Auto merge of #109010 - compiler-errors:rtn, r=eholk

Initial support for return type notation (RTN)

See: https://smallcultfollowing.com/babysteps/blog/2023/02/13/return-type-notation-send-bounds-part-2/

1. Only supports `T: Trait<method(): Send>` style bounds, not `<T as Trait>::method(): Send`. Checking validity and injecting an implicit binder for all of the late-bound method generics is harder to do for the latter.
    * I'd add this in a follow-up.
3. ~Doesn't support RTN in general type position, i.e. no `let x: <T as Trait>::method() = ...`~
    * I don't think we actually want this.
5. Doesn't add syntax for "eliding" the function args -- i.e. for now, we write `method(): Send` instead of `method(..): Send`.
    * May be a hazard if we try to add it in the future. I'll probably add it in a follow-up later, with a structured suggestion to change `method()` to `method(..)` once we add it.
7. ~I'm not in love with the feature gate name 😺~
    * I renamed it to `return_type_notation` :heavy_check_mark:

Follow-up PRs will probably add support for `where T::method(): Send` bounds. I'm not sure if we ever want to support return-type-notation in arbitrary type positions. I may also make the bounds require `..` in the args list later.

r? `@ghost`

2 files changed, 2 insertions, 0 deletions

diff --git a/compiler/rustc_resolve/src/late.rs b/compiler/rustc_resolve/src/late.rs
index 4ca54bab31a..f66bad1d429 100644
--- a/compiler/rustc_resolve/src/late.rs
+++ b/compiler/rustc_resolve/src/late.rs
@@ -1110,6 +1110,7 @@ impl<'a: 'ast, 'ast, 'tcx> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast,
                         }
                     }
                 }
+                GenericArgs::ReturnTypeNotation(_span) => {}
             }
         }
     }
diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
index 0e84432a5b4..99fad22d4a1 100644
--- a/compiler/rustc_resolve/src/lib.rs
+++ b/compiler/rustc_resolve/src/lib.rs
@@ -312,6 +312,7 @@ impl<'a> From<&'a ast::PathSegment> for Segment {
                     (args.span, found_lifetimes)
                 }
                 GenericArgs::Parenthesized(args) => (args.span, true),
+                GenericArgs::ReturnTypeNotation(span) => (*span, false),
             }
         } else {
             (DUMMY_SP, false)