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, 2 deletions

diff --git a/src/libsyntax/parse/parser/generics.rs b/src/libsyntax/parse/parser/generics.rs
index 51caae69c86..3c094750b4d 100644
--- a/src/libsyntax/parse/parser/generics.rs
+++ b/src/libsyntax/parse/parser/generics.rs
@@ -3,7 +3,8 @@ use super::{Parser, PResult};
 use crate::ast::{self, WhereClause, GenericParam, GenericParamKind, GenericBounds, Attribute};
 use crate::parse::token;
 use crate::source_map::DUMMY_SP;
-use crate::symbol::kw;
+
+use syntax_pos::symbol::{kw, sym};
 
 impl<'a> Parser<'a> {
     /// Parses bounds of a lifetime parameter `BOUND + BOUND + BOUND`, possibly with trailing `+`.
@@ -62,7 +63,7 @@ impl<'a> Parser<'a> {
         self.expect(&token::Colon)?;
         let ty = self.parse_ty()?;
 
-        self.sess.gated_spans.const_generics.borrow_mut().push(lo.to(self.prev_span));
+        self.sess.gated_spans.gate(sym::const_generics, lo.to(self.prev_span));
 
         Ok(GenericParam {
             ident,