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Add nightly style guide section for precise_capturing use<> syntax #126753

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merged 1 commit into from
Jul 1, 2024

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@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-style Relevant to the style team, which will review and decide on the PR/issue. labels Jun 20, 2024
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rustbot commented Jun 20, 2024

Some changes occurred in src/doc/style-guide

cc @rust-lang/style

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I'm going to go ahead and FCP merge this as-is, since I don't believe there is any additional consideration we need to have towards this syntax. Please register a concern if we should have more discussion.

@rfcbot fcp merge

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rfcbot commented Jun 20, 2024

Team member @compiler-errors has proposed to merge this. The next step is review by the rest of the tagged team members:

Concerns:

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

@rfcbot rfcbot added proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. disposition-merge This issue / PR is in PFCP or FCP with a disposition to merge it. labels Jun 20, 2024
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Tho let's get three checkboxes

@rfcbot concern checkboxes

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The job x86_64-gnu-tools failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)
#18 exporting to docker image format
#18 sending tarball 15.5s done
#18 DONE 25.6s

 3 warnings found (use --debug to expand):
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 81)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 86)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 101)
[CI_JOB_NAME=x86_64-gnu-tools]
[CI_JOB_NAME=x86_64-gnu-tools]
---
 finished in 43.356 seconds
Build completed successfully in 0:01:43
+ head -n 1 /tmp/browser-ui-test.version
+ npm install browser-ui-test@0.17.2 --unsafe-perm=true
npm ERR! code E502
npm ERR! 502 Bad Gateway - GET https://registry.npmjs.org/agent-base

npm ERR! A complete log of this run can be found in: /home/user/.npm/_logs/2024-06-20T18_42_24_417Z-debug-0.log
  network time: Thu, 20 Jun 2024 18:49:55 GMT
##[error]Process completed with exit code 1.
Post job cleanup.

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I can't resolve the concern but have checked my box

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@rfcbot resolve checkboxes

@rfcbot rfcbot added the final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. label Jun 21, 2024
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rfcbot commented Jun 21, 2024

🔔 This is now entering its final comment period, as per the review above. 🔔

@rfcbot rfcbot removed the proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. label Jun 21, 2024
workingjubilee added a commit to workingjubilee/rustc that referenced this pull request Jun 24, 2024
…ebcartwright

Implement `use<>` formatting in rustfmt

This PR implements formatting for precise-capturing `use<>` syntax as proposed in rust-lang#126753.

The syntax is implemented as-if the `use<>` bound were a trait bound but with the `use` keyword as its path segment identifier.

I opted to develop this in the rust-lang/rust tree since I'm not certain when the next rustfmt subtree sync is going to be, and I'd rather not block landing nightly support for `use<>` on something I have no control over. If `@rust-lang/rustfmt` would rather I move this PR over to that repository, then I would at least like to know when the next rustfmt->rust subtree sync is going to be, since stabilizing `precise_capturing` without formatting will be disruptive.

This implementation is otherwise rather straightforward.

Tracking:

- rust-lang#123432
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this pull request Jun 24, 2024
…ebcartwright

Implement `use<>` formatting in rustfmt

This PR implements formatting for precise-capturing `use<>` syntax as proposed in rust-lang#126753.

The syntax is implemented as-if the `use<>` bound were a trait bound but with the `use` keyword as its path segment identifier.

I opted to develop this in the rust-lang/rust tree since I'm not certain when the next rustfmt subtree sync is going to be, and I'd rather not block landing nightly support for `use<>` on something I have no control over. If ``@rust-lang/rustfmt`` would rather I move this PR over to that repository, then I would at least like to know when the next rustfmt->rust subtree sync is going to be, since stabilizing `precise_capturing` without formatting will be disruptive.

This implementation is otherwise rather straightforward.

Tracking:

- rust-lang#123432
rust-timer added a commit to rust-lang-ci/rust that referenced this pull request Jun 24, 2024
Rollup merge of rust-lang#126754 - compiler-errors:use-rustfmt, r=calebcartwright

Implement `use<>` formatting in rustfmt

This PR implements formatting for precise-capturing `use<>` syntax as proposed in rust-lang#126753.

The syntax is implemented as-if the `use<>` bound were a trait bound but with the `use` keyword as its path segment identifier.

I opted to develop this in the rust-lang/rust tree since I'm not certain when the next rustfmt subtree sync is going to be, and I'd rather not block landing nightly support for `use<>` on something I have no control over. If ``@rust-lang/rustfmt`` would rather I move this PR over to that repository, then I would at least like to know when the next rustfmt->rust subtree sync is going to be, since stabilizing `precise_capturing` without formatting will be disruptive.

This implementation is otherwise rather straightforward.

Tracking:

- rust-lang#123432
compiler-errors pushed a commit to compiler-errors/rustfmt that referenced this pull request Jun 25, 2024
Implement `use<>` formatting in rustfmt

This PR implements formatting for precise-capturing `use<>` syntax as proposed in rust-lang/rust#126753.

The syntax is implemented as-if the `use<>` bound were a trait bound but with the `use` keyword as its path segment identifier.

I opted to develop this in the rust-lang/rust tree since I'm not certain when the next rustfmt subtree sync is going to be, and I'd rather not block landing nightly support for `use<>` on something I have no control over. If ``@rust-lang/rustfmt`` would rather I move this PR over to that repository, then I would at least like to know when the next rustfmt->rust subtree sync is going to be, since stabilizing `precise_capturing` without formatting will be disruptive.

This implementation is otherwise rather straightforward.

Tracking:

- rust-lang/rust#123432
@rfcbot rfcbot added finished-final-comment-period The final comment period is finished for this PR / Issue. to-announce Announce this issue on triage meeting and removed final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. labels Jul 1, 2024
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rfcbot commented Jul 1, 2024

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

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@bors r=joshtriplett,calebcartwright

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bors commented Jul 1, 2024

📌 Commit 1031d4d has been approved by joshtriplett,calebcartwright

It is now in the queue for this repository.

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 1, 2024
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@bors rollup

GuillaumeGomez added a commit to GuillaumeGomez/rust that referenced this pull request Jul 1, 2024
…=joshtriplett,calebcartwright

Add nightly style guide section for `precise_capturing` `use<>` syntax

r? style

Tracking:

- rust-lang#123432
bors added a commit to rust-lang-ci/rust that referenced this pull request Jul 1, 2024
…llaumeGomez

Rollup of 7 pull requests

Successful merges:

 - rust-lang#126753 (Add nightly style guide section for `precise_capturing` `use<>` syntax)
 - rust-lang#126880 (Migrate `volatile-intrinsics`, `weird-output-filenames`, `wasm-override-linker`, `wasm-exceptions-nostd` to `rmake`)
 - rust-lang#126941 (Migrate `run-make/llvm-ident` to `rmake.rs`)
 - rust-lang#127128 (Stabilize `duration_abs_diff`)
 - rust-lang#127129 (Use full expr span for return suggestion on type error/ambiguity)
 - rust-lang#127188 ( improve the way bootstrap handles rustlib components)
 - rust-lang#127201 (Improve run-make-support API)

r? `@ghost`
`@rustbot` modify labels: rollup
bors added a commit to rust-lang-ci/rust that referenced this pull request Jul 1, 2024
…llaumeGomez

Rollup of 8 pull requests

Successful merges:

 - rust-lang#126732 (Stabilize `PanicInfo::message()` and `PanicMessage`)
 - rust-lang#126753 (Add nightly style guide section for `precise_capturing` `use<>` syntax)
 - rust-lang#126832 (linker: Refactor interface for passing arguments to linker)
 - rust-lang#126880 (Migrate `volatile-intrinsics`, `weird-output-filenames`, `wasm-override-linker`, `wasm-exceptions-nostd` to `rmake`)
 - rust-lang#127128 (Stabilize `duration_abs_diff`)
 - rust-lang#127129 (Use full expr span for return suggestion on type error/ambiguity)
 - rust-lang#127188 ( improve the way bootstrap handles rustlib components)
 - rust-lang#127201 (Improve run-make-support API)

r? `@ghost`
`@rustbot` modify labels: rollup
@bors bors merged commit c4baa3f into rust-lang:master Jul 1, 2024
5 of 6 checks passed
@rustbot rustbot added this to the 1.81.0 milestone Jul 1, 2024
rust-timer added a commit to rust-lang-ci/rust that referenced this pull request Jul 1, 2024
Rollup merge of rust-lang#126753 - compiler-errors:use-style-guide, r=joshtriplett,calebcartwright

Add nightly style guide section for `precise_capturing` `use<>` syntax

r? style

Tracking:

- rust-lang#123432
@apiraino apiraino removed the to-announce Announce this issue on triage meeting label Jul 4, 2024
bors added a commit to rust-lang-ci/rust that referenced this pull request Aug 20, 2024
…=spastorino

Stabilize opaque type precise capturing (RFC 3617)

This PR partially stabilizes opaque type *precise capturing*, which was specified in [RFC 3617](rust-lang/rfcs#3617), and whose syntax was amended by FCP in [rust-lang#125836](rust-lang#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](rust-lang/rfcs#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 [rust-lang#125836](rust-lang#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:

- rust-lang#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.
  - dtolnay/syn#1677
  - dtolnay/syn#1707
- [x] Look into `rustfmt` support.
  - rust-lang#126754
- [x] Look into `rust-analyzer` support.
  - rust-lang/rust-analyzer#17598
  - rust-lang/rust-analyzer#17676
- [x] Look into `rustdoc` support.
  - rust-lang#127228
  - rust-lang#127632
  - rust-lang#127658
- [x] Suggest this feature to RfL (a known nightly user).
- [x] Add a chapter to the edition guide.
  - rust-lang/edition-guide#316
- [x] Update the Reference.
  - rust-lang/reference#1577

### (Selected) implementation history

* rust-lang/rfcs#3498
* rust-lang/rfcs#3617
* rust-lang#123468
* rust-lang#125836
* rust-lang#126049
* rust-lang#126753

Closes rust-lang#123432.

cc `@rust-lang/lang` `@rust-lang/types`

`@rustbot` labels +T-lang +I-lang-nominated +A-impl-trait +F-precise_capturing

Tracking:

- rust-lang#123432

----

For the compiler reviewer, I'll leave some inline comments about diagnostics fallout :^)

r? compiler
bors added a commit to rust-lang-ci/rust that referenced this pull request Aug 20, 2024
…=spastorino

Stabilize opaque type precise capturing (RFC 3617)

This PR partially stabilizes opaque type *precise capturing*, which was specified in [RFC 3617](rust-lang/rfcs#3617), and whose syntax was amended by FCP in [rust-lang#125836](rust-lang#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](rust-lang/rfcs#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 [rust-lang#125836](rust-lang#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:

- rust-lang#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.
  - dtolnay/syn#1677
  - dtolnay/syn#1707
- [x] Look into `rustfmt` support.
  - rust-lang#126754
- [x] Look into `rust-analyzer` support.
  - rust-lang/rust-analyzer#17598
  - rust-lang/rust-analyzer#17676
- [x] Look into `rustdoc` support.
  - rust-lang#127228
  - rust-lang#127632
  - rust-lang#127658
- [x] Suggest this feature to RfL (a known nightly user).
- [x] Add a chapter to the edition guide.
  - rust-lang/edition-guide#316
- [x] Update the Reference.
  - rust-lang/reference#1577

### (Selected) implementation history

* rust-lang/rfcs#3498
* rust-lang/rfcs#3617
* rust-lang#123468
* rust-lang#125836
* rust-lang#126049
* rust-lang#126753

Closes rust-lang#123432.

cc `@rust-lang/lang` `@rust-lang/types`

`@rustbot` labels +T-lang +I-lang-nominated +A-impl-trait +F-precise_capturing

Tracking:

- rust-lang#123432

----

For the compiler reviewer, I'll leave some inline comments about diagnostics fallout :^)

r? compiler
github-actions bot pushed a commit to rust-lang/miri that referenced this pull request Aug 26, 2024
Stabilize opaque type precise capturing (RFC 3617)

This PR partially stabilizes opaque type *precise capturing*, which was specified in [RFC 3617](rust-lang/rfcs#3617), and whose syntax was amended by FCP in [#125836](rust-lang/rust#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](rust-lang/rfcs#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](rust-lang/rust#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:

- rust-lang/rust#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.
  - dtolnay/syn#1677
  - dtolnay/syn#1707
- [x] Look into `rustfmt` support.
  - rust-lang/rust#126754
- [x] Look into `rust-analyzer` support.
  - rust-lang/rust-analyzer#17598
  - rust-lang/rust-analyzer#17676
- [x] Look into `rustdoc` support.
  - rust-lang/rust#127228
  - rust-lang/rust#127632
  - rust-lang/rust#127658
- [x] Suggest this feature to RfL (a known nightly user).
- [x] Add a chapter to the edition guide.
  - rust-lang/edition-guide#316
- [x] Update the Reference.
  - rust-lang/reference#1577

### (Selected) implementation history

* rust-lang/rfcs#3498
* rust-lang/rfcs#3617
* rust-lang/rust#123468
* rust-lang/rust#125836
* rust-lang/rust#126049
* rust-lang/rust#126753

Closes #123432.

cc `@rust-lang/lang` `@rust-lang/types`

`@rustbot` labels +T-lang +I-lang-nominated +A-impl-trait +F-precise_capturing

Tracking:

- rust-lang/rust#123432

----

For the compiler reviewer, I'll leave some inline comments about diagnostics fallout :^)

r? compiler
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