Provide suggestion for missing > in a type parameter list

When encountering an inproperly terminated type parameter list, provide
a suggestion to close it after the last non-constraint type parameter
that was successfully parsed.

Fix #94058.

compiler/rustc_parse/src/parser/path.rs

@@ -272,7 +272,23 @@ impl<'a> Parser<'a> {
                         lo,
                         ty_generics,
                     )?;
-                    self.expect_gt()?;
+                    self.expect_gt().map_err(|mut err| {
+                        // Attempt to find places where a missing `>` might belong.
+                        if let Some(arg) = args
+                            .iter()
+                            .rev()
+                            .skip_while(|arg| matches!(arg, AngleBracketedArg::Constraint(_)))
+                            .next()
+                        {
+                            err.span_suggestion_verbose(
+                                arg.span().shrink_to_hi(),
+                                "you might have meant to end the type parameters here",
+                                ">".to_string(),
+                                Applicability::MaybeIncorrect,
+                            );
+                        }
+                        err
+                    })?;
                     let span = lo.to(self.prev_token.span);
                     AngleBracketedArgs { args, span }.into()
                 } else {