Inherit lifetimes for async fn instead of duplicating them. #91403
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r? @nagisa (rust-highfive has picked a reviewer for you, use r? to override) |
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@bors try @rust-timer queue |
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Awaiting bors try build completion. @rustbot label: +S-waiting-on-perf |
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⌛ Trying commit 88af837 with merge 5dc1f10578886a598f5fc415475961d726194f80... |
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☀️ Try build successful - checks-actions |
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Queued 5dc1f10578886a598f5fc415475961d726194f80 with parent f04a2f4, future comparison URL. |
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Finished benchmarking commit (5dc1f10578886a598f5fc415475961d726194f80): comparison url. Summary: This change led to moderate relevant mixed results 🤷 in compiler performance.
If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf. Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR led to changes in compiler perf. Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @bors rollup=never |
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Having seconds thoughts now, can you maybe extract the cleanup commits that doesn't change functionality into a separate PR?. That part can easily be merged and then we can continue discussing this with @oli-obk and @nikomatsakis |
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Simplify collection of in-band lifetimes Split from rust-lang#91403 r? `@oli-obk`
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@bors r+ No matter what final design we come up with, as mentioned previously, it reduces complexity |
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📌 Commit 10cf626 has been approved by |
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☀️ Test successful - checks-actions |
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Finished benchmarking commit (3cfa4de): comparison url. Summary: This benchmark run shows 2 relevant regressions 😿 to instruction counts.
If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf. Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression |
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We should definitely talk this out -- I don't think I want to avoid lowering where clauses twice, so that's worth talking over. =) |
It looks to me like the regression identified by the rust-timer run from 4 days ago (i.e. when this landed) is exactly the same as the regression identified back when the PR was first filed. But, I do not see any discussion of that performance regression. 😞 Part of the reason to do the timer runs is to encourage PR authors and reviewers to acknowledge the fallout of the changes they are making, and to answer questions like:
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Revert rust-lang#91403 fixes rust-lang#94004 r? `@pnkfelix` `@cjgillot`
Revert rust-lang#91403 fixes rust-lang#94004 r? `@pnkfelix` `@cjgillot`
Revert rust-lang#91403 fixes rust-lang#94004 r? `@pnkfelix` `@cjgillot`
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Status: Reverted in #94088 |
Stabilize `impl_trait_projections` Closes rust-lang#115659 ## TL;DR: This allows us to mention `Self` and `T::Assoc` in async fn and return-position `impl Trait`, as you would expect you'd be able to. Some examples: ```rust #![feature(return_position_impl_trait_in_trait, async_fn_in_trait)] // (just needed for final tests below) // ---------------------------------------- // struct Wrapper<'a, T>(&'a T); impl Wrapper<'_, ()> { async fn async_fn() -> Self { //^ Previously rejected because it returns `-> Self`, not `-> Wrapper<'_, ()>`. Wrapper(&()) } fn impl_trait() -> impl Iterator<Item = Self> { //^ Previously rejected because it mentions `Self`, not `Wrapper<'_, ()>`. std::iter::once(Wrapper(&())) } } // ---------------------------------------- // trait Trait<'a> { type Assoc; fn new() -> Self::Assoc; } impl Trait<'_> for () { type Assoc = (); fn new() {} } impl<'a, T: Trait<'a>> Wrapper<'a, T> { async fn mk_assoc() -> T::Assoc { //^ Previously rejected because `T::Assoc` doesn't mention `'a` in the HIR, // but ends up resolving to `<T as Trait<'a>>::Assoc`, which does rely on `'a`. // That's the important part -- the elided trait. T::new() } fn a_few_assocs() -> impl Iterator<Item = T::Assoc> { //^ Previously rejected for the same reason [T::new(), T::new(), T::new()].into_iter() } } // ---------------------------------------- // trait InTrait { async fn async_fn() -> Self; fn impl_trait() -> impl Iterator<Item = Self>; } impl InTrait for &() { async fn async_fn() -> Self { &() } //^ Previously rejected just like inherent impls fn impl_trait() -> impl Iterator<Item = Self> { //^ Previously rejected just like inherent impls [&()].into_iter() } } ``` ## Technical: Lifetimes in return-position `impl Trait` (and `async fn`) are duplicated as early-bound generics local to the opaque in order to make sure we are able to substitute any late-bound lifetimes from the function in the opaque's hidden type. (The [dev guide](https://rustc-dev-guide.rust-lang.org/return-position-impl-trait-in-trait.html#aside-opaque-lifetime-duplication) has a small section about why this is necessary -- this was written for RPITITs, but it applies to all RPITs) Prior to rust-lang#103491, all of the early-bound lifetimes not local to the opaque were replaced with `'static` to avoid issues where relating opaques caused their *non-captured* lifetimes to be related. This `'static` replacement led to strange and possibly unsound behaviors (rust-lang#61949 (comment)) (rust-lang#53613) when referencing the `Self` type alias in an impl or indirectly referencing a lifetime parameter via a projection type (via a `T::Assoc` projection without an explicit trait), since lifetime resolution is performed on the HIR, when neither `T::Assoc`-style projections or `Self` in impls are expanded. Therefore an error was implemented in rust-lang#62849 to deny this subtle behavior as a known limitation of the compiler. It was attempted by `@cjgillot` to fix this in rust-lang#91403, which was subsequently unlanded. Then it was re-attempted to much success (🎉) in rust-lang#103491, which is where we currently are in the compiler. The PR above (rust-lang#103491) fixed this issue technically by *not* replacing the opaque's parent lifetimes with `'static`, but instead using variance to properly track which lifetimes are captured and are not. The PR gated any of the "side-effects" of the PR behind a feature gate (`impl_trait_projections`) presumably to avoid having to involve T-lang or T-types in the PR as well. `@cjgillot` can clarify this if I'm misunderstanding what their intention was with the feature gate. Since we're not replacing (possibly *invariant*!) lifetimes with `'static` anymore, there are no more soundness concerns here. Therefore, this PR removes the feature gate. Tests: * `tests/ui/async-await/feature-self-return-type.rs` * `tests/ui/impl-trait/feature-self-return-type.rs` * `tests/ui/async-await/issues/issue-78600.rs` * `tests/ui/impl-trait/capture-lifetime-not-in-hir.rs` --- r? cjgillot on the impl (not much, just removing the feature gate) I'm gonna mark this as FCP for T-lang and T-types.
Stabilize `impl_trait_projections` Closes rust-lang#115659 ## TL;DR: This allows us to mention `Self` and `T::Assoc` in async fn and return-position `impl Trait`, as you would expect you'd be able to. Some examples: ```rust #![feature(return_position_impl_trait_in_trait, async_fn_in_trait)] // (just needed for final tests below) // ---------------------------------------- // struct Wrapper<'a, T>(&'a T); impl Wrapper<'_, ()> { async fn async_fn() -> Self { //^ Previously rejected because it returns `-> Self`, not `-> Wrapper<'_, ()>`. Wrapper(&()) } fn impl_trait() -> impl Iterator<Item = Self> { //^ Previously rejected because it mentions `Self`, not `Wrapper<'_, ()>`. std::iter::once(Wrapper(&())) } } // ---------------------------------------- // trait Trait<'a> { type Assoc; fn new() -> Self::Assoc; } impl Trait<'_> for () { type Assoc = (); fn new() {} } impl<'a, T: Trait<'a>> Wrapper<'a, T> { async fn mk_assoc() -> T::Assoc { //^ Previously rejected because `T::Assoc` doesn't mention `'a` in the HIR, // but ends up resolving to `<T as Trait<'a>>::Assoc`, which does rely on `'a`. // That's the important part -- the elided trait. T::new() } fn a_few_assocs() -> impl Iterator<Item = T::Assoc> { //^ Previously rejected for the same reason [T::new(), T::new(), T::new()].into_iter() } } // ---------------------------------------- // trait InTrait { async fn async_fn() -> Self; fn impl_trait() -> impl Iterator<Item = Self>; } impl InTrait for &() { async fn async_fn() -> Self { &() } //^ Previously rejected just like inherent impls fn impl_trait() -> impl Iterator<Item = Self> { //^ Previously rejected just like inherent impls [&()].into_iter() } } ``` ## Technical: Lifetimes in return-position `impl Trait` (and `async fn`) are duplicated as early-bound generics local to the opaque in order to make sure we are able to substitute any late-bound lifetimes from the function in the opaque's hidden type. (The [dev guide](https://rustc-dev-guide.rust-lang.org/return-position-impl-trait-in-trait.html#aside-opaque-lifetime-duplication) has a small section about why this is necessary -- this was written for RPITITs, but it applies to all RPITs) Prior to rust-lang#103491, all of the early-bound lifetimes not local to the opaque were replaced with `'static` to avoid issues where relating opaques caused their *non-captured* lifetimes to be related. This `'static` replacement led to strange and possibly unsound behaviors (rust-lang#61949 (comment)) (rust-lang#53613) when referencing the `Self` type alias in an impl or indirectly referencing a lifetime parameter via a projection type (via a `T::Assoc` projection without an explicit trait), since lifetime resolution is performed on the HIR, when neither `T::Assoc`-style projections or `Self` in impls are expanded. Therefore an error was implemented in rust-lang#62849 to deny this subtle behavior as a known limitation of the compiler. It was attempted by `@cjgillot` to fix this in rust-lang#91403, which was subsequently unlanded. Then it was re-attempted to much success (🎉) in rust-lang#103491, which is where we currently are in the compiler. The PR above (rust-lang#103491) fixed this issue technically by *not* replacing the opaque's parent lifetimes with `'static`, but instead using variance to properly track which lifetimes are captured and are not. The PR gated any of the "side-effects" of the PR behind a feature gate (`impl_trait_projections`) presumably to avoid having to involve T-lang or T-types in the PR as well. `@cjgillot` can clarify this if I'm misunderstanding what their intention was with the feature gate. Since we're not replacing (possibly *invariant*!) lifetimes with `'static` anymore, there are no more soundness concerns here. Therefore, this PR removes the feature gate. Tests: * `tests/ui/async-await/feature-self-return-type.rs` * `tests/ui/impl-trait/feature-self-return-type.rs` * `tests/ui/async-await/issues/issue-78600.rs` * `tests/ui/impl-trait/capture-lifetime-not-in-hir.rs` --- r? cjgillot on the impl (not much, just removing the feature gate) I'm gonna mark this as FCP for T-lang and T-types.
The current desugaring of
async fn foo<'a>(&usize) -> &u8is equivalent tofollowing the RPIT model.
Duplicating all the inherited lifetime parameters and setting the inherited version to
'staticmakes lowering more complex and causes issues like #61949. This PR removes the duplication of inherited lifetimes to directly usefollowing the TAIT model.
Fixes #61949