bors 7fe6f36224 Auto merge of #103491 - cjgillot:self-rpit, r=oli-obk ... Support using `Self` or projections inside an RPIT/async fn I reuse the same idea as https://github.com/rust-lang/rust/pull/103449 to use variances to encode whether a lifetime parameter is captured by impl-trait. The current implementation of async and RPIT replace all lifetimes from the parent generics by `'static`. This PR changes the scheme ```rust impl<'a> Foo<'a> { fn foo<'b, T>() -> impl Into<Self> + 'b { ... } } opaque Foo::<'_a>::foo::<'_b, T>::opaque<'b>: Into<Foo<'_a>> + 'b; impl<'a> Foo<'a> { // OLD fn foo<'b, T>() -> Foo::<'static>::foo::<'static, T>::opaque::<'b> { ... } ^^^^^^^ the `Self` becomes `Foo<'static>` // NEW fn foo<'b, T>() -> Foo::<'a>::foo::<'b, T>::opaque::<'b> { ... } ^^ the `Self` stays `Foo<'a>` } ``` There is the same issue with projections. In the example, substitute `Self` by `<T as Trait<'b>>::Assoc` in the sugared version, and `Foo<'_a>` by `<T as Trait<'_b>>::Assoc` in the desugared one. This allows to support `Self` in impl-trait, since we do not replace lifetimes by `'static` any more. The same trick allows to use projections like `T::Assoc` where `Self` is allowed. The feature is gated behind a `impl_trait_projections` feature gate. The implementation relies on 2 tweaking rules for opaques in 2 places: - we only relate substs that correspond to captured lifetimes during TypeRelation; - we only list captured lifetimes in choice region computation. For simplicity, I encoded the "capturedness" of lifetimes as a variance, `Bivariant` vs `Invariant` for unused vs captured lifetimes. The `variances_of` query used to ICE for opaques. Impl-trait that do not reference `Self` or projections will have their variances as: - `o` (invariant) for each parent type or const; - `*` (bivariant) for each parent lifetime --> will not participate in borrowck; - `o` (invariant) for each own lifetime. Impl-trait that does reference `Self` and/or projections will have some parent lifetimes marked as `o` (as the example above), and participate in type relation and borrowck. In the example above, `variances_of(opaque) = ['_a: o, '_b: *, T: o, 'b: o]`. r? types cc `@compiler-errors` , as you asked about the issue with `Self` and projections.		2022-11-21 12:17:03 +00:00
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src	Auto merge of #103491 - cjgillot:self-rpit, r=oli-obk	2022-11-21 12:17:03 +00:00
Cargo.toml	Migrate to 2021	2021-09-20 22:21:42 -04:00