Add temporary scope to assert_matches

Addresses https://github.com/rust-lang/rust/issues/154406 in part.
assert_eq will be done in a separate PR.

library/core/src/macros/mod.rs

@@ -168,7 +168,7 @@ macro_rules! assert_ne {
 #[allow_internal_unstable(panic_internals)]
 #[rustc_macro_transparency = "semiopaque"]
 pub macro assert_matches {
-    ($left:expr, $(|)? $( $pattern:pat_param )|+ $( if $guard: expr )? $(,)?) => {
+    ($left:expr, $(|)? $( $pattern:pat_param )|+ $( if $guard: expr )? $(,)?) => {{
         match $left {
             $( $pattern )|+ $( if $guard )? => {}
             ref left_val => {
@@ -179,8 +179,8 @@ macro_rules! assert_ne {
                 );
             }
         }
-    },
-    ($left:expr, $(|)? $( $pattern:pat_param )|+ $( if $guard: expr )?, $($arg:tt)+) => {
+    }},
+    ($left:expr, $(|)? $( $pattern:pat_param )|+ $( if $guard: expr )?, $($arg:tt)+) => {{
         match $left {
             $( $pattern )|+ $( if $guard )? => {}
             ref left_val => {
@@ -191,7 +191,7 @@ macro_rules! assert_ne {
                 );
             }
         }
-    },
+    }},
 }
 
 /// Selects code at compile-time based on `cfg` predicates.

library/coretests/tests/macros.rs

@@ -1,5 +1,7 @@
 #![allow(unused_must_use)]
 
+use std::{assert_matches, debug_assert_matches};
+
 #[allow(dead_code)]
 trait Trait {
     fn blah(&self);
@@ -219,3 +221,24 @@ fn _matches_does_not_trigger_non_exhaustive_omitted_patterns_lint(o: core::sync:
     // Ordering is a #[non_exhaustive] enum from a separate crate
     let _m = matches!(o, core::sync::atomic::Ordering::Relaxed);
 }
+
+struct MutRefWithDrop<'a>(&'a mut u32);
+
+// MutRefWithDrop needs to have a non-trivial drop to encounter potential lifetime issues if the
+// macros don't introduce a temporary scope.
+impl Drop for MutRefWithDrop<'_> {
+    fn drop(&mut self) {
+        *self.0 = u32::MAX;
+    }
+}
+
+#[test]
+fn temporary_scope_introduction() {
+    // Fails to compile if the macros don't introduce a temporary scope, since `&mut val` would
+    // create a second mutable borrow while `MutRefWithDrop` still holds a unique ref.
+    let mut val = 0;
+
+    (assert_matches!(*MutRefWithDrop(&mut val).0, 0), std::mem::take(&mut val));
+
+    (debug_assert_matches!(*MutRefWithDrop(&mut val).0, 0), std::mem::take(&mut val));
+}