add regression test

Author: Lukas Markeffsky

src/test/ui/matches2021.rs

@@ -0,0 +1,12 @@
+// run-pass
+// edition:2021
+// compile-flags: -Zunstable-options
+
+// regression test for https://github.com/rust-lang/rust/pull/85678
+
+#![feature(assert_matches)]
+
+fn main() {
+    assert!(matches!((), ()));
+    assert_matches!((), ());
+}