One way to check that this is doing something sensible is to write something like
```rust
let renamed = mtwt_resolve(label);
info!("ExprLoop label: {:?} -> {:?}", label, renamed);
bindings.get().insert(renamed, def_like);
```
and then run the compiler on the test cases like `RUST_LOG=rustc::middle::resolve=info .../bin/rustc testcase.rs` to check if it's being renamed or not.

(I suggest using `info!` rather than `debug!` for now so that you can avoid printing all the other `debug!` statements in this module and focus on just what you're interested in.)