rustc appears to hang on this RPITIT example

I found a small example where rustc appear to hang during type checking.

pub struct BoundDropDisposal<D>(D);

pub trait Observable {
    fn subscribe<OR>(self, observer: OR) -> BoundDropDisposal<impl Clone>;
}

fn test(source: impl Observable) {
    subscribe_unsub_after_termination(|observer| {
        source.subscribe(observer)
    });
}

pub fn subscribe_unsub_after_termination<D, F>(_builder: F)
where
    F: FnOnce(D) -> BoundDropDisposal<D>,
{
}

Running this does not finish for me and keeps CPU busy:

timeout 10 rustc --crate-type lib lib.rs

With the next solver, it reports an error immediately:

rustc +nightly -Znext-solver=globally --crate-type lib lib.rs

error[E0271]: type mismatch resolving `impl Clone == impl Clone`
 --> lib.rs:9:26
  |
9 |         source.subscribe(observer)
  |                          ^^^^^^^^ types differ

So I expected the current solver to also terminate with a type error instead of continuing to run.

Toolchain:

rustc 1.97.0 (2d8144b78 2026-07-07)
binary: rustc
commit-hash: 2d8144b7880597b6e6d3dfd63a9a9efae3f533d3
commit-date: 2026-07-07
host: aarch64-apple-darwin
release: 1.97.0
LLVM version: 22.1.6

Start with the self-contained example in lib.rs and reproduce the hang using timeout 10 rustc --crate-type lib lib.rs. Compare it with rustc +nightly -Znext-solver=globally --crate-type lib lib.rs; done means the current solver terminates with an appropriate type error instead of continuing to consume CPU.