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

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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
This commit is contained in:
Michael Goulet 2024-09-21 15:18:56 -04:00 • committed by GitHub
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48 changed files with 1493 additions and 229 deletions
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4
compiler/rustc_hir_analysis/messages.ftl
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@ -37,7 +37,7 @@ hir_analysis_assoc_kind_mismatch = expected {$expected}, found {$got}
hir_analysis_assoc_kind_mismatch_wrap_in_braces_sugg = consider adding braces here
hir_analysis_associated_type_trait_uninferred_generic_params = cannot use the associated type of a trait with uninferred generic parameters
hir_analysis_associated_type_trait_uninferred_generic_params = cannot use the associated {$what} of a trait with uninferred generic parameters
.suggestion = use a fully qualified path with inferred lifetimes
hir_analysis_associated_type_trait_uninferred_generic_params_multipart_suggestion = use a fully qualified path with explicit lifetimes
@ -48,6 +48,8 @@ hir_analysis_auto_deref_reached_recursion_limit = reached the recursion limit wh
hir_analysis_bad_precise_capture = expected {$kind} parameter in `use<...>` precise captures list, found {$found}
hir_analysis_bad_return_type_notation_position = return type notation not allowed in this position yet
hir_analysis_cannot_capture_late_bound_const =
cannot capture late-bound const parameter in {$what}
.label = parameter defined here