Warn unused_assignments for arguments

src/test/compile-fail/liveness-dead.rs

@@ -27,4 +27,14 @@ fn f3() {
     x = 4; //~ ERROR: value assigned to `x` is never read
 }
 
+fn f4(mut x: i32) { //~ ERROR: value passed to `x` is never read
+    x = 4;
+    x.clone();
+}
+
+fn f5(mut x: i32) {
+    x.clone();
+    x = 4; //~ ERROR: value assigned to `x` is never read
+}
+
 fn main() {}