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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
parent 736c675d2a b37feca804
commit 7fe6f36224
33 changed files with 570 additions and 341 deletions
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63
compiler/rustc_middle/src/ty/mod.rs
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@@ -1257,7 +1257,7 @@ impl<'tcx> InstantiatedPredicates<'tcx> {
}
}
#[derive(Copy, Clone, Debug, PartialEq, Eq, HashStable, TyEncodable, TyDecodable, Lift)]
#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, HashStable, TyEncodable, TyDecodable, Lift)]
#[derive(TypeFoldable, TypeVisitable)]
pub struct OpaqueTypeKey<'tcx> {
pub def_id: LocalDefId,
@@ -1332,6 +1332,9 @@ impl<'tcx> OpaqueHiddenType<'tcx> {
let id_substs = InternalSubsts::identity_for_item(tcx, def_id.to_def_id());
debug!(?id_substs);
// This zip may have several times the same lifetime in `substs` paired with a different
// lifetime from `id_substs`. Simply `collect`ing the iterator is the correct behaviour:
// it will pick the last one, which is the one we introduced in the impl-trait desugaring.
let map = substs.iter().zip(id_substs);
let map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>> = match origin {
@@ -1345,61 +1348,13 @@ impl<'tcx> OpaqueHiddenType<'tcx> {
// type Foo<'a, 'b, 'c> = impl Trait<'a> + 'b;
// ```
// we may not use `'c` in the hidden type.
struct OpaqueTypeLifetimeCollector<'tcx> {
lifetimes: FxHashSet<ty::Region<'tcx>>,
}
let variances = tcx.variances_of(def_id);
debug!(?variances);
impl<'tcx> ty::TypeVisitor<'tcx> for OpaqueTypeLifetimeCollector<'tcx> {
fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
self.lifetimes.insert(r);
r.super_visit_with(self)
}
}
let mut collector = OpaqueTypeLifetimeCollector { lifetimes: Default::default() };
for pred in tcx.bound_explicit_item_bounds(def_id.to_def_id()).transpose_iter() {
let pred = pred.map_bound(|(pred, _)| *pred).subst(tcx, id_substs);
trace!(pred=?pred.kind());
// We only ignore opaque type substs if the opaque type is the outermost type.
// The opaque type may be nested within itself via recursion in e.g.
// type Foo<'a> = impl PartialEq<Foo<'a>>;
// which thus mentions `'a` and should thus accept hidden types that borrow 'a
// instead of requiring an additional `+ 'a`.
match pred.kind().skip_binder() {
ty::PredicateKind::Trait(TraitPredicate {
trait_ref: ty::TraitRef { def_id: _, substs },
constness: _,
polarity: _,
}) => {
trace!(?substs);
for subst in &substs[1..] {
subst.visit_with(&mut collector);
}
}
ty::PredicateKind::Projection(ty::ProjectionPredicate {
projection_ty: ty::ProjectionTy { substs, item_def_id: _ },
term,
}) => {
for subst in &substs[1..] {
subst.visit_with(&mut collector);
}
term.visit_with(&mut collector);
}
_ => {
pred.visit_with(&mut collector);
}
}
}
let lifetimes = collector.lifetimes;
trace!(?lifetimes);
map.filter(|(_, v)| {
let ty::GenericArgKind::Lifetime(lt) = v.unpack() else {
return true;
};
lifetimes.contains(&lt)
let ty::GenericArgKind::Lifetime(lt) = v.unpack() else { return true };
let ty::ReEarlyBound(ebr) = lt.kind() else { bug!() };
variances[ebr.index as usize] == ty::Variance::Invariant
})
.collect()
}