Rollup merge of #78524 - tmiasko:source-files-borrow, r=Aaron1011

Avoid BorrowMutError with RUSTC_LOG=debug

```console
$ touch empty.rs
$ env RUSTC_LOG=debug rustc +stage1 --crate-type=lib empty.rs
```

Fails with a `BorrowMutError` because source map files are already
borrowed while `features_query` attempts to format a log message
containing a span.

Release the borrow before the query to avoid the issue.

compiler/rustc_data_structures/src/sync.rs

@@ -512,7 +512,7 @@ impl<T: Clone> Clone for Lock<T> {
     }
 }
 
-#[derive(Debug)]
+#[derive(Debug, Default)]
 pub struct RwLock<T>(InnerRwLock<T>);
 
 impl<T> RwLock<T> {