commit d72d44d8ed5d5e19a8bd7993a96a6f30ddeb1afe
author Michael Goulet <michael@errs.io> Sat Sep 21 15:18:56 2024 -0400
committer GitHub <noreply@github.com> Sat Sep 21 15:18:56 2024 -0400
tree 5c401c7c57dc62ff3e41ef311d59f32c547227e2
parent 2a9525bb90cd8079bb11233395d6e97fc6dcca19
parent 1de894f0c1882aed5b6fba6b665de7664626bd59

Rollup merge of #129629 - compiler-errors:rtn-in-path, r=jackh726

Implement Return Type Notation (RTN)'s path form in where clauses

Implement return type notation (RTN) in path position for where clauses. We already had RTN in associated type position ([e.g.](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=627a4fb8e2cb334863fbd08ed3722c09)), but per [the RFC](https://rust-lang.github.io/rfcs/3654-return-type-notation.html#where-rtn-can-be-used-for-now):

> As a standalone type, RTN can only be used as the Self type of a where-clause [...]

Specifically, in order to enable code like:

```rust
trait Foo {
    fn bar() -> impl Sized;
}

fn is_send(_: impl Send) {}

fn test<T>()
where
    T: Foo,
    T::bar(..): Send,
{
    is_send(T::bar());
}
```

* In the resolver, when we see a `TyKind::Path` whose final segment is `GenericArgs::ParenthesizedElided` (i.e. `(..)`), resolve that path in the *value* namespace, since we're looking for a method.
* When lowering where clauses in HIR lowering, we first try to intercept an RTN self type via `lower_ty_maybe_return_type_notation`. If we find an RTN type, we lower it manually in a way that respects its higher-ranked-ness (see below) and resolves to the corresponding RPITIT. Anywhere else, we'll emit the same "return type notation not allowed in this position yet" error we do when writing RTN in every other position.
* In `resolve_bound_vars`, we add some special treatment for RTN types in where clauses. Specifically, we need to add new lifetime variables to our binders for the early- and late-bound vars we encounter on the method. This implements the higher-ranked desugaring [laid out in the RFC](https://rust-lang.github.io/rfcs/3654-return-type-notation.html#converting-to-higher-ranked-trait-bounds).

This PR also adds a bunch of tests, mostly negative ones (testing error messages).

In a follow-up PR, I'm going to mark RTN as no longer incomplete, since this PR basically finishes the impl surface that we should initially stabilize, and the RFC was accepted.

cc [RFC 3654](https://github.com/rust-lang/rfcs/pull/3654) and https://github.com/rust-lang/rust/issues/109417