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, 14 insertions, 0 deletions

diff --git a/src/test/ui/parser/issue-65846-rollback-gating-failing-matcher.rs b/src/test/ui/parser/issue-65846-rollback-gating-failing-matcher.rs
new file mode 100644
index 00000000000..9d68a7bffde
--- /dev/null
+++ b/src/test/ui/parser/issue-65846-rollback-gating-failing-matcher.rs
@@ -0,0 +1,14 @@
+// run-pass
+
+// Test that failing macro matchers will not cause pre-expansion errors
+// even though they use a feature that is pre-expansion gated.
+
+macro_rules! m {
+    ($e:expr) => { 0 }; // This fails on the input below due to `, foo`.
+    ($e:expr,) => { 1 }; // This also fails to match due to `foo`.
+    (box $e:expr, foo) => { 2 }; // Successful matcher, we should get `2`.
+}
+
+fn main() {
+    assert_eq!(2, m!(box 42, foo));
+}