add lint test

closes #138069, add test program

Author: KaiTomotake

tests/ui/lint/unused/unused_assignments_across_match_guards.rs

@@ -0,0 +1,18 @@
+// #138069: This test ensures that unused_assignments is not incorrectly reported for assignments used within a match expression.
+//@ run-pass
+
+fn pnk(x: usize) -> &'static str {
+    let mut k1 = "k1";
+    let mut h1 = "h1";
+    match x & 3 {
+        3 if { k1 = "unused?"; false } => (),
+        _ if { h1 = k1; true } => (),
+        _ => (),
+    }
+    h1
+}
+
+#[deny(unused_assignments)]
+fn main() {
+    pnk(3);
+}