Rollup merge of #134161 - nnethercote:overhaul-token-cursors, r=spastorino

Overhaul token cursors

Some nice cleanups here.

r? `````@davidtwco`````

compiler/rustc_lint/src/builtin.rs

@@ -1828,7 +1828,7 @@ impl KeywordIdents {
     fn check_tokens(&mut self, cx: &EarlyContext<'_>, tokens: &TokenStream) {
         // Check if the preceding token is `$`, because we want to allow `$async`, etc.
         let mut prev_dollar = false;
-        for tt in tokens.trees() {
+        for tt in tokens.iter() {
             match tt {
                 // Only report non-raw idents.
                 TokenTree::Token(token, _) => {