rustdoc: support for precise capturing #127228

fmease · 2024-07-01T23:39:13Z

Source:

#![feature(precise_capturing)]

pub fn demo(_: &str) -> impl Sized + use<> {}

Current rendered output (both HIR-based and middle-based) below (issue: dropping the precise capturing arg):

pub fn demo(_: &str) -> impl Sized

Tracking:

Tracking issue for precise_capturing syntax #123432

The text was updated successfully, but these errors were encountered:

fmease · 2024-07-01T23:43:00Z

I consider this issue to be partially blocked: Please don't try to add use<> support for inlined cross-crate reexports — more generally: middle-based cleaning — before I fix #113015 (which should happen very soon!).

Note that use<> doesn't get preserved post-HIR IINM, so it'd be non-trivial to impl anyway. To clarify, while it's possible to recover the list of captured regions from rustc_middle::ty data (via variance data: bivariant means not captured IIRC — maybe that's only true for TAITs not RPITs, haven't double-checked), it won't necessarily coincide with what the user wrote literally.

fmease · 2024-07-01T23:55:56Z

Furthermore, there's some design wiggle room which we should probably explore in a T-rustdoc meeting (I might add it to the agenda later).

Namely, with the edition differences around opaque type captures it might make sense for the rendered output to deviate from the actual source code as written by the user and to provide a 'normalized view' that's edition-agnostic (e.g., by always listing captures explicitly, etc. etc.).

compiler-errors · 2024-07-02T01:14:34Z

Note that use<> doesn't get preserved post-HIR IINM, so it'd be non-trivial to impl anyway. To clarify, while it's possible to recover the list of captured regions from rustc_middle::ty data (via variance data: bivariant means not captured IIRC — maybe that's only true for TAITs not RPITs, haven't double-checked), it won't necessarily coincide with what the user wrote literally.

I think we could/should probably just expose it as a query which is encoded in the crate.

…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

…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

…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

…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

…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

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

…=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

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

fmease added T-rustdoc Relevant to the rustdoc team, which will review and decide on the PR/issue. C-bug Category: This is a bug. F-precise_capturing `#![feature(precise_capturing)]` labels Jul 1, 2024

rustbot added the needs-triage This issue may need triage. Remove it if it has been sufficiently triaged. label Jul 1, 2024

fmease removed the needs-triage This issue may need triage. Remove it if it has been sufficiently triaged. label Jul 1, 2024

traviscross mentioned this issue Jul 3, 2024

Tracking issue for precise_capturing syntax #123432

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compiler-errors mentioned this issue Jul 12, 2024

Implement precise_capturing support for rustdoc #127632

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fmease assigned compiler-errors Jul 12, 2024

compiler-errors mentioned this issue Jul 12, 2024

Add cross-crate precise capturing support to rustdoc #127658

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traviscross mentioned this issue Jul 15, 2024

Stabilize opaque type precise capturing (RFC 3617) #127672

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bors closed this as completed in 73a2281 Jul 20, 2024

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rustdoc: support for precise capturing #127228

rustdoc: support for precise capturing #127228

fmease commented Jul 1, 2024 •

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compiler-errors commented Jul 2, 2024

rustdoc: support for precise capturing #127228

rustdoc: support for precise capturing #127228

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