Rustc hang, likely due to infinite recusion

[jacob-greenfield-ll]

I tried this code:

trait Chain<'a, H: Chain<'a, H>> {
    fn run(head: &mut H, get: impl FnMut(&mut H) -> &mut Self);
}

struct FinalStage;
impl<'a, H: Chain<'a, H>> Chain<'a, H> for FinalStage {
    fn run(_head: &mut H, _get: impl FnMut(&mut H) -> &mut Self) {}
}

struct BasicStage<C> {
    next_stage: C,
}
impl<'a, H: Chain<'a, H>, C: Chain<'a, H>> Chain<'a, H> for BasicStage<C> {
    fn run(head: &mut H, mut get: impl FnMut(&mut H) -> &mut Self) {
        // let _this = get(head);
        // C::run(head, |h| &mut get(h).next_stage);
    }
}

fn run_chain<'a, H: Chain<'a, H>>(head: &mut H) {
    H::run(head, |h| h);
}

fn test_example_chain() {
    let mut chain = BasicStage {
        next_stage: FinalStage,
    };

    run_chain(&mut chain);
}

I expected to see this happen: compiler to exit, either successfully or unsuccessfully

Instead, this happened: rustc hangs indefinitely

(I will hazard a guess that this is probably just due to a missing check on the recursion limit, because similar versions of this code fail with overflow evaluating the requirement).

Meta

rustc --version --verbose:

rustc 1.88.0 (6b00bc388 2025-06-23)
binary: rustc
commit-hash: 6b00bc3880198600130e1cf62b8f8a93494488cc
commit-date: 2025-06-23
host: aarch64-apple-darwin
release: 1.88.0
LLVM version: 20.1.5

Backtrace: N/A.

Sampler output: Sample of rustc.txt

[lqd]

IIUC these cases of overflow should also be taken care of by the new trait solver

[theemathas]

Minimized:

trait Chain<'a, H> {}

struct FinalStage;
impl<'a, H: Chain<'a, H>> Chain<'a, H> for FinalStage {}

struct BasicStage;
impl<'a, H: Chain<'a, H>> Chain<'a, H> for BasicStage where FinalStage: Chain<'a, H> {}

fn run<'a, H: Chain<'a, BasicStage>>() {}

fn test_example_chain() {
    run::<'_, BasicStage>();
}