Extend support of `_` in type parameters

 - Account for `impl Trait<_>`.
 - Provide a reasonable `Span` for empty `Generics` in `impl`s.
 - Account for `fn foo<_>(_: _) {}` to suggest `fn foo<T>(_: T) {}`.
 - Fix #67995.

2 files changed, 6 insertions, 2 deletions

diff --git a/src/librustc_parse/parser/generics.rs b/src/librustc_parse/parser/generics.rs
index 1b816e2b90d..075583711f5 100644
--- a/src/librustc_parse/parser/generics.rs
+++ b/src/librustc_parse/parser/generics.rs
@@ -156,7 +156,7 @@ impl<'a> Parser<'a> {
             self.expect_gt()?;
             (params, span_lo.to(self.prev_span))
         } else {
-            (vec![], self.prev_span.between(self.token.span))
+            (vec![], self.prev_span.shrink_to_hi())
         };
         Ok(ast::Generics {
             params,
diff --git a/src/librustc_parse/parser/item.rs b/src/librustc_parse/parser/item.rs
index 918e826fc26..12dcf0391da 100644
--- a/src/librustc_parse/parser/item.rs
+++ b/src/librustc_parse/parser/item.rs
@@ -555,7 +555,11 @@ impl<'a> Parser<'a> {
         let mut generics = if self.choose_generics_over_qpath() {
             self.parse_generics()?
         } else {
-            Generics::default()
+            let mut generics = Generics::default();
+            // impl A for B {}
+            //    /\ this is where `generics.span` should point when there are no type params.
+            generics.span = self.prev_span.shrink_to_hi();
+            generics
         };
 
         // Disambiguate `impl !Trait for Type { ... }` and `impl ! { ... }` for the never type.