Stabilize opaque type precise capturing (RFC 3617)
This PR partially stabilizes opaque type *precise capturing*, which was specified in [RFC 3617](https://github.com/rust-lang/rfcs/pull/3617), and whose syntax was amended by FCP in [#125836](https://github.com/rust-lang/rust/issues/125836).
This feature, as stabilized here, gives us a way to explicitly specify the generic lifetime parameters that an RPIT-like opaque type captures. This solves the problem of overcapturing, for lifetime parameters in these opaque types, and will allow the Lifetime Capture Rules 2024 ([RFC 3498](https://github.com/rust-lang/rfcs/pull/3498)) to be fully stabilized for RPIT in Rust 2024.
### What are we stabilizing?
This PR stabilizes the use of a `use<'a, T>` bound in return-position impl Trait opaque types. Such a bound fully specifies the set of generic parameters captured by the RPIT opaque type, entirely overriding the implicit default behavior. E.g.:
```rust
fn does_not_capture<'a, 'b>() -> impl Sized + use<'a> {}
// ~~~~~~~~~~~~~~~~~~~~
// This RPIT opaque type does not capture `'b`.
```
The way we would suggest thinking of `impl Trait` types *without* an explicit `use<..>` bound is that the `use<..>` bound has been *elided*, and that the bound is filled in automatically by the compiler according to the edition-specific capture rules.
All non-`'static` lifetime parameters, named (i.e. non-APIT) type parameters, and const parameters in scope are valid to name, including an elided lifetime if such a lifetime would also be valid in an outlives bound, e.g.:
```rust
fn elided(x: &u8) -> impl Sized + use<'_> { x }
```
Lifetimes must be listed before type and const parameters, but otherwise the ordering is not relevant to the `use<..>` bound. Captured parameters may not be duplicated. For now, only one `use<..>` bound may appear in a bounds list. It may appear anywhere within the bounds list.
### How does this differ from the RFC?
This stabilization differs from the RFC in one respect: the RFC originally specified `use<'a, T>` as syntactically part of the RPIT type itself, e.g.:
```rust
fn capture<'a>() -> impl use<'a> Sized {}
```
However, settling on the final syntax was left as an open question. T-lang later decided via FCP in [#125836](https://github.com/rust-lang/rust/issues/125836) to treat `use<..>` as a syntactic bound instead, e.g.:
```rust
fn capture<'a>() -> impl Sized + use<'a> {}
```
### What aren't we stabilizing?
The key goal of this PR is to stabilize the parts of *precise capturing* that are needed to enable the migration to Rust 2024.
There are some capabilities of *precise capturing* that the RFC specifies but that we're not stabilizing here, as these require further work on the type system. We hope to lift these limitations later.
The limitations that are part of this PR were specified in the [RFC's stabilization strategy](https://rust-lang.github.io/rfcs/3617-precise-capturing.html#stabilization-strategy).
#### Not capturing type or const parameters
The RFC addresses the overcapturing of type and const parameters; that is, it allows for them to not be captured in opaque types. We're not stabilizing that in this PR. Since all in scope generic type and const parameters are implicitly captured in all editions, this is not needed for the migration to Rust 2024.
For now, when using `use<..>`, all in scope type and const parameters must be nameable (i.e., APIT cannot be used) and included as arguments. For example, this is an error because `T` is in scope and not included as an argument:
```rust
fn test<T>() -> impl Sized + use<> {}
//~^ ERROR `impl Trait` must mention all type parameters in scope in `use<...>`
```
This is due to certain current limitations in the type system related to how generic parameters are represented as captured (i.e. bivariance) and how inference operates.
We hope to relax this in the future, and this stabilization is forward compatible with doing so.
#### Precise capturing for return-position impl Trait **in trait** (RPITIT)
The RFC specifies precise capturing for RPITIT. We're not stabilizing that in this PR. Since RPITIT already adheres to the Lifetime Capture Rules 2024, this isn't needed for the migration to Rust 2024.
The effect of this is that the anonymous associated types created by RPITITs must continue to capture all of the lifetime parameters in scope, e.g.:
```rust
trait Foo<'a> {
fn test() -> impl Sized + use<Self>;
//~^ ERROR `use<...>` precise capturing syntax is currently not allowed in return-position `impl Trait` in traits
}
```
To allow this involves a meaningful amount of type system work related to adding variance to GATs or reworking how generics are represented in RPITITs. We plan to do this work separately from the stabilization. See:
- https://github.com/rust-lang/rust/pull/124029
Supporting precise capturing for RPITIT will also require us to implement a new algorithm for detecting refining capture behavior. This may involve looking through type parameters to detect cases where the impl Trait type in an implementation captures fewer lifetimes than the corresponding RPITIT in the trait definition, e.g.:
```rust
trait Foo {
fn rpit() -> impl Sized + use<Self>;
}
impl<'a> Foo for &'a () {
// This is "refining" due to not capturing `'a` which
// is implied by the trait's `use<Self>`.
fn rpit() -> impl Sized + use<>;
// This is not "refining".
fn rpit() -> impl Sized + use<'a>;
}
```
This stabilization is forward compatible with adding support for this later.
### The technical details
This bound is purely syntactical and does not lower to a [`Clause`](https://doc.rust-lang.org/1.79.0/nightly-rustc/rustc_middle/ty/type.ClauseKind.html) in the type system. For the purposes of the type system (and for the types team's curiosity regarding this stabilization), we have no current need to represent this as a `ClauseKind`.
Since opaques already capture a variable set of lifetimes depending on edition and their syntactical position (e.g. RPIT vs RPITIT), a `use<..>` bound is just a way to explicitly rather than implicitly specify that set of lifetimes, and this only affects opaque type lowering from AST to HIR.
### FCP plan
While there's much discussion of the type system here, the feature in this PR is implemented internally as a transformation that happens before lowering to the type system layer. We already support impl Trait types partially capturing the in scope lifetimes; we just currently only expose that implicitly.
So, in my (errs's) view as a types team member, there's nothing for types to weigh in on here with respect to the implementation being stabilized, and I'd suggest a lang-only proposed FCP (though we'll of course CC the team below).
### Authorship and acknowledgments
This stabilization report was coauthored by compiler-errors and TC.
TC would like to acknowledge the outstanding and speedy work that compiler-errors has done to make this feature happen.
compiler-errors thanks TC for authoring the RFC, for all of his involvement in this feature's development, and pushing the Rust 2024 edition forward.
### Open items
We're doing some things in parallel here. In signaling the intention to stabilize, we want to uncover any latent issues so we can be sure they get addressed. We want to give the maximum time for discussion here to happen by starting it while other remaining miscellaneous work proceeds. That work includes:
- [x] Look into `syn` support.
- https://github.com/dtolnay/syn/issues/1677
- https://github.com/dtolnay/syn/pull/1707
- [x] Look into `rustfmt` support.
- https://github.com/rust-lang/rust/pull/126754
- [x] Look into `rust-analyzer` support.
- https://github.com/rust-lang/rust-analyzer/issues/17598
- https://github.com/rust-lang/rust-analyzer/pull/17676
- [x] Look into `rustdoc` support.
- https://github.com/rust-lang/rust/issues/127228
- https://github.com/rust-lang/rust/pull/127632
- https://github.com/rust-lang/rust/pull/127658
- [x] Suggest this feature to RfL (a known nightly user).
- [x] Add a chapter to the edition guide.
- https://github.com/rust-lang/edition-guide/pull/316
- [x] Update the Reference.
- https://github.com/rust-lang/reference/pull/1577
### (Selected) implementation history
* https://github.com/rust-lang/rfcs/pull/3498
* https://github.com/rust-lang/rfcs/pull/3617
* https://github.com/rust-lang/rust/pull/123468
* https://github.com/rust-lang/rust/issues/125836
* https://github.com/rust-lang/rust/pull/126049
* https://github.com/rust-lang/rust/pull/126753Closes#123432.
cc `@rust-lang/lang` `@rust-lang/types`
`@rustbot` labels +T-lang +I-lang-nominated +A-impl-trait +F-precise_capturing
Tracking:
- https://github.com/rust-lang/rust/issues/123432
----
For the compiler reviewer, I'll leave some inline comments about diagnostics fallout :^)
r? compiler
Suggest adding Result return type for associated method in E0277.
Recommit #126515 because I messed up during rebase,
Suggest adding Result return type for associated method in E0277.
For following:
```rust
struct A;
impl A {
fn test4(&self) {
let mut _file = File::create("foo.txt")?;
//~^ ERROR the `?` operator can only be used in a method
}
```
Suggest:
```rust
impl A {
fn test4(&self) -> Result<(), Box<dyn std::error::Error>> {
let mut _file = File::create("foo.txt")?;
//~^ ERROR the `?` operator can only be used in a method
Ok(())
}
}
```
For #125997
r? `@cjgillot`
Use cnum for extern crate data key
Noticed this when fixing #129184. I still have yet to put up a fix for that (mostly because I'm too lazy to minimize a test, that will come soon though).
Use `FnSig` instead of raw `FnDecl` for `ForeignItemKind::Fn`, fix ICE for `Fn` trait error on safe foreign fn
Let's use `hir::FnSig` instead of `hir::FnDecl + hir::Safety` for `ForeignItemKind::Fn`. This consolidates some handling code between normal fns and foreign fns.
Separetly, fix an ICE where we weren't handling `Fn` trait errors for safe foreign fns.
If perf is bad for the first commit, I can rework the ICE fix to not rely on it. But if perf is good, I prefer we fix and clean up things all at once 👍
r? spastorino
Fixes#128764
Detect multiple crate versions on method not found
When a type comes indirectly from one crate version but the imported trait comes from a separate crate version, the called method won't be found. We now show additional context:
```
error[E0599]: no method named `foo` found for struct `dep_2_reexport::Type` in the current scope
--> multiple-dep-versions.rs:8:10
|
8 | Type.foo();
| ^^^ method not found in `Type`
|
note: there are multiple different versions of crate `dependency` in the dependency graph
--> multiple-dep-versions.rs:4:32
|
4 | use dependency::{do_something, Trait};
| ^^^^^ `dependency` imported here doesn't correspond to the right crate version
|
::: ~/rust/build/x86_64-unknown-linux-gnu/test/run-make/crate-loading/rmake_out/multiple-dep-versions-1.rs:4:1
|
4 | pub trait Trait {
| --------------- this is the trait that was imported
|
::: ~/rust/build/x86_64-unknown-linux-gnu/test/run-make/crate-loading/rmake_out/multiple-dep-versions-2.rs:4:1
|
4 | pub trait Trait {
| --------------- this is the trait that is needed
5 | fn foo(&self);
| --- the method is available for `dep_2_reexport::Type` here
```
Fix#128569, fix#110926, fix#109161, fix#81659, fix#51458, fix#32611. Follow up to #124944.
Shrink `TyKind::FnPtr`.
By splitting the `FnSig` within `TyKind::FnPtr` into `FnSigTys` and `FnHeader`, which can be packed more efficiently. This reduces the size of the hot `TyKind` type from 32 bytes to 24 bytes on 64-bit platforms. This reduces peak memory usage by a few percent on some benchmarks. It also reduces cache misses and page faults similarly, though this doesn't translate to clear cycles or wall-time improvements on CI.
r? `@compiler-errors`
Store `do_not_recommend`-ness in impl header
Alternative to #128674
It's less flexible, but also less invasive. Hopefully it's also performant. I'd recommend we think separately about the design for how to gate arbitrary diagnostic attributes moving forward.
Use more slice patterns inside the compiler
Nothing super noteworthy. Just replacing the common 'fragile' pattern of "length check followed by indexing or unwrap" with slice patterns for legibility and 'robustness'.
r? ghost
By splitting the `FnSig` within `TyKind::FnPtr` into `FnSigTys` and
`FnHeader`, which can be packed more efficiently. This reduces the size
of the hot `TyKind` type from 32 bytes to 24 bytes on 64-bit platforms.
This reduces peak memory usage by a few percent on some benchmarks. It
also reduces cache misses and page faults similarly, though this doesn't
translate to clear cycles or wall-time improvements on CI.
More information for fully-qualified suggestion when there are multiple impls
```
error[E0790]: cannot call associated function on trait without specifying the corresponding `impl` type
--> $DIR/E0283.rs:30:21
|
LL | fn create() -> u32;
| ------------------- `Coroutine::create` defined here
...
LL | let cont: u32 = Coroutine::create();
| ^^^^^^^^^^^^^^^^^^^ cannot call associated function of trait
|
help: use a fully-qualified path to a specific available implementation
|
LL | let cont: u32 = <Impl as Coroutine>::create();
| ++++++++ +
LL | let cont: u32 = <AnotherImpl as Coroutine>::create();
| +++++++++++++++ +
```
The two altered expectation messages both seem like improvements:
- `coerce-expect-unsized-ascribed.stderr` says you can go
`Box<char> -> Box<dyn Debug>`, which you can.
- `upcast_soundness_bug.stderr` used to say that you could go
`Box<dyn Trait<u8, u8>> -> Box<dyn Trait>`, which you can't,
because the type parameters are missing in the destination
and the only ones that work aren't what's needed.
When encountering an E0277, if the type and the trait both come from a crate with the same name but different crate number, we explain that there are multiple crate versions in the dependency tree.
If there's a type that fulfills the bound, and it has the same name as the passed in type and has the same crate name, we explain that the same type in two different versions of the same crate *are different*.
```
error[E0277]: the trait bound `Type: dependency::Trait` is not satisfied
--> src/main.rs:4:18
|
4 | do_something(Type);
| ------------ ^^^^ the trait `dependency::Trait` is not implemented for `Type`
| |
| required by a bound introduced by this call
|
help: you have multiple different versions of crate `dependency` in your dependency graph
--> src/main.rs:1:5
|
1 | use bar::do_something;
| ^^^ one version of crate `dependency` is used here, as a dependency of crate `bar`
2 | use dependency::Type;
| ^^^^^^^^^^ one version of crate `dependency` is used here, as a direct dependency of the current crate
note: two types coming from two different versions of the same crate are different types even if they look the same
--> /home/gh-estebank/crate_versions/baz-2/src/lib.rs:1:1
|
1 | pub struct Type;
| ^^^^^^^^^^^^^^^ this type doesn't implement the required trait
|
::: /home/gh-estebank/crate_versions/baz/src/lib.rs:1:1
|
1 | pub struct Type;
| ^^^^^^^^^^^^^^^ this type implements the required trait
2 | pub trait Trait {}
| --------------- this is the required trait
note: required by a bound in `bar::do_something`
--> /home/gh-estebank/crate_versions/baz/src/lib.rs:4:24
|
4 | pub fn do_something<X: Trait>(_: X) {}
| ^^^^^ required by this bound in `do_something`
```
Address #22750.
```
error[E0790]: cannot call associated function on trait without specifying the corresponding `impl` type
--> $DIR/E0283.rs:30:21
|
LL | fn create() -> u32;
| ------------------- `Coroutine::create` defined here
...
LL | let cont: u32 = Coroutine::create();
| ^^^^^^^^^^^^^^^^^^^ cannot call associated function of trait
|
help: use a fully-qualified path to a specific available implementation
|
LL | let cont: u32 = <Impl as Coroutine>::create();
| ++++++++ +
LL | let cont: u32 = <AnotherImpl as Coroutine>::create();
| +++++++++++++++ +
```
For following:
```rust
struct A;
impl A {
fn test4(&self) {
let mut _file = File::create("foo.txt")?;
//~^ ERROR the `?` operator can only be used in a method
}
```
Suggest:
```rust
impl A {
fn test4(&self) -> Result<(), Box<dyn std::error::Error>> {
let mut _file = File::create("foo.txt")?;
//~^ ERROR the `?` operator can only be used in a method
Ok(())
}
}
```
For #125997
When encountering `-> Trait`, suggest `-> Box<dyn Trait>` (instead of `-> Box<Trait>`.
If there's a single returned type within the `fn`, suggest `-> impl Trait`.
Invert infer `error_reporting` mod struture
Parallel change to #127493, which moves `rustc_infer::infer::error_reporting` to `rustc_infer::error_reporting::infer`. After this, we should just be able to merge this into `rustc_trait_selection::error_reporting::infer`, and pull down `TypeErrCtxt` into that crate. 👍
r? lcnr
This commit changes the error reporting mechanism for not implemented
traits to skip impl marked as `#[diagnostic::do_not_recommend]` in the
help part of the error message ("the following other types implement
trait `Foo`:"). The main use case here is to allow crate authors to skip
non-meaningful confusing suggestions. A common example for this are
fully generic impls on tuples.
