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Auto merge of #103491 - cjgillot:self-rpit, r=oli-obk

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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.
This commit is contained in:
bors 2022-11-21 12:17:03 +00:00
commit 7fe6f36224
33 changed files with 570 additions and 341 deletions
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18
compiler/rustc_ast_lowering/src/lib.rs
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@ -60,8 +60,8 @@ use rustc_hir::def_id::{LocalDefId, CRATE_DEF_ID};
use rustc_hir::definitions::DefPathData;
use rustc_hir::{ConstArg, GenericArg, ItemLocalId, ParamName, TraitCandidate};
use rustc_index::vec::{Idx, IndexVec};
use rustc_middle::span_bug;
use rustc_middle::ty::{ResolverAstLowering, TyCtxt};
use rustc_middle::{bug, span_bug};
use rustc_session::parse::feature_err;
use rustc_span::hygiene::MacroKind;
use rustc_span::source_map::DesugaringKind;
@ -1465,17 +1465,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
// frequently opened issues show.
let opaque_ty_span = self.mark_span_with_reason(DesugaringKind::OpaqueTy, span, None);
let opaque_ty_def_id = match origin {
hir::OpaqueTyOrigin::TyAlias => self.create_def(
self.current_hir_id_owner.def_id,
opaque_ty_node_id,
DefPathData::ImplTrait,
),
hir::OpaqueTyOrigin::FnReturn(fn_def_id) => {
self.create_def(fn_def_id, opaque_ty_node_id, DefPathData::ImplTrait)
}
hir::OpaqueTyOrigin::AsyncFn(..) => bug!("unreachable"),
};
let opaque_ty_def_id = self.create_def(
self.current_hir_id_owner.def_id,
opaque_ty_node_id,
DefPathData::ImplTrait,
);
debug!(?opaque_ty_def_id);
// Contains the new lifetime definitions created for the TAIT (if any).