Auto merge of #134185 - compiler-errors:impl-trait-in-bindings, r=oli-obk

(Re-)Implement `impl_trait_in_bindings`

This reimplements the `impl_trait_in_bindings` feature for local bindings.

"`impl Trait` in bindings" serve as a form of *trait* ascription, where the type basically functions as an infer var but additionally registering the `impl Trait`'s trait bounds for the infer type. These trait bounds can be used to enforce that predicates hold, and can guide inference (e.g. for closure signature inference):

```rust
let _: impl Fn(&u8) -> &u8 = |x| x;
```

They are implemented as an additional set of bounds that are registered when the type is lowered during typeck, and then these bounds are tied to a given `CanonicalUserTypeAscription` for borrowck. We enforce these `CanonicalUserTypeAscription` bounds during borrowck to make sure that the `impl Trait` types are sensitive to lifetimes:

```rust
trait Static: 'static {}
impl<T> Static for T where T: 'static {}

let local = 1;
let x: impl Static = &local;
//~^ ERROR `local` does not live long enough
```

r? oli-obk

cc #63065

---

Why can't we just use TAIT inference or something? Well, TAITs in bodies have the problem that they cannot reference lifetimes local to a body. For example:

```rust
type TAIT = impl Display;
let local = 0;
let x: TAIT = &local;
//~^ ERROR `local` does not live long enough
```

That's because TAITs requires us to do *opaque type inference* which is pretty strict, since we need to remap all of the lifetimes of the hidden type to universal regions. This is simply not possible here.

---

I consider this part of the "impl trait everywhere" experiment. I'm not certain if this needs yet another lang team experiment.

1 files changed, 6 insertions, 2 deletions

diff --git a/src/librustdoc/clean/mod.rs b/src/librustdoc/clean/mod.rs
index a201a9bbfed..25f88c8797f 100644
--- a/src/librustdoc/clean/mod.rs
+++ b/src/librustdoc/clean/mod.rs
@@ -1839,11 +1839,15 @@ pub(crate) fn clean_ty<'tcx>(ty: &hir::Ty<'tcx>, cx: &mut DocContext<'tcx>) -> T
             DynTrait(bounds, lifetime)
         }
         TyKind::BareFn(barefn) => BareFunction(Box::new(clean_bare_fn_ty(barefn, cx))),
-        // Rustdoc handles `TyKind::Err`s by turning them into `Type::Infer`s.
-        TyKind::Infer | TyKind::Err(_) | TyKind::Typeof(..) | TyKind::InferDelegation(..) => Infer,
         TyKind::UnsafeBinder(..) => {
             unimplemented!("unsafe binders are not supported yet")
         }
+        // Rustdoc handles `TyKind::Err`s by turning them into `Type::Infer`s.
+        TyKind::Infer
+        | TyKind::Err(_)
+        | TyKind::Typeof(..)
+        | TyKind::InferDelegation(..)
+        | TyKind::TraitAscription(_) => Infer,
     }
 }