ICE from inductive cycle in specialization

[theemathas]

Code

#![feature(specialization)]
#![allow(incomplete_features)]

trait Helper {}

struct Wrap<T>(T);
impl<T> Helper for Wrap<T> where Wrap<T>: Helper {}

trait Trait {
    fn method();
}
impl<T> Trait for T {
    default fn method() {}
}
impl<T: Helper> Trait for T {
    fn method() {}
}

fn call_method<T>() {
    T::method();
}

fn main() {
    call_method::<Wrap<i32>>();
}

My guess is that, after monomorphization, the trait solver tries to choose which trait impl of Trait to use. It finds the impl that goes through Helper. Then, the trait solver overflows while trying to figure out if the type implements Helper.

Using the next-solver does not fix this issue.

Meta

Reproducible on the playground with version 1.92.0-nightly (2025-10-08 b6f0945e4681bc4d2faa)

Error output

error: internal compiler error: compiler/rustc_middle/src/ty/instance.rs:579:21: failed to resolve instance for <Wrap<i32> as Trait>::method
  --> src/main.rs:20:5
   |
20 |     T::method();
   |     ^^^^^^^^^^^


thread 'rustc' (100) panicked at compiler/rustc_middle/src/ty/instance.rs:579:21:
Box<dyn Any>

Backtrace

stack backtrace:
   0: std::panicking::begin_panic::<rustc_errors::ExplicitBug>
   1: <rustc_errors::diagnostic::BugAbort as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
   2: <rustc_errors::DiagCtxtHandle>::span_bug::<rustc_span::span_encoding::Span, alloc::string::String>
   3: rustc_middle::util::bug::opt_span_bug_fmt::<rustc_span::span_encoding::Span>::{closure#0}
   4: rustc_middle::ty::context::tls::with_opt::<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}
   5: rustc_middle::ty::context::tls::with_context_opt::<rustc_middle::ty::context::tls::with_opt<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
   6: rustc_middle::util::bug::span_bug_fmt::<rustc_span::span_encoding::Span>
   7: <rustc_middle::ty::instance::Instance>::expect_resolve
   8: rustc_monomorphize::collector::items_of_instance
      [... omitted 1 frame ...]
   9: rustc_monomorphize::collector::collect_items_rec::{closure#0}
  10: rustc_monomorphize::collector::collect_items_rec
  11: rustc_monomorphize::collector::collect_items_rec
  12: rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}::{closure#0}
  13: rustc_monomorphize::partitioning::collect_and_partition_mono_items
      [... omitted 2 frames ...]
  14: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
  15: <rustc_interface::queries::Linker>::codegen_and_build_linker
  16: rustc_interface::passes::create_and_enter_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  17: <rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core::ops::function::FnOnce<(&rustc_session::session::Session, rustc_middle::ty::context::CurrentGcx, alloc::sync::Arc<rustc_data_structures::jobserver::Proxy>, &std::sync::once_lock::OnceLock<rustc_middle::ty::context::GlobalCtxt>, &rustc_data_structures::sync::worker_local::WorkerLocal<rustc_middle::arena::Arena>, &rustc_data_structures::sync::worker_local::WorkerLocal<rustc_hir::Arena>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
  18: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: please make sure that you have updated to the latest nightly

note: please attach the file at `/playground/rustc-ice-2025-10-09T04_33_22-98.txt` to your bug report

note: compiler flags: --crate-type bin -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
#0 [items_of_instance] collecting items used by `call_method::<Wrap<i32>>`
#1 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack
error: could not compile `playground` (bin "playground")

[theemathas]

This ICE is observable on stable with the following code:

#[derive(Clone)]
struct Wrap<T>(T);
impl<T: Copy> Copy for Wrap<T> where Wrap<T>: Copy {}

fn main() {
    let x = vec![Wrap(1i32)];
    let _ = x.clone();
}

This regressed from version 1.78.0 (compiling) to 1.79.0 (ICE).

[theemathas]

Another way to observe the ICE on stable:

use std::iter::FusedIterator;

struct Wrap<T>(T);
impl<T> Iterator for Wrap<T> {
    type Item = i32;
    fn next(&mut self) -> Option<i32> {
        None
    }
}
impl<T> FusedIterator for Wrap<T> where Wrap<T>: FusedIterator {}

fn main() {
    let _ = Wrap(1i64).fuse().next();
}

This regressed from version 1.78.0 (compiling) to 1.79.0 (ICE).