Implement always-fallible TryFrom for usize/isize conversions that are infallible on some platforms
This reverts commit 837d6c7023 "Remove TryFrom impls that might become conditionally-infallible with a portability lint".
This fixes#49415 by adding (restoring) missing `TryFrom` impls for integer conversions to or from `usize` or `isize`, by making them always fallible at the type system level (that is, with `Error=TryFromIntError`) even though they happen to be infallible on some platforms (for some values of `size_of::<usize>()`).
They had been removed to allow the possibility to conditionally having some of them be infallible `From` impls instead, depending on the platforms, and have the [portability lint](https://github.com/rust-lang/rfcs/pull/1868) warn when they are used in code that is not already opting into non-portability. For example `#[allow(some_lint)] usize::from(x: u64)` would be valid on code that only targets 64-bit platforms.
This PR gives up on this possiblity for two reasons:
* Based on discussion with @aturon, it seems that the portability lint is not happening any time soon. It’s better to have the conversions be available *at all* than keep blocking them for so long. Portability-lint-gated platform-specific APIs can always be added separately later.
* For code that is fine with fallibility, the alternative would force it to opt into "non-portability" even though there would be no real portability issue.
Stabilize Iterator::flatten in 1.29, fixes#48213.
This PR stabilizes [`Iterator::flatten`](https://doc.rust-lang.org/nightly/std/iter/trait.Iterator.html#method.flatten) in *version 1.29* (1.28 goes to beta in 10 days, I don't think there's enough time to land it in that time, but let's see...).
Tracking issue is: #48213.
cc @bluss re. itertools.
r? @SimonSapin
ping @pietroalbini -- let's do a crater run when this passes CI :)
Various changes to existing diagnostics
* [Add code to `invalid ABI` error, add span label, move list to help to make message shorter](https://github.com/rust-lang/rust/pull/51463/commits/23ae5af274defa9ff884f593e44a2bbcaf814a02):
```
error[E0697]: invalid ABI: found `路濫狼á́́`
--> $DIR/unicode.rs:11:8
|
LL | extern "路濫狼á́́" fn foo() {} //~ ERROR invalid ABI
| ^^^^^^^^^ invalid ABI
|
= help: valid ABIs: cdecl, stdcall, fastcall, vectorcall, thiscall, aapcs, win64, sysv64, ptx-kernel, msp430-interrupt, x86-interrupt, Rust, C, system, rust-intrinsic, rust-call, platform-intrinsic, unadjusted
```
* [Add code to incorrect `pub` restriction error](https://github.com/rust-lang/rust/pull/51463/commits/e96fdea8a38f39f99f8b9a4000a689187a457e08)
* [Add message to `rustc_on_unimplemented` attributes in core to have them set a custom message _and_ label](https://github.com/rust-lang/rust/pull/51463/commits/2cc7e5ed307aee936c20479cfdc7409d6b52a464):
```
error[E0277]: `W` does not have a constant size known at compile-time
--> $DIR/unsized-enum2.rs:33:8
|
LL | VA(W),
| ^ `W` does not have a constant size known at compile-time
|
= help: the trait `std::marker::Sized` is not implemented for `W`
= help: consider adding a `where W: std::marker::Sized` bound
= note: no field of an enum variant may have a dynamically sized type
```
```
error[E0277]: `Foo` cannot be sent between threads safely
--> $DIR/E0277-2.rs:26:5
|
LL | is_send::<Foo>();
| ^^^^^^^^^^^^^^ `Foo` cannot be sent between threads safely
|
= help: the trait `std::marker::Send` is not implemented for `Foo`
```
```
error[E0277]: can't compare `{integer}` with `std::string::String`
--> $DIR/binops.rs:16:7
|
LL | 5 < String::new();
| ^ no implementation for `{integer} < std::string::String` and `{integer} > std::string::String`
|
= help: the trait `std::cmp::PartialOrd<std::string::String>` is not implemented for `{integer}`
```
```
error[E0277]: can't compare `{integer}` with `std::result::Result<{integer}, _>`
--> $DIR/binops.rs:17:7
|
LL | 6 == Ok(1);
| ^^ no implementation for `{integer} == std::result::Result<{integer}, _>`
|
= help: the trait `std::cmp::PartialEq<std::result::Result<{integer}, _>>` is not implemented for `{integer}`
```
```
error[E0277]: a collection of type `i32` cannot be built from an iterator over elements of type `i32`
--> $DIR/type-check-defaults.rs:16:19
|
LL | struct WellFormed<Z = Foo<i32, i32>>(Z);
| ^ a collection of type `i32` cannot be built from `std::iter::Iterator<Item=i32>`
|
= help: the trait `std::iter::FromIterator<i32>` is not implemented for `i32`
note: required by `Foo`
--> $DIR/type-check-defaults.rs:15:1
|
LL | struct Foo<T, U: FromIterator<T>>(T, U);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```
* [Add link to book for `Sized` errors](https://github.com/rust-lang/rust/pull/51463/commits/1244dc7c283323aea1a3457a4458d590a3e160c8):
```
error[E0277]: `std::fmt::Debug + std::marker::Sync + 'static` does not have a constant size known at compile-time
--> $DIR/const-unsized.rs:13:29
|
LL | const CONST_0: Debug+Sync = *(&0 as &(Debug+Sync));
| ^^^^^^^^^^^^^^^^^^^^^^ `std::fmt::Debug + std::marker::Sync + 'static` does not have a constant size known at compile-time
|
= help: the trait `std::marker::Sized` is not implemented for `std::fmt::Debug + std::marker::Sync + 'static`
= note: to learn more, visit <https://doc.rust-lang.org/book/second-edition/ch19-04-advanced-types.html#dynamically-sized-types--sized>
= note: constant expressions must have a statically known size
```
* [Point to previous line for single expected token not found](https://github.com/rust-lang/rust/pull/51463/commits/48165168fb0f059d8536cd4a2276b609d4a7f721) (if the current token is in a different line)
the originally generated code was highly suboptimal
this brings it close to the same code or even exactly the same as a
manual while-loop by eliminating a branch and the
double stepping of n-1 + 1 steps
The intermediate trait lets us circumvent the specialization
type inference bugs
Correct a few stability attributes
* `const_indexing` language feature was stabilized in 1.26.0 by #46882
* `Display` impls for `PanicInfo` and `Location` were stabilized in 1.26.0 by #47687
* `TrustedLen` is still unstable so its impls should be as well even though `RangeInclusive` was stabilized by #47813
* `!Send` and `!Sync` for `Args` and `ArgsOs` were stabilized in 1.26.0 by #48005
* `EscapeDefault` has been stable since 1.0.0 so should continue to show that even though it was moved to core in #48735
This could be backported to beta like #49612
Add #[must_use] to a few standard library methods
Chosen to start a precedent of using it on ones that are potentially-expensive and where using it for side effects is particularly discouraged.
Discuss :)
```rust
warning: unused return value of `std::iter::Iterator::collect` which must be used: if you really need to exhaust the iterator, consider `.for_each(drop)` instead
--> $DIR/fn_must_use_stdlib.rs:19:5
|
LL | "1 2 3".split_whitespace().collect::<Vec<_>>();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
warning: unused return value of `std::borrow::ToOwned::to_owned` which must be used: cloning is often expensive and is not expected to have side effects
--> $DIR/fn_must_use_stdlib.rs:21:5
|
LL | "hello".to_owned();
| ^^^^^^^^^^^^^^^^^^^
warning: unused return value of `std::clone::Clone::clone` which must be used: cloning is often expensive and is not expected to have side effects
--> $DIR/fn_must_use_stdlib.rs:23:5
|
LL | String::from("world").clone();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```
cc https://github.com/rust-lang/rust/issues/48926
Add Iterator::find_map
I'd like to propose to add `find_map` method to the `Iterator`: an occasionally useful utility, which relates to `filter_map` in the same way that `find` relates to `filter`.
`find_map` takes an `Option`-returning function, applies it to the elements of the iterator, and returns the first non-`None` result. In other words, `find_map(f) == filter_map(f).next()`.
Why do we want to add a function to the `Iterator`, which can be trivially expressed as a combination of existing ones? Observe that `find(f) == filter(f).next()`, so, by the same logic, `find` itself is unnecessary!
The more positive argument is that desugaring of `find[_map]` in terms of `filter[_map]().next()` is not super obvious, because the `filter` operation reads as if it is applies to the whole collection, although in reality we are interested only in the first element. That is, the jump from "I need a **single** result" to "let's use a function which maps **many** values to **many** values" is a non-trivial speed-bump, and causes friction when reading and writing code.
Does the need for `find_map` arise in practice? Yes!
* Anecdotally, I've more than once searched the docs for the function with `[T] -> (T -> Option<U>) -> Option<U>` signature.
* The direct cause for this PR was [this](https://github.com/rust-lang/cargo/pull/5187/files/1291c50e86ed4b31db0c76de03a47a5d0074bbd7#r174934173) discussion in Cargo, which boils down to "there's some pattern that we try to express here, but current approaches looks non-pretty" (and the pattern is `filter_map`
* There are several `filter_map().next` combos in Cargo: [[1]](545a4a2c93/src/cargo/ops/cargo_new.rs (L585)), [[2]](545a4a2c93/src/cargo/core/resolver/mod.rs (L1130)), [[3]](545a4a2c93/src/cargo/ops/cargo_rustc/mod.rs (L1086)).
* I've also needed similar functionality in `Kotlin` several times. There, it is expressed as `mapNotNull {}.firstOrNull`, as can be seen [here](ee8bdb4e07/src/main/kotlin/org/rust/cargo/project/model/impl/CargoProjectImpl.kt (L154)), [here](ee8bdb4e07/src/main/kotlin/org/rust/lang/core/resolve/ImplLookup.kt (L444)) [here](ee8bdb4e07/src/main/kotlin/org/rust/ide/inspections/RsLint.kt (L38)) and [here](ee8bdb4e07/src/main/kotlin/org/rust/cargo/toolchain/RustToolchain.kt (L74)) (and maybe in some other cases as well)
Note that it is definitely not among the most popular functions (it definitely is less popular than `find`), but, for example it (in case of Cargo) seems to be more popular than `rposition` (1 occurrence), `step_by` (zero occurrences) and `nth` (three occurrences as `nth(0)` which probably should be replaced with `next`).
Do we necessary need this function in `std`? Could we move it to itertools? That is possible, but observe that `filter`, `filter_map`, `find` and `find_map` together really form a complete table:
|||
|-------|---------|
| filter| find|
|filter_map|find_map|
It would be somewhat unsatisfying to have one quarter of this table live elsewhere :) Also, if `Itertools` adds an `find_map` method, it would be more difficult to move it to std due to name collision.
Hm, at this point I've searched for `filter_map` the umpteenth time, and, strangely, this time I do find this RFC: https://github.com/rust-lang/rfcs/issues/1801. I guess this could be an implementation though? :)
To sum up:
Pro:
- complete the symmetry with existing method
- codify a somewhat common non-obvious pattern
Contra:
- niche use case
- we can, and do, live without it
Implement some trivial size_hints for various iterators
This also implements ExactSizeIterator where applicable.
Addresses most of the Iterator traits mentioned in #23708.
I intend to do more, but I don't want to make the PR too large.
Chosen to start a precedent of using it on ones that are potentially-expensive and where using it for side effects is particularly discouraged.
Discuss :)
The comment "the value passed on to the next iteration" confused me since it sounded more like what Haskell's [scanl](http://hackage.haskell.org/package/base-4.11.0.0/docs/Prelude.html#v:scanl) does where the closure's return value serves as both the "yielded value" *and* the new value of the "state".
I tried changing the example to make it clear that the closure's return value is decoupled from the state argument.
Stabilize FusedIterator
FusedIterator is a marker trait that promises that the implementing
iterator continues to return `None` from `.next()` once it has returned
`None` once (and/or `.next_back()`, if implemented).
The effects of FusedIterator are already widely available through
`.fuse()`, but with stable `FusedIterator`, stable Rust users can
implement this trait for their iterators when appropriate.
Closes#35602
FusedIterator is a marker trait that promises that the implementing
iterator continues to return `None` from `.next()` once it has returned
`None` once (and/or `.next_back()`, if implemented).
The effects of FusedIterator are already widely available through
`.fuse()`, but with stable `FusedIterator`, stable Rust users can
implement this trait for their iterators when appropriate.
Add Iterator::flatten
This adds the trait method `.flatten()` on `Iterator` which flattens one level of nesting from an iterator or (into)iterators. The method `.flat_fmap(f)` is then redefined as `.map(f).flatten()`. The implementation of `Flatten` is essentially that of what it was for `FlatMap` but removing the call to `f` at various places.
Hopefully the type alias approach should be OK as was indicated / alluded to by @bluss and @eddyb in https://github.com/rust-lang/rfcs/pull/2306#issuecomment-361391370.
cc @scottmcm
