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Autotrait bounds on dyn-safe trait methods #107082
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see the PR description for details @rfcbot fcp merge |
Team member @lcnr has proposed to merge this. The next step is review by the rest of the tagged team members: No concerns currently listed. 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. |
🔔 This is now entering its final comment period, as per the review above. 🔔 |
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. |
@bors r+ |
Autotrait bounds on dyn-safe trait methods This PR is a successor to rust-lang#106604 implementing the approach encouraged by rust-lang#106604 (comment). **I propose making it legal to use autotraits as trait bounds on the `Self` type of trait methods in a trait object.** rust-lang#51443 (comment) justifies why this use case is particularly important in the context of the async-trait crate. ```rust #![feature(auto_traits)] #![deny(where_clauses_object_safety)] auto trait AutoTrait {} trait MyTrait { fn f(&self) where Self: AutoTrait; } fn main() { let _: &dyn MyTrait; } ``` Previously this would fail with: ```console error: the trait `MyTrait` cannot be made into an object --> src/main.rs:7:8 | 7 | fn f(&self) where Self: AutoTrait; | ^ | = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release! = note: for more information, see issue rust-lang#51443 <rust-lang#51443> note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety> --> src/main.rs:7:8 | 6 | trait MyTrait { | ------- this trait cannot be made into an object... 7 | fn f(&self) where Self: AutoTrait; | ^ ...because method `f` references the `Self` type in its `where` clause = help: consider moving `f` to another trait ``` In order for this to be sound without hitting rust-lang#50781, **I further propose that we disallow handwritten autotrait impls that apply to trait objects.** Both of the following were previously allowed (_on nightly_) and no longer allowed in my proposal: ```rust auto trait AutoTrait {} trait MyTrait {} impl AutoTrait for dyn MyTrait {} // NOT ALLOWED impl<T: ?Sized> AutoTrait for T {} // NOT ALLOWED ``` (`impl<T> AutoTrait for T {}` remains allowed.) After this change, traits with a default impl are implemented for a trait object **if and only if** the autotrait is one of the trait object's trait bounds (or a supertrait of a bound). In other words `dyn Trait + AutoTrait` always implements AutoTrait while `dyn Trait` never implements AutoTrait. Fixes dtolnay/async-trait#228. r? `@lcnr`
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@bors r=lcnr |
Rollup of 6 pull requests Successful merges: - rust-lang#107082 (Autotrait bounds on dyn-safe trait methods) - rust-lang#107427 (Add candidates for DiscriminantKind builtin) - rust-lang#107539 (Emit warnings on unused parens in index expressions) - rust-lang#107544 (Improve `TokenCursor`.) - rust-lang#107585 (Don't cause a cycle when formatting query description that references a FnDef) - rust-lang#107633 (Fix suggestion for coercing Option<&String> to Option<&str>) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup
Pkgsrc changes: * Adjust patches and cargo checksums to new versions. * Sadly, the patch to reduce the cargo verbosity no longer applies, so I've asked upstream about the proper way to get the old result. (so the build log becomes Quite Bloated for now). Upstream changes: Version 1.69.0 (2023-04-20) ========================== Language -------- - [Deriving built-in traits on packed structs works with `Copy` fields.] (rust-lang/rust#104429) - [Stabilize the `cmpxchg16b` target feature on x86 and x86_64.] (rust-lang/rust#106774) - [Improve analysis of trait bounds for associated types.] (rust-lang/rust#103695) - [Allow associated types to be used as union fields.] (rust-lang/rust#106938) - [Allow `Self: Autotrait` bounds on dyn-safe trait methods.] (rust-lang/rust#107082) - [Treat `str` as containing `[u8]` for auto trait purposes.] (rust-lang/rust#107941) Compiler -------- - [Upgrade `*-pc-windows-gnu` on CI to mingw-w64 v10 and GCC 12.2.] (rust-lang/rust#100178) - [Rework min_choice algorithm of member constraints.] (rust-lang/rust#105300) - [Support `true` and `false` as boolean flags in compiler arguments.] (rust-lang/rust#107043) - [Default `repr(C)` enums to `c_int` size.] (rust-lang/rust#107592) Libraries --------- - [Implement the unstable `DispatchFromDyn` for cell types, allowing downstream experimentation with custom method receivers.] (rust-lang/rust#97373) - [Document that `fmt::Arguments::as_str()` may return `Some(_)` in more cases after optimization, subject to change.] (rust-lang/rust#106823) - [Implement `AsFd` and `AsRawFd` for `Rc`.] (rust-lang/rust#107317) Stabilized APIs --------------- - [`CStr::from_bytes_until_nul`] (https://doc.rust-lang.org/stable/core/ffi/struct.CStr.html#method.from_bytes_until_nul) - [`core::ffi::FromBytesUntilNulError`] (https://doc.rust-lang.org/stable/core/ffi/struct.FromBytesUntilNulError.html) These APIs are now stable in const contexts: - [`SocketAddr::new`] (https://doc.rust-lang.org/stable/std/net/enum.SocketAddr.html#method.new) - [`SocketAddr::ip`] (https://doc.rust-lang.org/stable/std/net/enum.SocketAddr.html#method.ip) - [`SocketAddr::port`] (https://doc.rust-lang.org/stable/std/net/enum.SocketAddr.html#method.port) - [`SocketAddr::is_ipv4`] (https://doc.rust-lang.org/stable/std/net/enum.SocketAddr.html#method.is_ipv4) - [`SocketAddr::is_ipv6`] (https://doc.rust-lang.org/stable/std/net/enum.SocketAddr.html#method.is_ipv6) - [`SocketAddrV4::new`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV4.html#method.new) - [`SocketAddrV4::ip`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV4.html#method.ip) - [`SocketAddrV4::port`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV4.html#method.port) - [`SocketAddrV6::new`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV6.html#method.new) - [`SocketAddrV6::ip`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV6.html#method.ip) - [`SocketAddrV6::port`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV6.html#method.port) - [`SocketAddrV6::flowinfo`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV6.html#method.flowinfo) - [`SocketAddrV6::scope_id`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV6.html#method.scope_id) Cargo ----- - [Cargo now suggests `cargo fix` or `cargo clippy --fix` when compilation warnings are auto-fixable.] (rust-lang/cargo#11558) - [Cargo now suggests `cargo add` if you try to install a library crate.] (rust-lang/cargo#11410) - [Cargo now sets the `CARGO_BIN_NAME` environment variable also for binary examples.] (rust-lang/cargo#11705) Rustdoc ----- - [Vertically compact trait bound formatting.] (rust-lang/rust#102842) - [Only include stable lints in `rustdoc::all` group.] (rust-lang/rust#106316) - [Compute maximum Levenshtein distance based on the query.] (rust-lang/rust#107141) - [Remove inconsistently-present sidebar tooltips.] (rust-lang/rust#107490) - [Search by macro when query ends with `!`.] (rust-lang/rust#108143) Compatibility Notes ------------------- - [The `rust-analysis` component from `rustup` now only contains a warning placeholder.] (rust-lang/rust#101841) This was primarily intended for RLS, and the corresponding `-Zsave-analysis` flag has been removed from the compiler as well. - [Unaligned references to packed fields are now a hard error.] (rust-lang/rust#102513) This has been a warning since 1.53, and denied by default with a future-compatibility warning since 1.62. - [Update the minimum external LLVM to 14.] (rust-lang/rust#107573) - [Cargo now emits errors on invalid characters in a registry token.] (rust-lang/cargo#11600) - [When `default-features` is set to false of a workspace dependency, and an inherited dependency of a member has `default-features = true`, Cargo will enable default features of that dependency.] (rust-lang/cargo#11409) - [Cargo denies `CARGO_HOME` in the `[env]` configuration table. Cargo itself doesn't pick up this value, but recursive calls to cargo would, which was not intended.] (rust-lang/cargo#11644) - [Debuginfo for build dependencies is now off if not explicitly set. This is expected to improve the overall build time.] (rust-lang/cargo#11252) Internal Changes ---------------- These changes do not affect any public interfaces of Rust, but they represent significant improvements to the performance or internals of rustc and related tools. - [Move `format_args!()` into AST (and expand it during AST lowering)] (rust-lang/rust#106745)
Pkgsrc changes: * Adjust patches and cargo checksums to new versions. Upstream changes: Version 1.69.0 (2023-04-20) ========================== Language -------- - [Deriving built-in traits on packed structs works with `Copy` fields.] (rust-lang/rust#104429) - [Stabilize the `cmpxchg16b` target feature on x86 and x86_64.] (rust-lang/rust#106774) - [Improve analysis of trait bounds for associated types.] (rust-lang/rust#103695) - [Allow associated types to be used as union fields.] (rust-lang/rust#106938) - [Allow `Self: Autotrait` bounds on dyn-safe trait methods.] (rust-lang/rust#107082) - [Treat `str` as containing `[u8]` for auto trait purposes.] (rust-lang/rust#107941) Compiler -------- - [Upgrade `*-pc-windows-gnu` on CI to mingw-w64 v10 and GCC 12.2.] (rust-lang/rust#100178) - [Rework min_choice algorithm of member constraints.] (rust-lang/rust#105300) - [Support `true` and `false` as boolean flags in compiler arguments.] (rust-lang/rust#107043) - [Default `repr(C)` enums to `c_int` size.] (rust-lang/rust#107592) Libraries --------- - [Implement the unstable `DispatchFromDyn` for cell types, allowing downstream experimentation with custom method receivers.] (rust-lang/rust#97373) - [Document that `fmt::Arguments::as_str()` may return `Some(_)` in more cases after optimization, subject to change.] (rust-lang/rust#106823) - [Implement `AsFd` and `AsRawFd` for `Rc`.] (rust-lang/rust#107317) Stabilized APIs --------------- - [`CStr::from_bytes_until_nul`] (https://doc.rust-lang.org/stable/core/ffi/struct.CStr.html#method.from_bytes_until_nul) - [`core::ffi::FromBytesUntilNulError`] (https://doc.rust-lang.org/stable/core/ffi/struct.FromBytesUntilNulError.html) These APIs are now stable in const contexts: - [`SocketAddr::new`] (https://doc.rust-lang.org/stable/std/net/enum.SocketAddr.html#method.new) - [`SocketAddr::ip`] (https://doc.rust-lang.org/stable/std/net/enum.SocketAddr.html#method.ip) - [`SocketAddr::port`] (https://doc.rust-lang.org/stable/std/net/enum.SocketAddr.html#method.port) - [`SocketAddr::is_ipv4`] (https://doc.rust-lang.org/stable/std/net/enum.SocketAddr.html#method.is_ipv4) - [`SocketAddr::is_ipv6`] (https://doc.rust-lang.org/stable/std/net/enum.SocketAddr.html#method.is_ipv6) - [`SocketAddrV4::new`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV4.html#method.new) - [`SocketAddrV4::ip`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV4.html#method.ip) - [`SocketAddrV4::port`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV4.html#method.port) - [`SocketAddrV6::new`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV6.html#method.new) - [`SocketAddrV6::ip`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV6.html#method.ip) - [`SocketAddrV6::port`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV6.html#method.port) - [`SocketAddrV6::flowinfo`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV6.html#method.flowinfo) - [`SocketAddrV6::scope_id`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV6.html#method.scope_id) Cargo ----- - [Cargo now suggests `cargo fix` or `cargo clippy --fix` when compilation warnings are auto-fixable.] (rust-lang/cargo#11558) - [Cargo now suggests `cargo add` if you try to install a library crate.] (rust-lang/cargo#11410) - [Cargo now sets the `CARGO_BIN_NAME` environment variable also for binary examples.] (rust-lang/cargo#11705) Rustdoc ----- - [Vertically compact trait bound formatting.] (rust-lang/rust#102842) - [Only include stable lints in `rustdoc::all` group.] (rust-lang/rust#106316) - [Compute maximum Levenshtein distance based on the query.] (rust-lang/rust#107141) - [Remove inconsistently-present sidebar tooltips.] (rust-lang/rust#107490) - [Search by macro when query ends with `!`.] (rust-lang/rust#108143) Compatibility Notes ------------------- - [The `rust-analysis` component from `rustup` now only contains a warning placeholder.] (rust-lang/rust#101841) This was primarily intended for RLS, and the corresponding `-Zsave-analysis` flag has been removed from the compiler as well. - [Unaligned references to packed fields are now a hard error.] (rust-lang/rust#102513) This has been a warning since 1.53, and denied by default with a future-compatibility warning since 1.62. - [Update the minimum external LLVM to 14.] (rust-lang/rust#107573) - [Cargo now emits errors on invalid characters in a registry token.] (rust-lang/cargo#11600) - [When `default-features` is set to false of a workspace dependency, and an inherited dependency of a member has `default-features = true`, Cargo will enable default features of that dependency.] (rust-lang/cargo#11409) - [Cargo denies `CARGO_HOME` in the `[env]` configuration table. Cargo itself doesn't pick up this value, but recursive calls to cargo would, which was not intended.] (rust-lang/cargo#11644) - [Debuginfo for build dependencies is now off if not explicitly set. This is expected to improve the overall build time.] (rust-lang/cargo#11252) Internal Changes ---------------- These changes do not affect any public interfaces of Rust, but they represent significant improvements to the performance or internals of rustc and related tools. - [Move `format_args!()` into AST (and expand it during AST lowering)] (rust-lang/rust#106745)
This PR is a successor to #106604 implementing the approach encouraged by #106604 (comment).
I propose making it legal to use autotraits as trait bounds on the
Self
type of trait methods in a trait object. #51443 (comment) justifies why this use case is particularly important in the context of the async-trait crate.Previously this would fail with:
In order for this to be sound without hitting #50781, I further propose that we disallow handwritten autotrait impls that apply to trait objects. Both of the following were previously allowed (on nightly) and no longer allowed in my proposal:
(
impl<T> AutoTrait for T {}
remains allowed.)After this change, traits with a default impl are implemented for a trait object if and only if the autotrait is one of the trait object's trait bounds (or a supertrait of a bound). In other words
dyn Trait + AutoTrait
always implements AutoTrait whiledyn Trait
never implements AutoTrait.Fixes dtolnay/async-trait#228.
r? @lcnr