Suppress unused field in test_borrow_in_map_key

    warning: field is never read: `map`
        --> tests/test.rs:2233:9
         |
    2233 |         map: BTreeMap<MyMapKey, ()>,
         |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
         |
         = note: `#[warn(dead_code)]` on by default

Author: dtolnay

Diff for: tests/test.rs

@@ -2230,6 +2230,7 @@ fn test_raw_invalid_utf8() {
 fn test_borrow_in_map_key() {
     #[derive(Deserialize, Debug)]
     struct Outer {
+        #[allow(dead_code)]
         map: BTreeMap<MyMapKey, ()>,
     }