stabilize member constraints #84701

nikomatsakis · 2021-04-29T15:26:38Z

Stabilizes the use of "member constraints" in solving impl Trait bindings. This is a step towards stabilizing a "MVP" of "named impl Trait".

Member constraint stabilization report

Info
Tracking issue	rust-lang/rust#61997
Implementation history	rust-lang/rust#61775
rustc-dev-guide coverage	link
Complications	rust-lang/rust#61773

Background

Member constraints are an extension to our region solver that was introduced to make async fn region solving tractable. There are used in situations like the following:

fn foo<'a, 'b>(...) -> impl Trait<'a, 'b> { .. }

The problem here is that every region R in the hidden type must be equal to either 'a or 'b (or 'static). This cannot be expressed simply via 'outlives constriants' like R: 'a. Therefore, we introduce a 'member constraint' R member of ['a, 'b].

These constraints were introduced in rust-lang/rust#61775. At the time, we kept them feature gated and used them only for impl Trait return types that are derived from async fn. The intention, however, was always to support them in other contexts once we had time to gain more experience with them.

In the time since their introduction, we have encountered no surprises or bugs due to these member constraints. They are tested extensively as part of every async function that involves multiple unrelated lifetimes in its arguments.

Tests

The behavior of member constraints is covered by the following tests:

src/test/ui/async-await/multiple-lifetimes -- tests using the async await, which are mostly already stabilized
src/test/ui/impl-trait/multiple-lifetimes.rs
src/test/ui/impl-trait/multiple-lifetimes/ordinary-bounds-unsuited.rs
src/test/ui/async-await/multiple-lifetimes/ret-impl-trait-fg.rs
src/test/ui/async-await/multiple-lifetimes/ret-impl-trait-one.rs

These tests cover a number of scenarios:

-> implTrait<'a, 'b> with unrelated lifetimes 'a and 'b, as described above
async fn that returns an impl Trait like the previous case, which desugars to a kind of "nested" impl trait like impl Future<Output = impl Trait<'a, 'b>>

Potential concerns

There is a potential interaction with impl Trait on local variables, described in rust-lang/rust#61773. The challenge is that if you have a program like:

trait Foo<'_> { }
impl Foo<'_> for &u32 { }

fn bar() {
  let x: impl Foo<'_> = &44; // let's call the region variable for `'_` `'1`
}

then we would wind up with '0 member of ['1, 'static], where '0 is the region variable in the hidden type (&'0 u32) and '1 is the region variable in the bounds Foo<'1>. This is tricky because both '0 and '1 are being inferred -- so making them equal may have other repercussions.

That said, impl Trait in bindings are not stable, and the implementation is pretty far from stabilization. Moreover, the difficulty highlighted here is not due to the presence of member constraints -- it's inherent to the design of the language. In other words, stabilizing member constraints does not actually cause us to accept anything that would make this problem any harder.

So I don't see this as a blocker to stabilization of member constraints; it is potentially a blocker to stablization of impl trait in let bindings.

rust-highfive · 2021-04-29T15:26:42Z

r? @varkor

(rust-highfive has picked a reviewer for you, use r? to override)

nikomatsakis · 2021-04-29T15:27:49Z

I've marked this for @rust-lang/lang, but I'd like to cc @rust-lang/compiler, @rust-lang/wg-traits, and especially @matthewjasper for comment.

jackh726

Small typo.

But overall, this seems extremely straightforward to me. From a language design standpoint, I don't see why the lifetimes in an impl Trait should have to relate to each other (we don't have that requirement for structs, for example). From a implementation standpoint, the fact that this has been effectively stable in async fns provides good evidence the correct implementation is as correct as it can be and that this is worth stabilizing more generally.

compiler/rustc_feature/src/accepted.rs

src/doc/unstable-book/src/language-features/member-constraints.md

nikomatsakis · 2021-05-06T14:47:44Z

@rfcbot fcp merge

As discussed in the lang-team meeting.

rfcbot · 2021-05-06T14:47:45Z

Team member @nikomatsakis 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.

rfcbot · 2021-05-11T18:05:49Z

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

jackh726

Stabilization PR here is pretty simple (as it should be). One comment, but r=me on this when FCP finishes

compiler/rustc_feature/src/accepted.rs

bors · 2021-05-19T07:47:46Z

☔ The latest upstream changes (presumably #85458) made this pull request unmergeable. Please resolve the merge conflicts.

rfcbot · 2021-05-21T18:06:42Z

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.

The RFC will be merged soon.

nikomatsakis · 2021-05-24T17:32:40Z

@bors r=jackh726

bors · 2021-05-24T17:32:42Z

📌 Commit a937f25 has been approved by jackh726

…raints-61997, r=jackh726 stabilize member constraints Stabilizes the use of "member constraints" in solving `impl Trait` bindings. This is a step towards stabilizing a "MVP" of "named impl Trait". # Member constraint stabilization report | Info | | | --- | --- | | Tracking issue | [rust-lang#61997](rust-lang#61997) | | Implementation history | [rust-lang#61775] | | rustc-dev-guide coverage | [link](https://rustc-dev-guide.rust-lang.org/borrow_check/region_inference/member_constraints.html) | | Complications | [rust-lang#61773] | [rust-lang#61775]: rust-lang#61775 [rust-lang#61773]: rust-lang#61773 ## Background Member constraints are an extension to our region solver that was introduced to make async fn region solving tractable. There are used in situations like the following: ```rust fn foo<'a, 'b>(...) -> impl Trait<'a, 'b> { .. } ``` The problem here is that every region R in the hidden type must be equal to *either* `'a` *or* `'b` (or `'static`). This cannot be expressed simply via 'outlives constriants' like `R: 'a`. Therefore, we introduce a 'member constraint' `R member of ['a, 'b]`. These constraints were introduced in [rust-lang#61775]. At the time, we kept them feature gated and used them only for `impl Trait` return types that are derived from `async fn`. The intention, however, was always to support them in other contexts once we had time to gain more experience with them. **In the time since their introduction, we have encountered no surprises or bugs due to these member constraints.** They are tested extensively as part of every async function that involves multiple unrelated lifetimes in its arguments. ## Tests The behavior of member constraints is covered by the following tests: * [`src/test/ui/async-await/multiple-lifetimes`](https://github.com/rust-lang/rust/tree/20e032e65007ff1376e8480c1fbdb0a5068028fa/src/test/ui/async-await/multiple-lifetimes) -- tests using the async await, which are mostly already stabilized * [`src/test/ui/impl-trait/multiple-lifetimes.rs`](https://github.com/rust-lang/rust/blob/20e032e65007ff1376e8480c1fbdb0a5068028fa/src/test/ui/impl-trait/multiple-lifetimes.rs) * [`src/test/ui/impl-trait/multiple-lifetimes/ordinary-bounds-unsuited.rs`](https://github.com/rust-lang/rust/blob/20e032e65007ff1376e8480c1fbdb0a5068028fa/src/test/ui/impl-trait/multiple-lifetimes/ordinary-bounds-unsuited.rs) * [`src/test/ui/async-await/multiple-lifetimes/ret-impl-trait-fg.rs`](https://github.com/rust-lang/rust/blob/20e032e65007ff1376e8480c1fbdb0a5068028fa/src/test/ui/async-await/multiple-lifetimes/ret-impl-trait-fg.rs) * [`src/test/ui/async-await/multiple-lifetimes/ret-impl-trait-one.rs`](https://github.com/rust-lang/rust/blob/20e032e65007ff1376e8480c1fbdb0a5068028fa/src/test/ui/async-await/multiple-lifetimes/ret-impl-trait-one.rs) These tests cover a number of scenarios: * `-> implTrait<'a, 'b>` with unrelated lifetimes `'a` and `'b`, as described above * `async fn` that returns an `impl Trait` like the previous case, which desugars to a kind of "nested" impl trait like `impl Future<Output = impl Trait<'a, 'b>>` ## Potential concerns There is a potential interaction with `impl Trait` on local variables, described in [rust-lang#61773]. The challenge is that if you have a program like: ```rust= trait Foo<'_> { } impl Foo<'_> for &u32 { } fn bar() { let x: impl Foo<'_> = &44; // let's call the region variable for `'_` `'1` } ``` then we would wind up with `'0 member of ['1, 'static]`, where `'0` is the region variable in the hidden type (`&'0 u32`) and `'1` is the region variable in the bounds `Foo<'1>`. This is tricky because both `'0` and `'1` are being inferred -- so making them equal may have other repercussions. That said, `impl Trait` in bindings are not stable, and the implementation is pretty far from stabilization. Moreover, the difficulty highlighted here is not due to the presence of member constraints -- it's inherent to the design of the language. In other words, stabilizing member constraints does not actually cause us to accept anything that would make this problem any harder. So I don't see this as a blocker to stabilization of member constraints; it is potentially a blocker to stablization of `impl trait` in let bindings.

bors · 2021-05-25T19:41:48Z

⌛ Testing commit a937f25 with merge 6ea940b75105c23bcd7049a64b6a2cfe8e9cf8aa...

bors · 2021-05-25T20:07:44Z

💔 Test failed - checks-actions

lqd · 2021-05-25T20:14:32Z

(the compare-mode=nll tests need blessing)

nikomatsakis · 2021-05-26T16:08:22Z

@bors r=jackh726

bors · 2021-05-26T16:08:23Z

📌 Commit b3054d2 has been approved by jackh726

…raints-61997, r=jackh726 stabilize member constraints Stabilizes the use of "member constraints" in solving `impl Trait` bindings. This is a step towards stabilizing a "MVP" of "named impl Trait". # Member constraint stabilization report | Info | | | --- | --- | | Tracking issue | [rust-lang#61997](rust-lang#61997) | | Implementation history | [rust-lang#61775] | | rustc-dev-guide coverage | [link](https://rustc-dev-guide.rust-lang.org/borrow_check/region_inference/member_constraints.html) | | Complications | [rust-lang#61773] | [rust-lang#61775]: rust-lang#61775 [rust-lang#61773]: rust-lang#61773 ## Background Member constraints are an extension to our region solver that was introduced to make async fn region solving tractable. There are used in situations like the following: ```rust fn foo<'a, 'b>(...) -> impl Trait<'a, 'b> { .. } ``` The problem here is that every region R in the hidden type must be equal to *either* `'a` *or* `'b` (or `'static`). This cannot be expressed simply via 'outlives constriants' like `R: 'a`. Therefore, we introduce a 'member constraint' `R member of ['a, 'b]`. These constraints were introduced in [rust-lang#61775]. At the time, we kept them feature gated and used them only for `impl Trait` return types that are derived from `async fn`. The intention, however, was always to support them in other contexts once we had time to gain more experience with them. **In the time since their introduction, we have encountered no surprises or bugs due to these member constraints.** They are tested extensively as part of every async function that involves multiple unrelated lifetimes in its arguments. ## Tests The behavior of member constraints is covered by the following tests: * [`src/test/ui/async-await/multiple-lifetimes`](https://github.com/rust-lang/rust/tree/20e032e65007ff1376e8480c1fbdb0a5068028fa/src/test/ui/async-await/multiple-lifetimes) -- tests using the async await, which are mostly already stabilized * [`src/test/ui/impl-trait/multiple-lifetimes.rs`](https://github.com/rust-lang/rust/blob/20e032e65007ff1376e8480c1fbdb0a5068028fa/src/test/ui/impl-trait/multiple-lifetimes.rs) * [`src/test/ui/impl-trait/multiple-lifetimes/ordinary-bounds-unsuited.rs`](https://github.com/rust-lang/rust/blob/20e032e65007ff1376e8480c1fbdb0a5068028fa/src/test/ui/impl-trait/multiple-lifetimes/ordinary-bounds-unsuited.rs) * [`src/test/ui/async-await/multiple-lifetimes/ret-impl-trait-fg.rs`](https://github.com/rust-lang/rust/blob/20e032e65007ff1376e8480c1fbdb0a5068028fa/src/test/ui/async-await/multiple-lifetimes/ret-impl-trait-fg.rs) * [`src/test/ui/async-await/multiple-lifetimes/ret-impl-trait-one.rs`](https://github.com/rust-lang/rust/blob/20e032e65007ff1376e8480c1fbdb0a5068028fa/src/test/ui/async-await/multiple-lifetimes/ret-impl-trait-one.rs) These tests cover a number of scenarios: * `-> implTrait<'a, 'b>` with unrelated lifetimes `'a` and `'b`, as described above * `async fn` that returns an `impl Trait` like the previous case, which desugars to a kind of "nested" impl trait like `impl Future<Output = impl Trait<'a, 'b>>` ## Potential concerns There is a potential interaction with `impl Trait` on local variables, described in [rust-lang#61773]. The challenge is that if you have a program like: ```rust= trait Foo<'_> { } impl Foo<'_> for &u32 { } fn bar() { let x: impl Foo<'_> = &44; // let's call the region variable for `'_` `'1` } ``` then we would wind up with `'0 member of ['1, 'static]`, where `'0` is the region variable in the hidden type (`&'0 u32`) and `'1` is the region variable in the bounds `Foo<'1>`. This is tricky because both `'0` and `'1` are being inferred -- so making them equal may have other repercussions. That said, `impl Trait` in bindings are not stable, and the implementation is pretty far from stabilization. Moreover, the difficulty highlighted here is not due to the presence of member constraints -- it's inherent to the design of the language. In other words, stabilizing member constraints does not actually cause us to accept anything that would make this problem any harder. So I don't see this as a blocker to stabilization of member constraints; it is potentially a blocker to stablization of `impl trait` in let bindings.

Rollup of 8 pull requests Successful merges: - rust-lang#84221 (E0599 suggestions and elision of generic argument if no canditate is found) - rust-lang#84701 (stabilize member constraints) - rust-lang#85564 ( readd capture disjoint fields gate) - rust-lang#85583 (Get rid of PreviousDepGraph.) - rust-lang#85649 (Update cc) - rust-lang#85689 (Remove Iterator #[rustc_on_unimplemented]s that no longer apply.) - rust-lang#85719 (Add inline attr to CString::into_inner so it can optimize out NonNull checks) - rust-lang#85725 (Remove unneeded workaround) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup

Pkgsrc changes: * Bump bootstrap requirements to 1.53.0. * Adjust patches, adapt to upstream changes, adjust cargo checksums * If using an external llvm, require >= 10.0 Upsteream changes: Version 1.54.0 (2021-07-29) ============================ Language ----------------------- - [You can now use macros for values in built-in attribute macros.][83366] While a seemingly minor addition on its own, this enables a lot of powerful functionality when combined correctly. Most notably you can now include external documentation in your crate by writing the following. ```rust #![doc = include_str!("README.md")] ``` You can also use this to include auto-generated modules: ```rust #[path = concat!(env!("OUT_DIR"), "/generated.rs")] mod generated; ``` - [You can now cast between unsized slice types (and types which contain unsized slices) in `const fn`.][85078] - [You can now use multiple generic lifetimes with `impl Trait` where the lifetimes don't explicitly outlive another.][84701] In code this means that you can now have `impl Trait<'a, 'b>` where as before you could only have `impl Trait<'a, 'b> where 'b: 'a`. Compiler ----------------------- - [Rustc will now search for custom JSON targets in `/lib/rustlib/<target-triple>/target.json` where `/` is the "sysroot" directory.][83800] You can find your sysroot directory by running `rustc --print sysroot`. - [Added `wasm` as a `target_family` for WebAssembly platforms.][84072] - [You can now use `#[target_feature]` on safe functions when targeting WebAssembly platforms.][84988] - [Improved debugger output for enums on Windows MSVC platforms.][85292] - [Added tier 3\* support for `bpfel-unknown-none` and `bpfeb-unknown-none`.][79608] \* Refer to Rust's [platform support page][platform-support-doc] for more information on Rust's tiered platform support. Libraries ----------------------- - [`panic::panic_any` will now `#[track_caller]`.][85745] - [Added `OutOfMemory` as a variant of `io::ErrorKind`.][84744] - [ `proc_macro::Literal` now implements `FromStr`.][84717] - [The implementations of vendor intrinsics in core::arch have been significantly refactored.][83278] The main user-visible changes are a 50% reduction in the size of libcore.rlib and stricter validation of constant operands passed to intrinsics. The latter is technically a breaking change, but allows Rust to more closely match the C vendor intrinsics API. Stabilized APIs --------------- - [`BTreeMap::into_keys`] - [`BTreeMap::into_values`] - [`HashMap::into_keys`] - [`HashMap::into_values`] - [`arch::wasm32`] - [`VecDeque::binary_search`] - [`VecDeque::binary_search_by`] - [`VecDeque::binary_search_by_key`] - [`VecDeque::partition_point`] Cargo ----- - [Added the `--prune <spec>` option to `cargo-tree` to remove a package from the dependency graph.][cargo/9520] - [Added the `--depth` option to `cargo-tree` to print only to a certain depth in the tree ][cargo/9499] - [Added the `no-proc-macro` value to `cargo-tree --edges` to hide procedural macro dependencies.][cargo/9488] - [A new environment variable named `CARGO_TARGET_TMPDIR` is available.][cargo/9375] This variable points to a directory that integration tests and benches can use as a "scratchpad" for testing filesystem operations. Compatibility Notes ------------------- - [Mixing Option and Result via `?` is no longer permitted in closures for inferred types.][86831] - [Previously unsound code is no longer permitted where different constructors in branches could require different lifetimes.][85574] - As previously mentioned the [`std::arch` instrinsics now uses stricter const checking][83278] than before and may reject some previously accepted code. - [`i128` multiplication on Cortex M0+ platforms currently unconditionally causes overflow when compiled with `codegen-units = 1`.][86063] [85574]: rust-lang/rust#85574 [86831]: rust-lang/rust#86831 [86063]: rust-lang/rust#86063 [86831]: rust-lang/rust#86831 [79608]: rust-lang/rust#79608 [84988]: rust-lang/rust#84988 [84701]: rust-lang/rust#84701 [84072]: rust-lang/rust#84072 [85745]: rust-lang/rust#85745 [84744]: rust-lang/rust#84744 [85078]: rust-lang/rust#85078 [84717]: rust-lang/rust#84717 [83800]: rust-lang/rust#83800 [83366]: rust-lang/rust#83366 [83278]: rust-lang/rust#83278 [85292]: rust-lang/rust#85292 [cargo/9520]: rust-lang/cargo#9520 [cargo/9499]: rust-lang/cargo#9499 [cargo/9488]: rust-lang/cargo#9488 [cargo/9375]: rust-lang/cargo#9375 [`BTreeMap::into_keys`]: https://doc.rust-lang.org/std/collections/struct.BTreeMap.html#method.into_keys [`BTreeMap::into_values`]: https://doc.rust-lang.org/std/collections/struct.BTreeMap.html#method.into_values [`HashMap::into_keys`]: https://doc.rust-lang.org/std/collections/struct.HashMap.html#method.into_keys [`HashMap::into_values`]: https://doc.rust-lang.org/std/collections/struct.HashMap.html#method.into_values [`arch::wasm32`]: https://doc.rust-lang.org/core/arch/wasm32/index.html [`VecDeque::binary_search`]: https://doc.rust-lang.org/std/collections/struct.VecDeque.html#method.binary_search [`VecDeque::binary_search_by`]: https://doc.rust-lang.org/std/collections/struct.VecDeque.html#method.binary_search_by [`VecDeque::binary_search_by_key`]: https://doc.rust-lang.org/std/collections/struct.VecDeque.html#method.binary_search_by_key [`VecDeque::partition_point`]: https://doc.rust-lang.org/std/collections/struct.VecDeque.html#method.partition_point

rust-highfive assigned varkor Apr 29, 2021

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stabilize member constraints

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nikomatsakis force-pushed the stabilize-member-constraints-61997 branch from be4ae9d to 128d385 Compare May 26, 2021 10:01

bless compare-mode=nll output

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Rollup of 9 pull requests #85727

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Tracking issue for member constraints in region inference #61997

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nikomatsakis mentioned this pull request Jul 13, 2021

Loop over all opaque types instead of looking at just the first one with the same DefId #87107

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Implement "intersection lifetimes" to improve diagnostics and accept rare impl trait cases #89394

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