 10px;
}
.rust {
color: #262625;
background-color: #e1e1db;
/* Inline code */
}
.rust .content .header:link,
.rust .content .header:visited {
color: #262625;
pointer: cursor;
}
.rust .content .header:link:hover,
.rust .content .header:visited:hover {
text-decoration: none;
}
.rust .sidebar {
background-color: #3b2e2a;
color: #c8c9db;
}
.rust .chapter li {
color: #505254;
}
.rust .chapter li a {
color: #c8c9db;
}
.rust .chapter li .active,
.rust .chapter li a:hover {
/* Animate color change */
color: #e69f67;
}
.rust .chapter .spacer {
background-color: #45373a;
}
.rust .menu-bar,
.rust .menu-bar:visited,
.rust .nav-chapters,
.rust .nav-chapters:visited,
.rust .mobile-nav-chapters,
.rust .mobile-nav-chapters:visited {
color: #737480;
}
.rust .menu-bar i:hover,
.rust .nav-chapters:hover,
.rust .mobile-nav-chapters i:hover {
color: #262625;
}
.rust .mobile-nav-chapters i:hover {
color: #c8c9db;
}
.rust .mobile-nav-chapters {
background-color: #3b2e2a;
}
.rust .content a:link,
.rust a:visited {
color: #2b79a2;
}
.rust .theme-popup {
color: #262625;
background: #e1e1db;
border: 1px solid #b38f6b;
}
.rust .theme-popup .theme:hover {
background-color: #99908a;
}
.rust .theme-popup .default {
color: #737480;
}
.rust blockquote {
margin: 20px 0;
padding: 0 20px;
color: #262625;
background-color: #c1c1bb;
border-top: 0.1em solid #b8b8b1;
border-bottom: 0.1em solid #b8b8b1;
}
.rust table td {
border-color: #d7d7cf;
}
.rust table tbody tr:nth-child(2n) {
background: #dbdbd4;
}
.rust table thead {
background: #b3a497;
}
.rust table thead td {
border: none;
}
.rust table thead tr {
border: 1px #b3a497 solid;
}
.rust :not(pre) > .hljs {
display: inline-block;
vertical-align: middle;
padding: 0.1em 0.3em;
-webkit-border-radius: 3px;
border-radius: 3px;
}
.rust pre {
position: relative;
}
.rust pre > .buttons {
position: absolute;
right: 5px;
top: 5px;
color: #c8c9db;
cursor: pointer;
}
.rust pre > .buttons :hover {
color: #e69f67;
}
.rust pre > .buttons i {
margin-left: 8px;
}
.rust pre > .result {
margin-top: 10px;
}
@media print {
#sidebar {
display: none;
}
#page-wrapper {
left: 0;
overflow-y: initial;
}
#content {
max-width: none;
margin: 0;
padding: 0;
}
#menu-bar {
display: none;
}
.page {
overflow-y: initial;
}
.nav-chapters {
display: none;
}
.mobile-nav-chapters {
display: none;
}
}
div.footnote-definition p {
display: inline;
}

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# Tokens
Tokens are primitive productions in the grammar defined by regular
(non-recursive) languages. "Simple" tokens are given in [string table
production] form, and occur in the rest of the
grammar as double-quoted strings. Other tokens have exact rules given.
[string table production]: string-table-productions.html
## Literals
A literal is an expression consisting of a single token, rather than a sequence
of tokens, that immediately and directly denotes the value it evaluates to,
rather than referring to it by name or some other evaluation rule. A literal is
a form of constant expression, so is evaluated (primarily) at compile time.
### Examples
#### Characters and strings
| | Example | `#` sets | Characters | Escapes |
|----------------------------------------------|-----------------|------------|-------------|---------------------|
| [Character](#character-literals) | `'H'` | `N/A` | All Unicode | [Quote](#quote-escapes) & [Byte](#byte-escapes) & [Unicode](#unicode-escapes) |
| [String](#string-literals) | `"hello"` | `N/A` | All Unicode | [Quote](#quote-escapes) & [Byte](#byte-escapes) & [Unicode](#unicode-escapes) |
| [Raw](#raw-string-literals) | `r#"hello"#` | `0...` | All Unicode | `N/A` |
| [Byte](#byte-literals) | `b'H'` | `N/A` | All ASCII | [Quote](#quote-escapes) & [Byte](#byte-escapes) |
| [Byte string](#byte-string-literals) | `b"hello"` | `N/A` | All ASCII | [Quote](#quote-escapes) & [Byte](#byte-escapes) |
| [Raw byte string](#raw-byte-string-literals) | `br#"hello"#` | `0...` | All ASCII | `N/A` |
#### Byte escapes
| | Name |
|---|------|
| `\x7F` | 8-bit character code (exactly 2 digits) |
| `\n` | Newline |
| `\r` | Carriage return |
| `\t` | Tab |
| `\\` | Backslash |
| `\0` | Null |
#### Unicode escapes
| | Name |
|---|------|
| `\u{7FFF}` | 24-bit Unicode character code (up to 6 digits) |
#### Quote escapes
| | Name |
|---|------|
| `\'` | Single quote |
| `\"` | Double quote |
#### Numbers
| [Number literals](#number-literals)`*` | Example | Exponentiation | Suffixes |
|----------------------------------------|---------|----------------|----------|
| Decimal integer | `98_222` | `N/A` | Integer suffixes |
| Hex integer | `0xff` | `N/A` | Integer suffixes |
| Octal integer | `0o77` | `N/A` | Integer suffixes |
| Binary integer | `0b1111_0000` | `N/A` | Integer suffixes |
| Floating-point | `123.0E+77` | `Optional` | Floating-point suffixes |
`*` All number literals allow `_` as a visual separator: `1_234.0E+18f64`
#### Suffixes
| Integer | Floating-point |
|---------|----------------|
| `u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`, `i64`, `isize`, `usize` | `f32`, `f64` |
### Character and string literals
#### Character literals
A _character literal_ is a single Unicode character enclosed within two
`U+0027` (single-quote) characters, with the exception of `U+0027` itself,
which must be _escaped_ by a preceding `U+005C` character (`\`).
#### String literals
A _string literal_ is a sequence of any Unicode characters enclosed within two
`U+0022` (double-quote) characters, with the exception of `U+0022` itself,
which must be _escaped_ by a preceding `U+005C` character (`\`).
Line-break characters are allowed in string literals. Normally they represent
themselves (i.e. no translation), but as a special exception, when an unescaped
`U+005C` character (`\`) occurs immediately before the newline (`U+000A`), the
`U+005C` character, the newline, and all whitespace at the beginning of the
next line are ignored. Thus `a` and `b` are equal:
```rust
let a = "foobar";
let b = "foo\
bar";
assert_eq!(a,b);
```
#### Character escapes
Some additional _escapes_ are available in either character or non-raw string
literals. An escape starts with a `U+005C` (`\`) and continues with one of the
following forms:
* An _8-bit code point escape_ starts with `U+0078` (`x`) and is
followed by exactly two _hex digits_. It denotes the Unicode code point
equal to the provided hex value.
* A _24-bit code point escape_ starts with `U+0075` (`u`) and is followed
by up to six _hex digits_ surrounded by braces `U+007B` (`{`) and `U+007D`
(`}`). It denotes the Unicode code point equal to the provided hex value.
* A _whitespace escape_ is one of the characters `U+006E` (`n`), `U+0072`
(`r`), or `U+0074` (`t`), denoting the Unicode values `U+000A` (LF),
`U+000D` (CR) or `U+0009` (HT) respectively.
* The _null escape_ is the character `U+0030` (`0`) and denotes the Unicode
value `U+0000` (NUL).
* The _backslash escape_ is the character `U+005C` (`\`) which must be
escaped in order to denote *itself*.
#### Raw string literals
Raw string literals do not process any escapes. They start with the character
`U+0072` (`r`), followed by zero or more of the character `U+0023` (`#`) and a
`U+0022` (double-quote) character. The _raw string body_ can contain any sequence
of Unicode characters and is terminated only by another `U+0022` (double-quote)
character, followed by the same number of `U+0023` (`#`) characters that preceded
the opening `U+0022` (double-quote) character.
All Unicode characters contained in the raw string body represent themselves,
the characters `U+0022` (double-quote) (except when followed by at least as
many `U+0023` (`#`) characters as were used to start the raw string literal) or
`U+005C` (`\`) do not have any special meaning.
Examples for string literals:
```
"foo"; r"foo"; // foo
"\"foo\""; r#""foo""#; // "foo"
"foo #\"# bar";
r##"foo #"# bar"##; // foo #"# bar
"\x52"; "R"; r"R"; // R
"\\x52"; r"\x52"; // \x52
```
### Byte and byte string literals
#### Byte literals
A _byte literal_ is a single ASCII character (in the `U+0000` to `U+007F`
range) or a single _escape_ preceded by the characters `U+0062` (`b`) and
`U+0027` (single-quote), and followed by the character `U+0027`. If the character
`U+0027` is present within the literal, it must be _escaped_ by a preceding
`U+005C` (`\`) character. It is equivalent to a `u8` unsigned 8-bit integer
_number literal_.
#### Byte string literals
A non-raw _byte string literal_ is a sequence of ASCII characters and _escapes_,
preceded by the characters `U+0062` (`b`) and `U+0022` (double-quote), and
followed by the character `U+0022`. If the character `U+0022` is present within
the literal, it must be _escaped_ by a preceding `U+005C` (`\`) character.
Alternatively, a byte string literal can be a _raw byte string literal_, defined
below. A byte string literal of length `n` is equivalent to a `&'static [u8; n]` borrowed fixed-sized array
of unsigned 8-bit integers.
Some additional _escapes_ are available in either byte or non-raw byte string
literals. An escape starts with a `U+005C` (`\`) and continues with one of the
following forms:
* A _byte escape_ escape starts with `U+0078` (`x`) and is
followed by exactly two _hex digits_. It denotes the byte
equal to the provided hex value.
* A _whitespace escape_ is one of the characters `U+006E` (`n`), `U+0072`
(`r`), or `U+0074` (`t`), denoting the bytes values `0x0A` (ASCII LF),
`0x0D` (ASCII CR) or `0x09` (ASCII HT) respectively.
* The _null escape_ is the character `U+0030` (`0`) and denotes the byte
value `0x00` (ASCII NUL).
* The _backslash escape_ is the character `U+005C` (`\`) which must be
escaped in order to denote its ASCII encoding `0x5C`.
#### Raw byte string literals
Raw byte string literals do not process any escapes. They start with the
character `U+0062` (`b`), followed by `U+0072` (`r`), followed by zero or more
of the character `U+0023` (`#`), and a `U+0022` (double-quote) character. The
_raw string body_ can contain any sequence of ASCII characters and is terminated
only by another `U+0022` (double-quote) character, followed by the same number of
`U+0023` (`#`) characters that preceded the opening `U+0022` (double-quote)
character. A raw byte string literal can not contain any non-ASCII byte.
All characters contained in the raw string body represent their ASCII encoding,
the characters `U+0022` (double-quote) (except when followed by at least as
many `U+0023` (`#`) characters as were used to start the raw string literal) or
`U+005C` (`\`) do not have any special meaning.
Examples for byte string literals:
```
b"foo"; br"foo"; // foo
b"\"foo\""; br#""foo""#; // "foo"
b"foo #\"# bar";
br##"foo #"# bar"##; // foo #"# bar
b"\x52"; b"R"; br"R"; // R
b"\\x52"; br"\x52"; // \x52
```
### Number literals
A _number literal_ is either an _integer literal_ or a _floating-point
literal_. The grammar for recognizing the two kinds of literals is mixed.
#### Integer literals
An _integer literal_ has one of four forms:
* A _decimal literal_ starts with a *decimal digit* and continues with any
mixture of *decimal digits* and _underscores_.
* A _hex literal_ starts with the character sequence `U+0030` `U+0078`
(`0x`) and continues as any mixture of hex digits and underscores.
* An _octal literal_ starts with the character sequence `U+0030` `U+006F`
(`0o`) and continues as any mixture of octal digits and underscores.
* A _binary literal_ starts with the character sequence `U+0030` `U+0062`
(`0b`) and continues as any mixture of binary digits and underscores.
Like any literal, an integer literal may be followed (immediately,
without any spaces) by an _integer suffix_, which forcibly sets the
type of the literal. The integer suffix must be the name of one of the
integral types: `u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`, `i64`,
`isize`, or `usize`.
The type of an _unsuffixed_ integer literal is determined by type inference:
* If an integer type can be _uniquely_ determined from the surrounding
program context, the unsuffixed integer literal has that type.
* If the program context under-constrains the type, it defaults to the
signed 32-bit integer `i32`.
* If the program context over-constrains the type, it is considered a
static type error.
Examples of integer literals of various forms:
```
123i32; // type i32
123u32; // type u32
123_u32; // type u32
0xff_u8; // type u8
0o70_i16; // type i16
0b1111_1111_1001_0000_i32; // type i32
0usize; // type usize
```
Note that the Rust syntax considers `-1i8` as an application of the [unary minus
operator] to an integer literal `1i8`, rather than
a single integer literal.
[unary minus operator]: expressions.html#unary-operator-expressions
#### Floating-point literals
A _floating-point literal_ has one of two forms:
* A _decimal literal_ followed by a period character `U+002E` (`.`). This is
optionally followed by another decimal literal, with an optional _exponent_.
* A single _decimal literal_ followed by an _exponent_.
Like integer literals, a floating-point literal may be followed by a
suffix, so long as the pre-suffix part does not end with `U+002E` (`.`).
The suffix forcibly sets the type of the literal. There are two valid
_floating-point suffixes_, `f32` and `f64` (the 32-bit and 64-bit floating point
types), which explicitly determine the type of the literal.
The type of an _unsuffixed_ floating-point literal is determined by
type inference:
* If a floating-point type can be _uniquely_ determined from the
surrounding program context, the unsuffixed floating-point literal
has that type.
* If the program context under-constrains the type, it defaults to `f64`.
* If the program context over-constrains the type, it is considered a
static type error.
Examples of floating-point literals of various forms:
```
123.0f64; // type f64
0.1f64; // type f64
0.1f32; // type f32
12E+99_f64; // type f64
let x: f64 = 2.; // type f64
```
This last example is different because it is not possible to use the suffix
syntax with a floating point literal ending in a period. `2.f64` would attempt
to call a method named `f64` on `2`.
The representation semantics of floating-point numbers are described in
["Machine Types"].
["Machine Types"]: types.html#machine-types
### Boolean literals
The two values of the boolean type are written `true` and `false`.
## Symbols
Symbols are a general class of printable [tokens] that play structural
roles in a variety of grammar productions. They are a
set of remaining miscellaneous printable tokens that do not
otherwise appear as [unary operators], [binary
operators], or [keywords].
They are catalogued in [the Symbols section][symbols] of the Grammar document.
[unary operators]: expressions.html#unary-operator-expressions
[binary operators]: expressions.html#binary-operator-expressions
[tokens]: #tokens
[symbols]: ../grammar.html#symbols
[keywords]: ../grammar.html#keywords

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# Type coercions
Coercions are defined in [RFC 401]. A coercion is implicit and has no syntax.
[RFC 401]: https://github.com/rust-lang/rfcs/blob/master/text/0401-coercions.md
## Coercion sites
A coercion can only occur at certain coercion sites in a program; these are
typically places where the desired type is explicit or can be derived by
propagation from explicit types (without type inference). Possible coercion
sites are:
* `let` statements where an explicit type is given.
For example, `42` is coerced to have type `i8` in the following:
```rust
let _: i8 = 42;
```
* `static` and `const` statements (similar to `let` statements).
* Arguments for function calls
The value being coerced is the actual parameter, and it is coerced to
the type of the formal parameter.
For example, `42` is coerced to have type `i8` in the following:
```rust
fn bar(_: i8) { }
fn main() {
bar(42);
}
```
* Instantiations of struct or variant fields
For example, `42` is coerced to have type `i8` in the following:
```rust
struct Foo { x: i8 }
fn main() {
Foo { x: 42 };
}
```
* Function results, either the final line of a block if it is not
semicolon-terminated or any expression in a `return` statement
For example, `42` is coerced to have type `i8` in the following:
```rust
fn foo() -> i8 {
42
}
```
If the expression in one of these coercion sites is a coercion-propagating
expression, then the relevant sub-expressions in that expression are also
coercion sites. Propagation recurses from these new coercion sites.
Propagating expressions and their relevant sub-expressions are:
* Array literals, where the array has type `[U; n]`. Each sub-expression in
the array literal is a coercion site for coercion to type `U`.
* Array literals with repeating syntax, where the array has type `[U; n]`. The
repeated sub-expression is a coercion site for coercion to type `U`.
* Tuples, where a tuple is a coercion site to type `(U_0, U_1, ..., U_n)`.
Each sub-expression is a coercion site to the respective type, e.g. the
zeroth sub-expression is a coercion site to type `U_0`.
* Parenthesized sub-expressions (`(e)`): if the expression has type `U`, then
the sub-expression is a coercion site to `U`.
* Blocks: if a block has type `U`, then the last expression in the block (if
it is not semicolon-terminated) is a coercion site to `U`. This includes
blocks which are part of control flow statements, such as `if`/`else`, if
the block has a known type.
## Coercion types
Coercion is allowed between the following types:
* `T` to `U` if `T` is a subtype of `U` (*reflexive case*)
* `T_1` to `T_3` where `T_1` coerces to `T_2` and `T_2` coerces to `T_3`
(*transitive case*)
Note that this is not fully supported yet
* `&mut T` to `&T`
* `*mut T` to `*const T`
* `&T` to `*const T`
* `&mut T` to `*mut T`
* `&T` to `&U` if `T` implements `Deref<Target = U>`. For example:
```rust
use std::ops::Deref;
struct CharContainer {
value: char,
}
impl Deref for CharContainer {
type Target = char;
fn deref<'a>(&'a self) -> &'a char {
&self.value
}
}
fn foo(arg: &char) {}
fn main() {
let x = &mut CharContainer { value: 'y' };
foo(x); //&mut CharContainer is coerced to &char.
}
```
* `&mut T` to `&mut U` if `T` implements `DerefMut<Target = U>`.
* TyCtor(`T`) to TyCtor(coerce_inner(`T`)), where TyCtor(`T`) is one of
- `&T`
- `&mut T`
- `*const T`
- `*mut T`
- `Box<T>`
and where
- coerce_inner(`[T, ..n]`) = `[T]`
- coerce_inner(`T`) = `U` where `T` is a concrete type which implements the
trait `U`.
In the future, coerce_inner will be recursively extended to tuples and
structs. In addition, coercions from sub-traits to super-traits will be
added. See [RFC 401] for more details.

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# Type system

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# Types
Every variable, item and value in a Rust program has a type. The _type_ of a
*value* defines the interpretation of the memory holding it.
Built-in types and type-constructors are tightly integrated into the language,
in nontrivial ways that are not possible to emulate in user-defined types.
User-defined types have limited capabilities.
## Primitive types
The primitive types are the following:
* The boolean type `bool` with values `true` and `false`.
* The machine types (integer and floating-point).
* The machine-dependent integer types.
* Arrays
* Tuples
* Slices
* Function pointers
### Machine types
The machine types are the following:
* The unsigned word types `u8`, `u16`, `u32` and `u64`, with values drawn from
the integer intervals [0, 2^8 - 1], [0, 2^16 - 1], [0, 2^32 - 1] and
[0, 2^64 - 1] respectively.
* The signed two's complement word types `i8`, `i16`, `i32` and `i64`, with
values drawn from the integer intervals [-(2^(7)), 2^7 - 1],
[-(2^(15)), 2^15 - 1], [-(2^(31)), 2^31 - 1], [-(2^(63)), 2^63 - 1]
respectively.
* The IEEE 754-2008 `binary32` and `binary64` floating-point types: `f32` and
`f64`, respectively.
### Machine-dependent integer types
The `usize` type is an unsigned integer type with the same number of bits as the
platform's pointer type. It can represent every memory address in the process.
The `isize` type is a signed integer type with the same number of bits as the
platform's pointer type. The theoretical upper bound on object and array size
is the maximum `isize` value. This ensures that `isize` can be used to calculate
differences between pointers into an object or array and can address every byte
within an object along with one byte past the end.
## Textual types
The types `char` and `str` hold textual data.
A value of type `char` is a [Unicode scalar value](
http://www.unicode.org/glossary/#unicode_scalar_value) (i.e. a code point that
is not a surrogate), represented as a 32-bit unsigned word in the 0x0000 to
0xD7FF or 0xE000 to 0x10FFFF range. A `[char]` array is effectively an UCS-4 /
UTF-32 string.
A value of type `str` is a Unicode string, represented as an array of 8-bit
unsigned bytes holding a sequence of UTF-8 code points. Since `str` is of
unknown size, it is not a _first-class_ type, but can only be instantiated
through a pointer type, such as `&str`.
## Tuple types
A tuple *type* is a heterogeneous product of other types, called the *elements*
of the tuple. It has no nominal name and is instead structurally typed.
Tuple types and values are denoted by listing the types or values of their
elements, respectively, in a parenthesized, comma-separated list.
Because tuple elements don't have a name, they can only be accessed by
pattern-matching or by using `N` directly as a field to access the
`N`th element.
An example of a tuple type and its use:
```
type Pair<'a> = (i32, &'a str);
let p: Pair<'static> = (10, "ten");
let (a, b) = p;
assert_eq!(a, 10);
assert_eq!(b, "ten");
assert_eq!(p.0, 10);
assert_eq!(p.1, "ten");
```
For historical reasons and convenience, the tuple type with no elements (`()`)
is often called ‘unit’ or ‘the unit type’.
## Array, and Slice types
Rust has two different types for a list of items:
* `[T; N]`, an 'array'
* `&[T]`, a 'slice'
An array has a fixed size, and can be allocated on either the stack or the
heap.
A slice is a 'view' into an array. It doesn't own the data it points
to, it borrows it.
Examples:
```{rust}
// A stack-allocated array
let array: [i32; 3] = [1, 2, 3];
// A heap-allocated array
let vector: Vec<i32> = vec![1, 2, 3];
// A slice into an array
let slice: &[i32] = &vector[..];
```
As you can see, the `vec!` macro allows you to create a `Vec<T>` easily. The
`vec!` macro is also part of the standard library, rather than the language.
All in-bounds elements of arrays and slices are always initialized, and access
to an array or slice is always bounds-checked.
## Struct types
A `struct` *type* is a heterogeneous product of other types, called the
*fields* of the type.[^structtype]
[^structtype]: `struct` types are analogous to `struct` types in C,
the *record* types of the ML family,
or the *struct* types of the Lisp family.
New instances of a `struct` can be constructed with a [struct
expression](expressions.html#struct-expressions).
The memory layout of a `struct` is undefined by default to allow for compiler
optimizations like field reordering, but it can be fixed with the
`#[repr(...)]` attribute. In either case, fields may be given in any order in
a corresponding struct *expression*; the resulting `struct` value will always
have the same memory layout.
The fields of a `struct` may be qualified by [visibility
modifiers](visibility-and-privacy.html), to allow access to data in a
struct outside a module.
A _tuple struct_ type is just like a struct type, except that the fields are
anonymous.
A _unit-like struct_ type is like a struct type, except that it has no
fields. The one value constructed by the associated [struct
expression](expressions.html#struct-expressions) is the only value that inhabits such a
type.
## Enumerated types
An *enumerated type* is a nominal, heterogeneous disjoint union type, denoted
by the name of an [`enum` item](items.html#enumerations). [^enumtype]
[^enumtype]: The `enum` type is analogous to a `data` constructor declaration in
ML, or a *pick ADT* in Limbo.
An [`enum` item](items.html#enumerations) declares both the type and a number of *variant
constructors*, each of which is independently named and takes an optional tuple
of arguments.
New instances of an `enum` can be constructed by calling one of the variant
constructors, in a [call expression](expressions.html#call-expressions).
Any `enum` value consumes as much memory as the largest variant constructor for
its corresponding `enum` type.
Enum types cannot be denoted *structurally* as types, but must be denoted by
named reference to an [`enum` item](items.html#enumerations).
## Recursive types
Nominal types &mdash; [enumerations](#enumerated-types) and
[structs](#struct-types) &mdash; may be recursive. That is, each `enum`
constructor or `struct` field may refer, directly or indirectly, to the
enclosing `enum` or `struct` type itself. Such recursion has restrictions:
* Recursive types must include a nominal type in the recursion
(not mere [type definitions](../grammar.html#type-definitions),
or other structural types such as [arrays](#array-and-slice-types) or [tuples](#tuple-types)).
* A recursive `enum` item must have at least one non-recursive constructor
(in order to give the recursion a basis case).
* The size of a recursive type must be finite;
in other words the recursive fields of the type must be [pointer types](#pointer-types).
* Recursive type definitions can cross module boundaries, but not module *visibility* boundaries,
or crate boundaries (in order to simplify the module system and type checker).
An example of a *recursive* type and its use:
```
enum List<T> {
Nil,
Cons(T, Box<List<T>>)
}
let a: List<i32> = List::Cons(7, Box::new(List::Cons(13, Box::new(List::Nil))));
```
## Pointer types
All pointers in Rust are explicit first-class values. They can be copied,
stored into data structs, and returned from functions. There are two
varieties of pointer in Rust:
* References (`&`)
: These point to memory _owned by some other value_.
A reference type is written `&type`,
or `&'a type` when you need to specify an explicit lifetime.
Copying a reference is a "shallow" operation:
it involves only copying the pointer itself.
Releasing a reference has no effect on the value it points to,
but a reference of a temporary value will keep it alive during the scope
of the reference itself.
* Raw pointers (`*`)
: Raw pointers are pointers without safety or liveness guarantees.
Raw pointers are written as `*const T` or `*mut T`,
for example `*const i32` means a raw pointer to a 32-bit integer.
Copying or dropping a raw pointer has no effect on the lifecycle of any
other value. Dereferencing a raw pointer or converting it to any other
pointer type is an [`unsafe` operation](unsafe-functions.html).
Raw pointers are generally discouraged in Rust code;
they exist to support interoperability with foreign code,
and writing performance-critical or low-level functions.
The standard library contains additional 'smart pointer' types beyond references
and raw pointers.
## Function types
The function type constructor `fn` forms new function types. A function type
consists of a possibly-empty set of function-type modifiers (such as `unsafe`
or `extern`), a sequence of input types and an output type.
An example of a `fn` type:
```
fn add(x: i32, y: i32) -> i32 {
x + y
}
let mut x = add(5,7);
type Binop = fn(i32, i32) -> i32;
let bo: Binop = add;
x = bo(5,7);
```
### Function types for specific items
Internal to the compiler, there are also function types that are specific to a particular
function item. In the following snippet, for example, the internal types of the functions
`foo` and `bar` are different, despite the fact that they have the same signature:
```
fn foo() { }
fn bar() { }
```
The types of `foo` and `bar` can both be implicitly coerced to the fn
pointer type `fn()`. There is currently no syntax for unique fn types,
though the compiler will emit a type like `fn() {foo}` in error
messages to indicate "the unique fn type for the function `foo`".
## Closure types
A [lambda expression](expressions.html#lambda-expressions) produces a closure
value with a unique, anonymous type that cannot be written out.
Depending on the requirements of the closure, its type implements one or
more of the closure traits:
* `FnOnce`
: The closure can be called once. A closure called as `FnOnce`
can move out values from its environment.
* `FnMut`
: The closure can be called multiple times as mutable. A closure called as
`FnMut` can mutate values from its environment. `FnMut` inherits from
`FnOnce` (i.e. anything implementing `FnMut` also implements `FnOnce`).
* `Fn`
: The closure can be called multiple times through a shared reference.
A closure called as `Fn` can neither move out from nor mutate values
from its environment. `Fn` inherits from `FnMut`, which itself
inherits from `FnOnce`.
## Trait objects
In Rust, a type like `&SomeTrait` or `Box<SomeTrait>` is called a _trait object_.
Each instance of a trait object includes:
- a pointer to an instance of a type `T` that implements `SomeTrait`
- a _virtual method table_, often just called a _vtable_, which contains, for
each method of `SomeTrait` that `T` implements, a pointer to `T`'s
implementation (i.e. a function pointer).
The purpose of trait objects is to permit "late binding" of methods. Calling a
method on a trait object results in virtual dispatch at runtime: that is, a
function pointer is loaded from the trait object vtable and invoked indirectly.
The actual implementation for each vtable entry can vary on an object-by-object
basis.
Note that for a trait object to be instantiated, the trait must be
_object-safe_. Object safety rules are defined in [RFC 255].
[RFC 255]: https://github.com/rust-lang/rfcs/blob/master/text/0255-object-safety.md
Given a pointer-typed expression `E` of type `&T` or `Box<T>`, where `T`
implements trait `R`, casting `E` to the corresponding pointer type `&R` or
`Box<R>` results in a value of the _trait object_ `R`. This result is
represented as a pair of pointers: the vtable pointer for the `T`
implementation of `R`, and the pointer value of `E`.
An example of a trait object:
```
trait Printable {
fn stringify(&self) -> String;
}
impl Printable for i32 {
fn stringify(&self) -> String { self.to_string() }
}
fn print(a: Box<Printable>) {
println!("{}", a.stringify());
}
fn main() {
print(Box::new(10) as Box<Printable>);
}
```
In this example, the trait `Printable` occurs as a trait object in both the
type signature of `print`, and the cast expression in `main`.
### Type parameters
Within the body of an item that has type parameter declarations, the names of
its type parameters are types:
```ignore
fn to_vec<A: Clone>(xs: &[A]) -> Vec<A> {
if xs.is_empty() {
return vec![];
}
let first: A = xs[0].clone();
let mut rest: Vec<A> = to_vec(&xs[1..]);
rest.insert(0, first);
rest
}
```
Here, `first` has type `A`, referring to `to_vec`'s `A` type parameter; and `rest`
has type `Vec<A>`, a vector with element type `A`.
## Self types
The special type `Self` has a meaning within traits and impls. In a trait definition, it refers
to an implicit type parameter representing the "implementing" type. In an impl,
it is an alias for the implementing type. For example, in:
```
pub trait From<T> {
fn from(T) -> Self;
}
impl From<i32> for String {
fn from(x: i32) -> Self {
x.to_string()
}
}
```
The notation `Self` in the impl refers to the implementing type: `String`. In another
example:
```
trait Printable {
fn make_string(&self) -> String;
}
impl Printable for String {
fn make_string(&self) -> String {
(*self).clone()
}
}
```
The notation `&self` is a shorthand for `self: &Self`. In this case,
in the impl, `Self` refers to the value of type `String` that is the
receiver for a call to the method `make_string`.

9
src/doc/reference/src/unicode-productions.md Normal file
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@ -0,0 +1,9 @@
# Unicode productions
A few productions in Rust's grammar permit Unicode code points outside the
ASCII range. We define these productions in terms of character properties
specified in the Unicode standard, rather than in terms of ASCII-range code
points. The grammar has a [Special Unicode Productions][unicodeproductions]
section that lists these productions.
[unicodeproductions]: ../grammar.html#special-unicode-productions

22
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@ -0,0 +1,22 @@
# Unsafe blocks
A block of code can be prefixed with the `unsafe` keyword, to permit calling
`unsafe` functions or dereferencing raw pointers within a safe function.
When a programmer has sufficient conviction that a sequence of potentially
unsafe operations is actually safe, they can encapsulate that sequence (taken
as a whole) within an `unsafe` block. The compiler will consider uses of such
code safe, in the surrounding context.
Unsafe blocks are used to wrap foreign libraries, make direct use of hardware
or implement features not directly present in the language. For example, Rust
provides the language features necessary to implement memory-safe concurrency
in the language but the implementation of threads and message passing is in the
standard library.
Rust's type system is a conservative approximation of the dynamic safety
requirements, so in some cases there is a performance cost to using safe code.
For example, a doubly-linked list is not a tree structure and can only be
represented with reference-counted pointers in safe code. By using `unsafe`
blocks to represent the reverse links as raw pointers, it can be implemented
with only boxes.

5
src/doc/reference/src/unsafe-functions.md Normal file
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@ -0,0 +1,5 @@
# Unsafe functions
Unsafe functions are functions that are not safe in all contexts and/or for all
possible inputs. Such a function must be prefixed with the keyword `unsafe` and
can only be called from an `unsafe` block or another `unsafe` function.

11
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@ -0,0 +1,11 @@
# Unsafety
Unsafe operations are those that potentially violate the memory-safety
guarantees of Rust's static semantics.
The following language level features cannot be used in the safe subset of
Rust:
- Dereferencing a [raw pointer](types.html#pointer-types).
- Reading or writing a [mutable static variable](items.html#mutable-statics).
- Calling an unsafe function (including an intrinsic or foreign function).

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src/doc/reference/src/variables.md Normal file
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@ -0,0 +1,31 @@
# Variables
A _variable_ is a component of a stack frame, either a named function parameter,
an anonymous [temporary](expressions.html#lvalues-rvalues-and-temporaries), or a named local
variable.
A _local variable_ (or *stack-local* allocation) holds a value directly,
allocated within the stack's memory. The value is a part of the stack frame.
Local variables are immutable unless declared otherwise like: `let mut x = ...`.
Function parameters are immutable unless declared with `mut`. The `mut` keyword
applies only to the following parameter (so `|mut x, y|` and `fn f(mut x:
Box<i32>, y: Box<i32>)` declare one mutable variable `x` and one immutable
variable `y`).
Methods that take either `self` or `Box<Self>` can optionally place them in a
mutable variable by prefixing them with `mut` (similar to regular arguments):
```
trait Changer: Sized {
fn change(mut self) {}
fn modify(mut self: Box<Self>) {}
}
```
Local variables are not initialized when allocated; the entire frame worth of
local variables are allocated at once, on frame-entry, in an uninitialized
state. Subsequent statements within a function may or may not initialize the
local variables. Local variables can be used only after they have been
initialized; this is enforced by the compiler.

160
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@ -0,0 +1,160 @@
# Visibility and Privacy
These two terms are often used interchangeably, and what they are attempting to
convey is the answer to the question "Can this item be used at this location?"
Rust's name resolution operates on a global hierarchy of namespaces. Each level
in the hierarchy can be thought of as some item. The items are one of those
mentioned above, but also include external crates. Declaring or defining a new
module can be thought of as inserting a new tree into the hierarchy at the
location of the definition.
To control whether interfaces can be used across modules, Rust checks each use
of an item to see whether it should be allowed or not. This is where privacy
warnings are generated, or otherwise "you used a private item of another module
and weren't allowed to."
By default, everything in Rust is *private*, with two exceptions: Associated
items in a `pub` Trait are public by default; Enum variants
in a `pub` enum are also public by default. When an item is declared as `pub`,
it can be thought of as being accessible to the outside world. For example:
```
# fn main() {}
// Declare a private struct
struct Foo;
// Declare a public struct with a private field
pub struct Bar {
field: i32,
}
// Declare a public enum with two public variants
pub enum State {
PubliclyAccessibleState,
PubliclyAccessibleState2,
}
```
With the notion of an item being either public or private, Rust allows item
accesses in two cases:
1. If an item is public, then it can be used externally through any of its
public ancestors.
2. If an item is private, it may be accessed by the current module and its
descendants.
These two cases are surprisingly powerful for creating module hierarchies
exposing public APIs while hiding internal implementation details. To help
explain, here's a few use cases and what they would entail:
* A library developer needs to expose functionality to crates which link
against their library. As a consequence of the first case, this means that
anything which is usable externally must be `pub` from the root down to the
destination item. Any private item in the chain will disallow external
accesses.
* A crate needs a global available "helper module" to itself, but it doesn't
want to expose the helper module as a public API. To accomplish this, the
root of the crate's hierarchy would have a private module which then
internally has a "public API". Because the entire crate is a descendant of
the root, then the entire local crate can access this private module through
the second case.
* When writing unit tests for a module, it's often a common idiom to have an
immediate child of the module to-be-tested named `mod test`. This module
could access any items of the parent module through the second case, meaning
that internal implementation details could also be seamlessly tested from the
child module.
In the second case, it mentions that a private item "can be accessed" by the
current module and its descendants, but the exact meaning of accessing an item
depends on what the item is. Accessing a module, for example, would mean
looking inside of it (to import more items). On the other hand, accessing a
function would mean that it is invoked. Additionally, path expressions and
import statements are considered to access an item in the sense that the
import/expression is only valid if the destination is in the current visibility
scope.
Here's an example of a program which exemplifies the three cases outlined
above:
```
// This module is private, meaning that no external crate can access this
// module. Because it is private at the root of this current crate, however, any
// module in the crate may access any publicly visible item in this module.
mod crate_helper_module {
// This function can be used by anything in the current crate
pub fn crate_helper() {}
// This function *cannot* be used by anything else in the crate. It is not
// publicly visible outside of the `crate_helper_module`, so only this
// current module and its descendants may access it.
fn implementation_detail() {}
}
// This function is "public to the root" meaning that it's available to external
// crates linking against this one.
pub fn public_api() {}
// Similarly to 'public_api', this module is public so external crates may look
// inside of it.
pub mod submodule {
use crate_helper_module;
pub fn my_method() {
// Any item in the local crate may invoke the helper module's public
// interface through a combination of the two rules above.
crate_helper_module::crate_helper();
}
// This function is hidden to any module which is not a descendant of
// `submodule`
fn my_implementation() {}
#[cfg(test)]
mod test {
#[test]
fn test_my_implementation() {
// Because this module is a descendant of `submodule`, it's allowed
// to access private items inside of `submodule` without a privacy
// violation.
super::my_implementation();
}
}
}
# fn main() {}
```
For a Rust program to pass the privacy checking pass, all paths must be valid
accesses given the two rules above. This includes all use statements,
expressions, types, etc.
## Re-exporting and Visibility
Rust allows publicly re-exporting items through a `pub use` directive. Because
this is a public directive, this allows the item to be used in the current
module through the rules above. It essentially allows public access into the
re-exported item. For example, this program is valid:
```
pub use self::implementation::api;
mod implementation {
pub mod api {
pub fn f() {}
}
}
# fn main() {}
```
This means that any external crate referencing `implementation::api::f` would
receive a privacy violation, while the path `api::f` would be allowed.
When re-exporting a private item, it can be thought of as allowing the "privacy
chain" being short-circuited through the reexport instead of passing through
the namespace hierarchy as it normally would.

22
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@ -0,0 +1,22 @@
# Whitespace
Whitespace is any non-empty string containing only characters that have the
`Pattern_White_Space` Unicode property, namely:
- `U+0009` (horizontal tab, `'\t'`)
- `U+000A` (line feed, `'\n'`)
- `U+000B` (vertical tab)
- `U+000C` (form feed)
- `U+000D` (carriage return, `'\r'`)
- `U+0020` (space, `' '`)
- `U+0085` (next line)
- `U+200E` (left-to-right mark)
- `U+200F` (right-to-left mark)
- `U+2028` (line separator)
- `U+2029` (paragraph separator)
Rust is a "free-form" language, meaning that all forms of whitespace serve only
to separate _tokens_ in the grammar, and have no semantic significance.
A Rust program has identical meaning if each whitespace element is replaced
with any other legal whitespace element, such as a single space character.

2
src/liballoc/arc.rs
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@ -102,7 +102,7 @@ const MAX_REFCOUNT: usize = (isize::MAX) as usize;
/// [downgrade]: struct.Arc.html#method.downgrade
/// [upgrade]: struct.Weak.html#method.upgrade
/// [`None`]: ../../std/option/enum.Option.html#variant.None
/// [assoc]: ../../book/method-syntax.html#Associated%20functions
/// [assoc]: ../../book/method-syntax.html#associated-functions
///
/// # Examples
///

2
src/liballoc/rc.rs
View file

@ -215,7 +215,7 @@
//! [downgrade]: struct.Rc.html#method.downgrade
//! [upgrade]: struct.Weak.html#method.upgrade
//! [`None`]: ../../std/option/enum.Option.html#variant.None
//! [assoc]: ../../book/method-syntax.html#Associated%20functions
//! [assoc]: ../../book/method-syntax.html#associated-functions
//! [mutability]: ../../std/cell/index.html#introducing-mutability-inside-of-something-immutable
#![stable(feature = "rust1", since = "1.0.0")]

18
src/libcollections/string.rs
View file

@ -1250,17 +1250,17 @@ impl String {
self.len() == 0
}
/// Divide one string into two at an index.
/// Splits the string into two at the given index.
///
/// The argument, `mid`, should be a byte offset from the start of the string. It must also
/// be on the boundary of a UTF-8 code point.
/// Returns a newly allocated `String`. `self` contains bytes `[0, at)`, and
/// the returned `String` contains bytes `[at, len)`. `at` must be on the
/// boundary of a UTF-8 code point.
///
/// The two strings returned go from the start of the string to `mid`, and from `mid` to the end
/// of the string.
/// Note that the capacity of `self` does not change.
///
/// # Panics
///
/// Panics if `mid` is not on a `UTF-8` code point boundary, or if it is beyond the last
/// Panics if `at` is not on a `UTF-8` code point boundary, or if it is beyond the last
/// code point of the string.
///
/// # Examples
@ -1275,9 +1275,9 @@ impl String {
/// ```
#[inline]
#[stable(feature = "string_split_off", since = "1.16.0")]
pub fn split_off(&mut self, mid: usize) -> String {
assert!(self.is_char_boundary(mid));
let other = self.vec.split_off(mid);
pub fn split_off(&mut self, at: usize) -> String {
assert!(self.is_char_boundary(at));
let other = self.vec.split_off(at);
unsafe { String::from_utf8_unchecked(other) }
}

2
src/libcore/intrinsics.rs
View file

@ -687,7 +687,7 @@ extern "rust-intrinsic" {
/// The [nomicon](../../nomicon/transmutes.html) has additional
/// documentation.
///
/// [ub]: ../../reference.html#behavior-considered-undefined
/// [ub]: ../../reference/behavior-considered-undefined.html
///
/// # Examples
///

34
src/libcore/iter/iterator.rs
View file

@ -1603,12 +1603,12 @@ pub trait Iterator {
let mut i = self.len();
while let Some(v) = self.next_back() {
if predicate(v) {
return Some(i - 1);
}
// No need for an overflow check here, because `ExactSizeIterator`
// implies that the number of elements fits into a `usize`.
i -= 1;
if predicate(v) {
return Some(i);
}
}
None
}
@ -1616,7 +1616,9 @@ pub trait Iterator {
/// Returns the maximum element of an iterator.
///
/// If several elements are equally maximum, the last element is
/// returned.
/// returned. If the iterator is empty, [`None`] is returned.
///
/// [`None`]: ../../std/option/enum.Option.html#variant.None
///
/// # Examples
///
@ -1624,8 +1626,10 @@ pub trait Iterator {
///
/// ```
/// let a = [1, 2, 3];
/// let b: Vec<u32> = Vec::new();
///
/// assert_eq!(a.iter().max(), Some(&3));
/// assert_eq!(b.iter().max(), None);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
@ -1642,7 +1646,9 @@ pub trait Iterator {
/// Returns the minimum element of an iterator.
///
/// If several elements are equally minimum, the first element is
/// returned.
/// returned. If the iterator is empty, [`None`] is returned.
///
/// [`None`]: ../../std/option/enum.Option.html#variant.None
///
/// # Examples
///
@ -1650,8 +1656,10 @@ pub trait Iterator {
///
/// ```
/// let a = [1, 2, 3];
/// let b: Vec<u32> = Vec::new();
///
/// assert_eq!(a.iter().min(), Some(&1));
/// assert_eq!(b.iter().min(), None);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
@ -1669,7 +1677,9 @@ pub trait Iterator {
/// specified function.
///
/// If several elements are equally maximum, the last element is
/// returned.
/// returned. If the iterator is empty, [`None`] is returned.
///
/// [`None`]: ../../std/option/enum.Option.html#variant.None
///
/// # Examples
///
@ -1694,7 +1704,9 @@ pub trait Iterator {
/// specified comparison function.
///
/// If several elements are equally maximum, the last element is
/// returned.
/// returned. If the iterator is empty, [`None`] is returned.
///
/// [`None`]: ../../std/option/enum.Option.html#variant.None
///
/// # Examples
///
@ -1719,7 +1731,9 @@ pub trait Iterator {
/// specified function.
///
/// If several elements are equally minimum, the first element is
/// returned.
/// returned. If the iterator is empty, [`None`] is returned.
///
/// [`None`]: ../../std/option/enum.Option.html#variant.None
///
/// # Examples
///
@ -1743,7 +1757,9 @@ pub trait Iterator {
/// specified comparison function.
///
/// If several elements are equally minimum, the first element is
/// returned.
/// returned. If the iterator is empty, [`None`] is returned.
///
/// [`None`]: ../../std/option/enum.Option.html#variant.None
///
/// # Examples
///

4
src/libcore/marker.rs
View file

@ -36,7 +36,7 @@ use hash::Hasher;
///
/// [`Rc`]: ../../std/rc/struct.Rc.html
/// [arc]: ../../std/sync/struct.Arc.html
/// [ub]: ../../reference.html#behavior-considered-undefined
/// [ub]: ../../reference/behavior-considered-undefined.html
#[stable(feature = "rust1", since = "1.0.0")]
#[lang = "send"]
#[rustc_on_unimplemented = "`{Self}` cannot be sent between threads safely"]
@ -338,7 +338,7 @@ pub trait Copy : Clone {
/// [mutex]: ../../std/sync/struct.Mutex.html
/// [rwlock]: ../../std/sync/struct.RwLock.html
/// [unsafecell]: ../cell/struct.UnsafeCell.html
/// [ub]: ../../reference.html#behavior-considered-undefined
/// [ub]: ../../reference/behavior-considered-undefined.html
/// [transmute]: ../../std/mem/fn.transmute.html
#[stable(feature = "rust1", since = "1.0.0")]
#[lang = "sync"]

10
src/libcore/mem.rs
View file

@ -167,7 +167,7 @@ pub use intrinsics::transmute;
/// [FFI]: ../../book/ffi.html
/// [box]: ../../std/boxed/struct.Box.html
/// [into_raw]: ../../std/boxed/struct.Box.html#method.into_raw
/// [ub]: ../../reference.html#behavior-considered-undefined
/// [ub]: ../../reference/behavior-considered-undefined.html
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn forget<T>(t: T) {
@ -318,7 +318,7 @@ pub fn align_of_val<T: ?Sized>(val: &T) -> usize {
///
/// [uninit]: fn.uninitialized.html
/// [FFI]: ../../book/ffi.html
/// [ub]: ../../reference.html#behavior-considered-undefined
/// [ub]: ../../reference/behavior-considered-undefined.html
///
/// # Examples
///
@ -417,7 +417,7 @@ pub unsafe fn zeroed<T>() -> T {
/// [`Vec`]: ../../std/vec/struct.Vec.html
/// [`vec!`]: ../../std/macro.vec.html
/// [`Clone`]: ../../std/clone/trait.Clone.html
/// [ub]: ../../reference.html#behavior-considered-undefined
/// [ub]: ../../reference/behavior-considered-undefined.html
/// [write]: ../ptr/fn.write.html
/// [copy]: ../intrinsics/fn.copy.html
/// [copy_no]: ../intrinsics/fn.copy_nonoverlapping.html
@ -525,7 +525,7 @@ pub fn replace<T>(dest: &mut T, mut src: T) -> T {
/// it will not release any borrows, as borrows are based on lexical scope.
///
/// This effectively does nothing for
/// [types which implement `Copy`](../../book/ownership.html#Copy%20types),
/// [types which implement `Copy`](../../book/ownership.html#copy-types),
/// e.g. integers. Such values are copied and _then_ moved into the function,
/// so the value persists after this function call.
///
@ -626,7 +626,7 @@ pub fn drop<T>(_x: T) { }
/// same size. This function triggers [undefined behavior][ub] if `U` is larger than
/// `T`.
///
/// [ub]: ../../reference.html#behavior-considered-undefined
/// [ub]: ../../reference/behavior-considered-undefined.html
/// [size_of]: fn.size_of.html
///
/// # Examples

2
src/libcore/raw.rs
View file

@ -25,7 +25,7 @@
/// Book][moreinfo] contains more details about the precise nature of
/// these internals.
///
/// [moreinfo]: ../../book/trait-objects.html#Representation
/// [moreinfo]: ../../book/trait-objects.html#representation
///
/// `TraitObject` is guaranteed to match layouts, but it is not the
/// type of trait objects (e.g. the fields are not directly accessible

46
src/librustc/dep_graph/dep_node.rs
View file

@ -131,7 +131,37 @@ pub enum DepNode<D: Clone + Debug> {
// which would yield an overly conservative dep-graph.
TraitItems(D),
ReprHints(D),
TraitSelect(Vec<D>),
// Trait selection cache is a little funny. Given a trait
// reference like `Foo: SomeTrait<Bar>`, there could be
// arbitrarily many def-ids to map on in there (e.g., `Foo`,
// `SomeTrait`, `Bar`). We could have a vector of them, but it
// requires heap-allocation, and trait sel in general can be a
// surprisingly hot path. So instead we pick two def-ids: the
// trait def-id, and the first def-id in the input types. If there
// is no def-id in the input types, then we use the trait def-id
// again. So for example:
//
// - `i32: Clone` -> `TraitSelect { trait_def_id: Clone, self_def_id: Clone }`
// - `u32: Clone` -> `TraitSelect { trait_def_id: Clone, self_def_id: Clone }`
// - `Clone: Clone` -> `TraitSelect { trait_def_id: Clone, self_def_id: Clone }`
// - `Vec<i32>: Clone` -> `TraitSelect { trait_def_id: Clone, self_def_id: Vec }`
// - `String: Clone` -> `TraitSelect { trait_def_id: Clone, self_def_id: String }`
// - `Foo: Trait<Bar>` -> `TraitSelect { trait_def_id: Trait, self_def_id: Foo }`
// - `Foo: Trait<i32>` -> `TraitSelect { trait_def_id: Trait, self_def_id: Foo }`
// - `(Foo, Bar): Trait` -> `TraitSelect { trait_def_id: Trait, self_def_id: Foo }`
// - `i32: Trait<Foo>` -> `TraitSelect { trait_def_id: Trait, self_def_id: Foo }`
//
// You can see that we map many trait refs to the same
// trait-select node. This is not a problem, it just means
// imprecision in our dep-graph tracking. The important thing is
// that for any given trait-ref, we always map to the **same**
// trait-select node.
TraitSelect { trait_def_id: D, input_def_id: D },
// For proj. cache, we just keep a list of all def-ids, since it is
// not a hotspot.
ProjectionCache { def_ids: Vec<D> },
}
impl<D: Clone + Debug> DepNode<D> {
@ -236,9 +266,17 @@ impl<D: Clone + Debug> DepNode<D> {
TraitImpls(ref d) => op(d).map(TraitImpls),
TraitItems(ref d) => op(d).map(TraitItems),
ReprHints(ref d) => op(d).map(ReprHints),
TraitSelect(ref type_ds) => {
let type_ds = try_opt!(type_ds.iter().map(|d| op(d)).collect());
Some(TraitSelect(type_ds))
TraitSelect { ref trait_def_id, ref input_def_id } => {
op(trait_def_id).and_then(|trait_def_id| {
op(input_def_id).and_then(|input_def_id| {
Some(TraitSelect { trait_def_id: trait_def_id,
input_def_id: input_def_id })
})
})
}
ProjectionCache { ref def_ids } => {
let def_ids: Option<Vec<E>> = def_ids.iter().map(op).collect();
def_ids.map(|d| ProjectionCache { def_ids: d })
}
}
}

39
src/librustc/hir/def.rs
View file

@ -59,32 +59,45 @@ pub enum Def {
Err,
}
/// The result of resolving a path.
/// Before type checking completes, `depth` represents the number of
/// trailing segments which are yet unresolved. Afterwards, if there
/// were no errors, all paths should be fully resolved, with `depth`
/// set to `0` and `base_def` representing the final resolution.
///
/// The result of resolving a path before lowering to HIR.
/// `base_def` is definition of resolved part of the
/// path, `unresolved_segments` is the number of unresolved
/// segments.
/// module::Type::AssocX::AssocY::MethodOrAssocType
/// ^~~~~~~~~~~~ ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/// base_def depth = 3
/// base_def unresolved_segments = 3
///
/// <T as Trait>::AssocX::AssocY::MethodOrAssocType
/// ^~~~~~~~~~~~~~ ^~~~~~~~~~~~~~~~~~~~~~~~~
/// base_def depth = 2
/// base_def unresolved_segments = 2
#[derive(Copy, Clone, Debug)]
pub struct PathResolution {
pub base_def: Def,
pub depth: usize
base_def: Def,
unresolved_segments: usize,
}
impl PathResolution {
pub fn new(def: Def) -> PathResolution {
PathResolution { base_def: def, depth: 0 }
pub fn new(def: Def) -> Self {
PathResolution { base_def: def, unresolved_segments: 0 }
}
pub fn with_unresolved_segments(def: Def, mut unresolved_segments: usize) -> Self {
if def == Def::Err { unresolved_segments = 0 }
PathResolution { base_def: def, unresolved_segments: unresolved_segments }
}
#[inline]
pub fn base_def(&self) -> Def {
self.base_def
}
#[inline]
pub fn unresolved_segments(&self) -> usize {
self.unresolved_segments
}
pub fn kind_name(&self) -> &'static str {
if self.depth != 0 {
if self.unresolved_segments != 0 {
"associated item"
} else {
self.base_def.kind_name()

16
src/librustc/hir/lowering.rs
View file

@ -217,10 +217,10 @@ impl<'a> LoweringContext<'a> {
fn expect_full_def(&mut self, id: NodeId) -> Def {
self.resolver.get_resolution(id).map_or(Def::Err, |pr| {
if pr.depth != 0 {
if pr.unresolved_segments() != 0 {
bug!("path not fully resolved: {:?}", pr);
}
pr.base_def
pr.base_def()
})
}
@ -421,9 +421,9 @@ impl<'a> LoweringContext<'a> {
let resolution = self.resolver.get_resolution(id)
.unwrap_or(PathResolution::new(Def::Err));
let proj_start = p.segments.len() - resolution.depth;
let proj_start = p.segments.len() - resolution.unresolved_segments();
let path = P(hir::Path {
def: resolution.base_def,
def: resolution.base_def(),
segments: p.segments[..proj_start].iter().enumerate().map(|(i, segment)| {
let param_mode = match (qself_position, param_mode) {
(Some(j), ParamMode::Optional) if i < j => {
@ -443,7 +443,7 @@ impl<'a> LoweringContext<'a> {
index: this.def_key(def_id).parent.expect("missing parent")
}
};
let type_def_id = match resolution.base_def {
let type_def_id = match resolution.base_def() {
Def::AssociatedTy(def_id) if i + 2 == proj_start => {
Some(parent_def_id(self, def_id))
}
@ -474,7 +474,7 @@ impl<'a> LoweringContext<'a> {
// Simple case, either no projections, or only fully-qualified.
// E.g. `std::mem::size_of` or `<I as Iterator>::Item`.
if resolution.depth == 0 {
if resolution.unresolved_segments() == 0 {
return hir::QPath::Resolved(qself, path);
}
@ -749,7 +749,7 @@ impl<'a> LoweringContext<'a> {
bound_pred.bound_lifetimes.is_empty() => {
if let Some(Def::TyParam(def_id)) =
self.resolver.get_resolution(bound_pred.bounded_ty.id)
.map(|d| d.base_def) {
.map(|d| d.base_def()) {
if let Some(node_id) =
self.resolver.definitions().as_local_node_id(def_id) {
for ty_param in &g.ty_params {
@ -1295,7 +1295,7 @@ impl<'a> LoweringContext<'a> {
PatKind::Wild => hir::PatKind::Wild,
PatKind::Ident(ref binding_mode, pth1, ref sub) => {
self.with_parent_def(p.id, |this| {
match this.resolver.get_resolution(p.id).map(|d| d.base_def) {
match this.resolver.get_resolution(p.id).map(|d| d.base_def()) {
// `None` can occur in body-less function signatures
def @ None | def @ Some(Def::Local(_)) => {
let def_id = def.map(|d| d.def_id()).unwrap_or_else(|| {

243
src/librustc/infer/error_reporting.rs
View file

@ -65,9 +65,6 @@ use super::region_inference::ConcreteFailure;
use super::region_inference::SubSupConflict;
use super::region_inference::GenericBoundFailure;
use super::region_inference::GenericKind;
use super::region_inference::ProcessedErrors;
use super::region_inference::ProcessedErrorOrigin;
use super::region_inference::SameRegions;
use hir::map as hir_map;
use hir;
@ -77,11 +74,10 @@ use infer;
use middle::region;
use traits::{ObligationCause, ObligationCauseCode};
use ty::{self, TyCtxt, TypeFoldable};
use ty::{Region, ReFree, Issue32330};
use ty::{Region, Issue32330};
use ty::error::TypeError;
use std::fmt;
use syntax::ast;
use syntax_pos::{Pos, Span};
use errors::DiagnosticBuilder;
@ -255,8 +251,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
// try to pre-process the errors, which will group some of them
// together into a `ProcessedErrors` group:
let processed_errors = self.process_errors(errors);
let errors = processed_errors.as_ref().unwrap_or(errors);
let errors = self.process_errors(errors);
debug!("report_region_errors: {} errors after preprocessing", errors.len());
@ -278,13 +273,6 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
sub_origin, sub_r,
sup_origin, sup_r);
}
ProcessedErrors(ref origins,
ref same_regions) => {
if !same_regions.is_empty() {
self.report_processed_errors(origins);
}
}
}
}
}
@ -300,202 +288,31 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
// duplicates that will be unhelpful to the end-user. But
// obviously it never weeds out ALL errors.
fn process_errors(&self, errors: &Vec<RegionResolutionError<'tcx>>)
-> Option<Vec<RegionResolutionError<'tcx>>> {
-> Vec<RegionResolutionError<'tcx>> {
debug!("process_errors()");
let mut origins = Vec::new();
// we collect up ConcreteFailures and SubSupConflicts that are
// relating free-regions bound on the fn-header and group them
// together into this vector
let mut same_regions = Vec::new();
// We want to avoid reporting generic-bound failures if we can
// avoid it: these have a very high rate of being unhelpful in
// practice. This is because they are basically secondary
// checks that test the state of the region graph after the
// rest of inference is done, and the other kinds of errors
// indicate that the region constraint graph is internally
// inconsistent, so these test results are likely to be
// meaningless.
//
// Therefore, we filter them out of the list unless they are
// the only thing in the list.
// here we put errors that we will not be able to process nicely
let mut other_errors = Vec::new();
let is_bound_failure = |e: &RegionResolutionError<'tcx>| match *e {
ConcreteFailure(..) => false,
SubSupConflict(..) => false,
GenericBoundFailure(..) => true,
};
// we collect up GenericBoundFailures in here.
let mut bound_failures = Vec::new();
for error in errors {
// Check whether we can process this error into some other
// form; if not, fall through.
match *error {
ConcreteFailure(ref origin, sub, sup) => {
debug!("processing ConcreteFailure");
if let SubregionOrigin::CompareImplMethodObligation { .. } = *origin {
// When comparing an impl method against a
// trait method, it is not helpful to suggest
// changes to the impl method. This is
// because the impl method signature is being
// checked using the trait's environment, so
// usually the changes we suggest would
// actually have to be applied to the *trait*
// method (and it's not clear that the trait
// method is even under the user's control).
} else if let Some(same_frs) = free_regions_from_same_fn(self.tcx, sub, sup) {
origins.push(
ProcessedErrorOrigin::ConcreteFailure(
origin.clone(),
sub,
sup));
append_to_same_regions(&mut same_regions, &same_frs);
continue;
}
}
SubSupConflict(ref var_origin, ref sub_origin, sub, ref sup_origin, sup) => {
debug!("processing SubSupConflict sub: {:?} sup: {:?}", sub, sup);
match (sub_origin, sup_origin) {
(&SubregionOrigin::CompareImplMethodObligation { .. }, _) => {
// As above, when comparing an impl method
// against a trait method, it is not helpful
// to suggest changes to the impl method.
}
(_, &SubregionOrigin::CompareImplMethodObligation { .. }) => {
// See above.
}
_ => {
if let Some(same_frs) = free_regions_from_same_fn(self.tcx, sub, sup) {
origins.push(
ProcessedErrorOrigin::VariableFailure(
var_origin.clone()));
append_to_same_regions(&mut same_regions, &same_frs);
continue;
}
}
}
}
GenericBoundFailure(ref origin, ref kind, region) => {
bound_failures.push((origin.clone(), kind.clone(), region));
continue;
}
ProcessedErrors(..) => {
bug!("should not encounter a `ProcessedErrors` yet: {:?}", error)
}
}
// No changes to this error.
other_errors.push(error.clone());
}
// ok, let's pull together the errors, sorted in an order that
// we think will help user the best
let mut processed_errors = vec![];
// first, put the processed errors, if any
if !same_regions.is_empty() {
let common_scope_id = same_regions[0].scope_id;
for sr in &same_regions {
// Since ProcessedErrors is used to reconstruct the function
// declaration, we want to make sure that they are, in fact,
// from the same scope
if sr.scope_id != common_scope_id {
debug!("returning empty result from process_errors because
{} != {}", sr.scope_id, common_scope_id);
return None;
}
}
assert!(origins.len() > 0);
let pe = ProcessedErrors(origins, same_regions);
debug!("errors processed: {:?}", pe);
processed_errors.push(pe);
}
// next, put the other misc errors
processed_errors.extend(other_errors);
// finally, put the `T: 'a` errors, but only if there were no
// other errors. otherwise, these have a very high rate of
// being unhelpful in practice. This is because they are
// basically secondary checks that test the state of the
// region graph after the rest of inference is done, and the
// other kinds of errors indicate that the region constraint
// graph is internally inconsistent, so these test results are
// likely to be meaningless.
if processed_errors.is_empty() {
for (origin, kind, region) in bound_failures {
processed_errors.push(GenericBoundFailure(origin, kind, region));
}
}
// we should always wind up with SOME errors, unless there were no
// errors to start
assert!(if errors.len() > 0 {processed_errors.len() > 0} else {true});
return Some(processed_errors);
#[derive(Debug)]
struct FreeRegionsFromSameFn {
sub_fr: ty::FreeRegion,
sup_fr: ty::FreeRegion,
scope_id: ast::NodeId
}
impl FreeRegionsFromSameFn {
fn new(sub_fr: ty::FreeRegion,
sup_fr: ty::FreeRegion,
scope_id: ast::NodeId)
-> FreeRegionsFromSameFn {
FreeRegionsFromSameFn {
sub_fr: sub_fr,
sup_fr: sup_fr,
scope_id: scope_id
}
}
}
fn free_regions_from_same_fn<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
sub: &'tcx Region,
sup: &'tcx Region)
-> Option<FreeRegionsFromSameFn> {
debug!("free_regions_from_same_fn(sub={:?}, sup={:?})", sub, sup);
let (scope_id, fr1, fr2) = match (sub, sup) {
(&ReFree(fr1), &ReFree(fr2)) => {
if fr1.scope != fr2.scope {
return None
}
assert!(fr1.scope == fr2.scope);
(fr1.scope.node_id(&tcx.region_maps), fr1, fr2)
},
_ => return None
};
let parent = tcx.hir.get_parent(scope_id);
let parent_node = tcx.hir.find(parent);
match parent_node {
Some(node) => match node {
hir_map::NodeItem(item) => match item.node {
hir::ItemFn(..) => {
Some(FreeRegionsFromSameFn::new(fr1, fr2, scope_id))
},
_ => None
},
hir_map::NodeImplItem(..) |
hir_map::NodeTraitItem(..) => {
Some(FreeRegionsFromSameFn::new(fr1, fr2, scope_id))
},
_ => None
},
None => {
debug!("no parent node of scope_id {}", scope_id);
None
}
}
}
fn append_to_same_regions(same_regions: &mut Vec<SameRegions>,
same_frs: &FreeRegionsFromSameFn) {
debug!("append_to_same_regions(same_regions={:?}, same_frs={:?})",
same_regions, same_frs);
let scope_id = same_frs.scope_id;
let (sub_fr, sup_fr) = (same_frs.sub_fr, same_frs.sup_fr);
for sr in same_regions.iter_mut() {
if sr.contains(&sup_fr.bound_region) && scope_id == sr.scope_id {
sr.push(sub_fr.bound_region);
return
}
}
same_regions.push(SameRegions {
scope_id: scope_id,
regions: vec![sub_fr.bound_region, sup_fr.bound_region]
})
if errors.iter().all(|e| is_bound_failure(e)) {
errors.clone()
} else {
errors.iter().filter(|&e| !is_bound_failure(e)).cloned().collect()
}
}
@ -1072,20 +889,6 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
self.note_region_origin(&mut err, &sub_origin);
err.emit();
}
fn report_processed_errors(&self,
origins: &[ProcessedErrorOrigin<'tcx>]) {
for origin in origins.iter() {
let mut err = match *origin {
ProcessedErrorOrigin::VariableFailure(ref var_origin) =>
self.report_inference_failure(var_origin.clone()),
ProcessedErrorOrigin::ConcreteFailure(ref sr_origin, sub, sup) =>
self.report_concrete_failure(sr_origin.clone(), sub, sup),
};
err.emit();
}
}
}
impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {

36
src/librustc/infer/region_inference/mod.rs
View file

@ -24,7 +24,7 @@ use rustc_data_structures::graph::{self, Direction, NodeIndex, OUTGOING};
use rustc_data_structures::unify::{self, UnificationTable};
use middle::free_region::FreeRegionMap;
use ty::{self, Ty, TyCtxt};
use ty::{BoundRegion, Region, RegionVid};
use ty::{Region, RegionVid};
use ty::{ReEmpty, ReStatic, ReFree, ReEarlyBound, ReErased};
use ty::{ReLateBound, ReScope, ReVar, ReSkolemized, BrFresh};
@ -171,13 +171,6 @@ pub enum RegionResolutionError<'tcx> {
&'tcx Region,
SubregionOrigin<'tcx>,
&'tcx Region),
/// For subsets of `ConcreteFailure` and `SubSupConflict`, we can derive
/// more specific errors message by suggesting to the user where they
/// should put a lifetime. In those cases we process and put those errors
/// into `ProcessedErrors` before we do any reporting.
ProcessedErrors(Vec<ProcessedErrorOrigin<'tcx>>,
Vec<SameRegions>),
}
#[derive(Clone, Debug)]
@ -186,33 +179,6 @@ pub enum ProcessedErrorOrigin<'tcx> {
VariableFailure(RegionVariableOrigin),
}
/// SameRegions is used to group regions that we think are the same and would
/// like to indicate so to the user.
/// For example, the following function
/// ```
/// struct Foo { bar: i32 }
/// fn foo2<'a, 'b>(x: &'a Foo) -> &'b i32 {
/// &x.bar
/// }
/// ```
/// would report an error because we expect 'a and 'b to match, and so we group
/// 'a and 'b together inside a SameRegions struct
#[derive(Clone, Debug)]
pub struct SameRegions {
pub scope_id: ast::NodeId,
pub regions: Vec<BoundRegion>,
}
impl SameRegions {
pub fn contains(&self, other: &BoundRegion) -> bool {
self.regions.contains(other)
}
pub fn push(&mut self, other: BoundRegion) {
self.regions.push(other);
}
}
pub type CombineMap<'tcx> = FxHashMap<TwoRegions<'tcx>, RegionVid>;
pub struct RegionVarBindings<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {

21
src/librustc/mir/mod.rs
View file

@ -777,6 +777,12 @@ pub enum StatementKind<'tcx> {
/// End the current live range for the storage of the local.
StorageDead(Lvalue<'tcx>),
InlineAsm {
asm: InlineAsm,
outputs: Vec<Lvalue<'tcx>>,
inputs: Vec<Operand<'tcx>>
},
/// No-op. Useful for deleting instructions without affecting statement indices.
Nop,
}
@ -790,7 +796,10 @@ impl<'tcx> Debug for Statement<'tcx> {
StorageDead(ref lv) => write!(fmt, "StorageDead({:?})", lv),
SetDiscriminant{lvalue: ref lv, variant_index: index} => {
write!(fmt, "discriminant({:?}) = {:?}", lv, index)
}
},
InlineAsm { ref asm, ref outputs, ref inputs } => {
write!(fmt, "asm!({:?} : {:?} : {:?})", asm, outputs, inputs)
},
Nop => write!(fmt, "nop"),
}
}
@ -1004,12 +1013,6 @@ pub enum Rvalue<'tcx> {
/// that `Foo` has a destructor. These rvalues can be optimized
/// away after type-checking and before lowering.
Aggregate(AggregateKind<'tcx>, Vec<Operand<'tcx>>),
InlineAsm {
asm: InlineAsm,
outputs: Vec<Lvalue<'tcx>>,
inputs: Vec<Operand<'tcx>>
}
}
#[derive(Clone, Copy, Debug, PartialEq, Eq, RustcEncodable, RustcDecodable)]
@ -1111,10 +1114,6 @@ impl<'tcx> Debug for Rvalue<'tcx> {
UnaryOp(ref op, ref a) => write!(fmt, "{:?}({:?})", op, a),
Discriminant(ref lval) => write!(fmt, "discriminant({:?})", lval),
Box(ref t) => write!(fmt, "Box({:?})", t),
InlineAsm { ref asm, ref outputs, ref inputs } => {
write!(fmt, "asm!({:?} : {:?} : {:?})", asm, outputs, inputs)
}
Ref(_, borrow_kind, ref lv) => {
let kind_str = match borrow_kind {
BorrowKind::Shared => "",

1
src/librustc/mir/tcx.rs
View file

@ -207,7 +207,6 @@ impl<'tcx> Rvalue<'tcx> {
}
}
}
Rvalue::InlineAsm { .. } => None
}
}
}

21
src/librustc/mir/visit.rs
View file

@ -333,6 +333,16 @@ macro_rules! make_mir_visitor {
StatementKind::StorageDead(ref $($mutability)* lvalue) => {
self.visit_lvalue(lvalue, LvalueContext::StorageDead, location);
}
StatementKind::InlineAsm { ref $($mutability)* outputs,
ref $($mutability)* inputs,
asm: _ } => {
for output in & $($mutability)* outputs[..] {
self.visit_lvalue(output, LvalueContext::Store, location);
}
for input in & $($mutability)* inputs[..] {
self.visit_operand(input, location);
}
}
StatementKind::Nop => {}
}
}
@ -526,17 +536,6 @@ macro_rules! make_mir_visitor {
self.visit_operand(operand, location);
}
}
Rvalue::InlineAsm { ref $($mutability)* outputs,
ref $($mutability)* inputs,
asm: _ } => {
for output in & $($mutability)* outputs[..] {
self.visit_lvalue(output, LvalueContext::Store, location);
}
for input in & $($mutability)* inputs[..] {
self.visit_operand(input, location);
}
}
}
}

5
src/librustc/traits/mod.rs
View file

@ -628,6 +628,11 @@ pub fn get_vtable_methods<'a, 'tcx>(
|_, _| tcx.mk_region(ty::ReErased),
|def, _| trait_ref.substs().type_for_def(def));
// the trait type may have higher-ranked lifetimes in it;
// so erase them if they appear, so that we get the type
// at some particular call site
let substs = tcx.erase_late_bound_regions_and_normalize(&ty::Binder(substs));
// It's possible that the method relies on where clauses that
// do not hold for this particular set of type parameters.
// Note that this method could then never be called, so we

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