Support "for loop" MIR lowering

crates/ide-diagnostics/src/handlers/mutability_errors.rs

@@ -507,6 +507,22 @@ fn f(x: i32) {
    x = 5;
  //^^^^^ 💡 error: cannot mutate immutable variable `x`
 }
+"#,
+        );
+    }
+
+    #[test]
+    fn for_loop() {
+        check_diagnostics(
+            r#"
+//- minicore: iterators
+fn f(x: [(i32, u8); 10]) {
+    for (a, mut b) in x {
+          //^^^^^ 💡 weak: remove this `mut`
+        a = 2;
+      //^^^^^ 💡 error: cannot mutate immutable variable `a`
+    }
+}
 "#,
         );
     }