SIGBUS on aarch64-apple-darwin during deep nominal trait projection with elevated recursion_limit

Note / Full transparency: I was running an experiment with an AI coding agent stress-testing compiler recursion limits on my Mac, and the agent produced and minimized this crash reproducer. If this is already a known stack-overflow issue under high recursion_limit or considered unintended AI noise/slop, please feel free to close/delete it right away! I just wanted to share the minimal reproducer in case it's helpful.

Code

Save the following 10 lines as reproduce.rs:

#![recursion_limit = "10000000"]
pub struct S<T>(std::marker::PhantomData<T>);
pub trait Trait { type Out; }
impl Trait for () { type Out = (); }
impl<T: Trait> Trait for S<T> { type Out = S<<T as Trait>::Out>; }
type N1<T> = S<S<S<S<S<S<S<S<S<S<T>>>>>>>>>>;
type N2<T> = N1<N1<N1<N1<N1<N1<N1<N1<N1<N1<T>>>>>>>>>>;
type N3<T> = N2<N2<N2<N2<N2<N2<N2<N2<N2<N2<T>>>>>>>>>>;
type N4<T> = N3<N3<N3<N3<N3<N3<N3<N3<N3<N3<T>>>>>>>>>>;
pub type Trigger = <N4<N4<()>> as Trait>::Out;

Command:

rustc --crate-type=lib reproduce.rs

Meta

rustc --version --verbose:

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

Error output

error: rustc interrupted by SIGBUS, printing backtrace

0   librustc_driver-22cdaff06538ddcd.dylib   _RNvNtCs3Z9OGp4qESS_17rustc_driver_impl14signal_handler17print_stack_trace + 140
1   libsystem_platform.dylib                 _sigtramp + 56
2   librustc_driver-22cdaff06538ddcd.dylib   _RNvXs_NtCs4scqkk2JIvp_10rustc_lint5early...
...

Process exits with exit code 138 (Signal 10 / SIGBUS).

Note / Observation

When recursion_limit is raised, trait projection in rustc_trait_selection::traits::project recurses until the thread stack exhausts the Darwin 8 MB main thread stack and hits the guard page (KERN_PROTECTION_FAILURE). Ideally, the compiler would detect stack depth before triggering a hard OS bus error.

Save the supplied reproducer as reproduce.rs and run rustc --crate-type=lib reproduce.rs on the reported target to confirm the SIGBUS. Read rustc_trait_selection::traits::project and the recursion-limit or stack-depth handling it reaches. Done means the compiler handles this deep projection without a hard SIGBUS, with appropriate diagnostic or controlled failure behavior.