Rollup merge of #65974 - Centril:matcher-friendly-gating, r=petrochenkov

A scheme for more macro-matcher friendly pre-expansion gating

Pre-expansion gating will now avoid gating macro matchers that did not result in `Success(...)`. That is, the following is now OK despite `box 42` being a valid `expr` and that form being pre-expansion gated:

```rust
macro_rules! m {
    ($e:expr) => { 0 }; // This fails on the input below due to `, foo`.
    (box $e:expr, foo) => { 1 }; // Successful matcher, we should get `2`.
}

fn main() {
    assert_eq!(1, m!(box 42, foo));
}
```

Closes https://github.com/rust-lang/rust/issues/65846.

r? @petrochenkov
cc @Mark-Simulacrum

1 files changed, 3 insertions, 5 deletions

diff --git a/src/libsyntax/parse/parser/item.rs b/src/libsyntax/parse/parser/item.rs
index ebb1cf12996..9d543055f23 100644
--- a/src/libsyntax/parse/parser/item.rs
+++ b/src/libsyntax/parse/parser/item.rs
@@ -146,9 +146,7 @@ impl<'a> Parser<'a> {
                 let unsafety = self.parse_unsafety();
 
                 if self.check_keyword(kw::Extern) {
-                    self.sess.gated_spans.const_extern_fn.borrow_mut().push(
-                        lo.to(self.token.span)
-                    );
+                    self.sess.gated_spans.gate(sym::const_extern_fn, lo.to(self.token.span));
                 }
                 let abi = self.parse_extern_abi()?;
                 self.bump(); // `fn`
@@ -831,7 +829,7 @@ impl<'a> Parser<'a> {
                     .emit();
             }
 
-            self.sess.gated_spans.trait_alias.borrow_mut().push(whole_span);
+            self.sess.gated_spans.gate(sym::trait_alias, whole_span);
 
             Ok((ident, ItemKind::TraitAlias(tps, bounds), None))
         } else {
@@ -1711,7 +1709,7 @@ impl<'a> Parser<'a> {
         let span = lo.to(self.prev_span);
 
         if !def.legacy {
-            self.sess.gated_spans.decl_macro.borrow_mut().push(span);
+            self.sess.gated_spans.gate(sym::decl_macro, span);
         }
 
         Ok(Some(self.mk_item(span, ident, ItemKind::MacroDef(def), vis.clone(), attrs.to_vec())))