Rollup merge of #107544 - nnethercote:improve-TokenCursor, r=petrochenkov

Improve `TokenCursor`.

Some small improvements, for things that were bugging me.

Best reviewed one commit at a time.

r? ``@petrochenkov``

compiler/rustc_parse/src/parser/expr.rs

@@ -2141,7 +2141,7 @@ impl<'a> Parser<'a> {
         }
 
         if self.token.kind == TokenKind::Semi
-            && matches!(self.token_cursor.frame.delim_sp, Some((Delimiter::Parenthesis, _)))
+            && matches!(self.token_cursor.stack.last(), Some((_, Delimiter::Parenthesis, _)))
             && self.may_recover()
         {
             // It is likely that the closure body is a block but where the