Rollup merge of #127092 - compiler-errors:rtn-dots-redux, r=estebank

Change return-type-notation to use `(..)`

Aligns the syntax with the current wording of [RFC 3654](https://github.com/rust-lang/rfcs/pull/3654). Also implements rustfmt support (along with making a match exhaustive).

Tracking:
* https://github.com/rust-lang/rust/issues/109417

compiler/rustc_resolve/src/late.rs

@@ -1221,6 +1221,7 @@ impl<'a: 'ast, 'ast, 'tcx> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast,
                         }
                     }
                 }
+                GenericArgs::ParenthesizedElided(_) => {}
             }
         }
     }

compiler/rustc_resolve/src/lib.rs

@@ -350,6 +350,7 @@ impl<'a> From<&'a ast::PathSegment> for Segment {
                     (args.span, found_lifetimes)
                 }
                 GenericArgs::Parenthesized(args) => (args.span, true),
+                GenericArgs::ParenthesizedElided(span) => (*span, true),
             }
         } else {
             (DUMMY_SP, false)