Streamline lex_token_trees error handling.

- Use iterators instead of `for` loops.
- Use `if`/`else` instead of `match`.

compiler/rustc_parse/src/lexer/mod.rs

@@ -72,29 +72,23 @@ pub(crate) fn lex_token_trees<'psess, 'src>(
     let (_open_spacing, stream, res) = lexer.lex_token_trees(/* is_delimited */ false);
     let unmatched_delims = lexer.diag_info.unmatched_delims;
 
-    match res {
+    if res.is_ok() && unmatched_delims.is_empty() {
-        Ok(()) if unmatched_delims.is_empty() => Ok(stream),
+        Ok(stream)
-        _ => {
+    } else {
         // Return error if there are unmatched delimiters or unclosed delimiters.
         // We emit delimiter mismatch errors first, then emit the unclosing delimiter mismatch
         // because the delimiter mismatch is more likely to be the root cause of error
-
+        let mut buffer: Vec<_> = unmatched_delims
-            let mut buffer = Vec::with_capacity(1);
+            .into_iter()
-            for unmatched in unmatched_delims {
+            .filter_map(|unmatched_delim| make_unclosed_delims_error(unmatched_delim, psess))
-                if let Some(err) = make_unclosed_delims_error(unmatched, psess) {
+            .collect();
-                    buffer.push(err);
-                }
-            }
         if let Err(errs) = res {
             // Add unclosing delimiter or diff marker errors
-                for err in errs {
+            buffer.extend(errs);
-                    buffer.push(err);
-                }
         }
         Err(buffer)
     }
 }
-}
 
 struct Lexer<'psess, 'src> {
     psess: &'psess ParseSess,