Auto merge of #58061 - nnethercote:overhaul-syntax-Folder, r=petrochenkov

Overhaul `syntax::fold::Folder`.

This PR changes `syntax::fold::Folder` from a functional style
(where most methods take a `T` and produce a new `T`) to a more
imperative style (where most methods take and modify a `&mut T`), and
renames it `syntax::mut_visit::MutVisitor`.

This makes the code faster and more concise.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 1c02a80df46..bbd1770e9c6 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -7054,7 +7054,8 @@ impl<'a> Parser<'a> {
                 sess: self.sess,
                 features: None, // don't perform gated feature checking
             };
-            let outer_attrs = strip_unconfigured.process_cfg_attrs(outer_attrs.to_owned());
+            let mut outer_attrs = outer_attrs.to_owned();
+            strip_unconfigured.process_cfg_attrs(&mut outer_attrs);
             (!self.cfg_mods || strip_unconfigured.in_cfg(&outer_attrs), outer_attrs)
         };