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Implement precise_capturing support for rustdoc #127632

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merged 2 commits into from
Jul 13, 2024

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@compiler-errors compiler-errors commented Jul 11, 2024

Implements rustdoc (+json) support for local (i.e. non-cross-crate-inlined) RPITs with use<...> precise capturing syntax.

Tests kinda suck. They're really hard to write 😰

r? @fmease or re-roll if you're too busy!
also cc @aDotInTheVoid for the json side

Tracking:

@rustbot rustbot added A-meta Area: Issues & PRs about the rust-lang/rust repository itself A-rustdoc-json Area: Rustdoc JSON backend S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-rustdoc Relevant to the rustdoc team, which will review and decide on the PR/issue. labels Jul 11, 2024
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rustbot commented Jul 11, 2024

Some changes occurred in src/librustdoc/clean/types.rs

cc @camelid

rustdoc-json-types is a public (although nightly-only) API. If possible, consider changing src/librustdoc/json/conversions.rs; otherwise, make sure you bump the FORMAT_VERSION constant.

cc @CraftSpider, @aDotInTheVoid, @Enselic, @obi1kenobi

@@ -22,6 +22,7 @@ ignore = [
"/tests/rustdoc-ui/", # Some have syntax errors, some are whitespace-sensitive.
"/tests/ui/", # Some have syntax errors, some are whitespace-sensitive.
"/tests/ui-fulldeps/", # Some are whitespace-sensitive (e.g. `// ~ERROR` comments).
"/tests/rustdoc-json/impl-trait-precise-capturing.rs",
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rustfmt on beta doesn't yet support + use<>.

@compiler-errors compiler-errors force-pushed the precise-capturing-rustdoc branch from db0136d to 8be6f1c Compare July 11, 2024 23:59
@compiler-errors compiler-errors added the F-precise_capturing `#![feature(precise_capturing)]` label Jul 12, 2024
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r=me after nits addressed

JSON looks good, too.

src/librustdoc/clean/mod.rs Outdated Show resolved Hide resolved
src/librustdoc/html/format.rs Outdated Show resolved Hide resolved
src/librustdoc/html/format.rs Outdated Show resolved Hide resolved
@compiler-errors compiler-errors force-pushed the precise-capturing-rustdoc branch from 8be6f1c to caf1457 Compare July 12, 2024 00:38
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@bors r=fmease

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bors commented Jul 12, 2024

📌 Commit caf1457 has been approved by fmease

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 12, 2024
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this pull request Jul 12, 2024
…rustdoc, r=fmease

Implement `precise_capturing` support for rustdoc

Implements rustdoc (+json) support for local (i.e. non-cross-crate-inlined) RPITs with `use<...>` precise capturing syntax.

Tests kinda suck. They're really hard to write 😰

r? `@fmease` or re-roll if you're too busy!
also cc `@aDotInTheVoid` for the json side

Tracking:
* rust-lang#127228 (comment) (not fully fixed for cross-crate-inlined opaques)
* rust-lang#123432
bors added a commit to rust-lang-ci/rust that referenced this pull request Jul 12, 2024
…iaskrgr

Rollup of 7 pull requests

Successful merges:

 - rust-lang#124980 (Generalize `fn allocator` for Rc/Arc.)
 - rust-lang#126639 (Add AMX target-features and `x86_amx_intrinsics` feature flag)
 - rust-lang#126827 (Use pidfd_spawn for faster process spawning when a PidFd is requested)
 - rust-lang#127397 (fix interleaved output in the default panic hook when multiple threads panic simultaneously)
 - rust-lang#127433 (Stabilize const_cstr_from_ptr (CStr::from_ptr, CStr::count_bytes))
 - rust-lang#127613 (Update dist-riscv64-linux to binutils 2.40)
 - rust-lang#127632 (Implement `precise_capturing` support for rustdoc)

r? `@ghost`
`@rustbot` modify labels: rollup
workingjubilee added a commit to workingjubilee/rustc that referenced this pull request Jul 12, 2024
…rustdoc, r=fmease

Implement `precise_capturing` support for rustdoc

Implements rustdoc (+json) support for local (i.e. non-cross-crate-inlined) RPITs with `use<...>` precise capturing syntax.

Tests kinda suck. They're really hard to write 😰

r? ``@fmease`` or re-roll if you're too busy!
also cc ``@aDotInTheVoid`` for the json side

Tracking:
* rust-lang#127228 (comment) (not fully fixed for cross-crate-inlined opaques)
* rust-lang#123432
src/librustdoc/html/format.rs Outdated Show resolved Hide resolved
@rustbot rustbot added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Jul 12, 2024
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fmease commented Jul 12, 2024

@bors r-

@bors bors removed the S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. label Jul 12, 2024
@compiler-errors compiler-errors force-pushed the precise-capturing-rustdoc branch from caf1457 to bd135e4 Compare July 12, 2024 09:43
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Sorry, fixed and rebased

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@bors r=fmease

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bors commented Jul 12, 2024

📌 Commit bd135e4 has been approved by fmease

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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jul 12, 2024
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this pull request Jul 12, 2024
…rustdoc, r=fmease

Implement `precise_capturing` support for rustdoc

Implements rustdoc (+json) support for local (i.e. non-cross-crate-inlined) RPITs with `use<...>` precise capturing syntax.

Tests kinda suck. They're really hard to write 😰

r? `@fmease` or re-roll if you're too busy!
also cc `@aDotInTheVoid` for the json side

Tracking:
* rust-lang#127228 (comment) (not fully fixed for cross-crate-inlined opaques)
* rust-lang#123432
@compiler-errors
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@bors rollup

bors added a commit to rust-lang-ci/rust that referenced this pull request Jul 12, 2024
…stdoc-cross, r=<try>

Add cross-crate precise capturing support to rustdoc

Follow-up to rust-lang#127632. Fixes rust-lang#127228.

r? `@fmease`

Tracking:
* rust-lang#123432
bors added a commit to rust-lang-ci/rust that referenced this pull request Jul 12, 2024
…stdoc-cross, r=<try>

Add cross-crate precise capturing support to rustdoc

Follow-up to rust-lang#127632. Fixes rust-lang#127228.

r? `@fmease`

Tracking:
* rust-lang#123432
bors added a commit to rust-lang-ci/rust that referenced this pull request Jul 12, 2024
…iaskrgr

Rollup of 8 pull requests

Successful merges:

 - rust-lang#126502 (Ignore allocation bytes in some mir-opt tests)
 - rust-lang#126606 (Guard against calling `libc::exit` multiple times on Linux.)
 - rust-lang#126922 (add lint for inline asm labels that look like binary)
 - rust-lang#127295 (CFI: Support provided methods on traits)
 - rust-lang#127310 (Fix import suggestion ice)
 - rust-lang#127535 (Fire unsafe_code lint on unsafe extern blocks)
 - rust-lang#127631 (Remove `fully_normalize`)
 - rust-lang#127632 (Implement `precise_capturing` support for rustdoc)

r? `@ghost`
`@rustbot` modify labels: rollup
bors added a commit to rust-lang-ci/rust that referenced this pull request Jul 12, 2024
…kingjubilee

Rollup of 11 pull requests

Successful merges:

 - rust-lang#126502 (Ignore allocation bytes in some mir-opt tests)
 - rust-lang#126922 (add lint for inline asm labels that look like binary)
 - rust-lang#127209 (Added the `xop` target-feature and the `xop_target_feature` feature gate)
 - rust-lang#127310 (Fix import suggestion ice)
 - rust-lang#127338 (Migrate `extra-filename-with-temp-outputs` and `issue-85019-moved-src-dir` `run-make` tests to rmake)
 - rust-lang#127381 (Migrate `issue-83045`, `rustc-macro-dep-files` and `env-dep-info` `run-make` tests to rmake)
 - rust-lang#127535 (Fire unsafe_code lint on unsafe extern blocks)
 - rust-lang#127619 (Suggest using precise capturing for hidden type that captures region)
 - rust-lang#127631 (Remove `fully_normalize`)
 - rust-lang#127632 (Implement `precise_capturing` support for rustdoc)
 - rust-lang#127660 (Rename the internal `const_strlen` to just `strlen`)

r? `@ghost`
`@rustbot` modify labels: rollup
@bors bors merged commit c0d9499 into rust-lang:master Jul 13, 2024
6 checks passed
@rustbot rustbot added this to the 1.81.0 milestone Jul 13, 2024
rust-timer added a commit to rust-lang-ci/rust that referenced this pull request Jul 13, 2024
Rollup merge of rust-lang#127632 - compiler-errors:precise-capturing-rustdoc, r=fmease

Implement `precise_capturing` support for rustdoc

Implements rustdoc (+json) support for local (i.e. non-cross-crate-inlined) RPITs with `use<...>` precise capturing syntax.

Tests kinda suck. They're really hard to write 😰

r? `@fmease` or re-roll if you're too busy!
also cc `@aDotInTheVoid` for the json side

Tracking:
* rust-lang#127228 (comment) (not fully fixed for cross-crate-inlined opaques)
* rust-lang#123432
bors added a commit to rust-lang-ci/rust that referenced this pull request Jul 20, 2024
…stdoc-cross, r=fmease

Add cross-crate precise capturing support to rustdoc

Follow-up to rust-lang#127632. Fixes rust-lang#127228.

r? `@fmease`

Tracking:
* rust-lang#123432
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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