ICE from inductive cycle in specialization

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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.

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")

Reproduce the ICE with the minimized specialization example, then inspect compiler/rustc_middle/src/ty/instance.rs at expect_resolve and the rustc_monomorphize collector's items_of_instance query. Trace how the Trait implementation is selected for Wrap and how the Helper cycle is handled; done means this case no longer reaches the internal compiler error.

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ICE from inductive cycle in specialization

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Code

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Error output

Contributor guide

First steps

Research direction

Assessment

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