ICE : collection encountered polymorphic constant: `Unevaluated`

[trait-Cong]

This issue was discovered by a fuzzer that generates code by adding trait bounds. Specifically, adding a recursive trait bound (where T: SpecializedTrait) to a specialized implementation seems to confuse the resolution of associated constants.

I tried this code:

#![feature(specialization)]
trait Marker {}
trait SpecializedTrait {
    const CONST_BOOL: bool;
    const CONST_STR: &'static str;
    fn method() -> &'static str;
}
impl<T> SpecializedTrait for T {
    default const CONST_BOOL: bool = false;
    default const CONST_STR: &'static str = "in default impl";
    #[inline(always)]
    default fn method() -> &'static str {
        "in default impl"
    }
}
impl<T: Marker> SpecializedTrait for T
where
    T: SpecializedTrait,
{
    const CONST_BOOL: bool = true;
    const CONST_STR: &'static str = "in specialized impl";
    fn method() -> &'static str {
        "in specialized impl"
    }
}
fn const_bool<T>() -> &'static str {
    if <T as SpecializedTrait>::CONST_BOOL {
        "in specialized impl"
    } else {
        "in default impl"
    }
}
fn const_str<T>() -> &'static str {
    <T as SpecializedTrait>::CONST_STR
}
fn run_method<T>() -> &'static str {
    <T as SpecializedTrait>::method()
}
struct TypeA;
impl Marker for TypeA {}
struct TypeB;
#[inline(never)]
fn exit_if_not_eq(left: &str, right: &str) {
    if left != right {
        std::process::exit(1);
    }
}
pub fn main() {
    exit_if_not_eq("in specialized impl", const_bool::<TypeA>());
}

I expected to see this happen:
The code may need to be correctly rejected.

Instead, this happened:
ICE

Meta

rustc --version --verbose:

rustc 1.94.0-nightly (e29fcf45e 2026-01-04)
binary: rustc
commit-hash: e29fcf45e4ae686d77b490bf07320f0d3a2cf35f
commit-date: 2026-01-04
host: x86_64-unknown-linux-gnu
release: 1.94.0-nightly
LLVM version: 21.1.8

Backtrace

thread 'rustc' panicked at /rustc-dev/e29fcf45e4ae686d77b490bf07320f0d3a2cf35f/compiler/rustc_monomorphize/src/collector.rs:718:50:
Box<dyn Any>
stack backtrace:
   0:     0x7ff8ea419c0b - <std[eaa43e369792efb9]::backtrace::Backtrace>::create
   1:     0x7ff8ea419b55 - <std[eaa43e369792efb9]::backtrace::Backtrace>::force_capture
   2:     0x7ff8e94115b7 - std[eaa43e369792efb9]::panicking::update_hook::<alloc[83e7320d2ee5a6b1]::boxed::Box<rustc_driver_impl[5af842e777d449e6]::install_ice_hook::{closure#1}>>::{closure#0}
   3:     0x7ff8ea42c4b2 - std[eaa43e369792efb9]::panicking::panic_with_hook
   4:     0x7ff8e944de81 - std[eaa43e369792efb9]::panicking::begin_panic::<rustc_errors[3cfa44cd230a06eb]::ExplicitBug>::{closure#0}
   5:     0x7ff8e943e4e6 - std[eaa43e369792efb9]::sys::backtrace::__rust_end_short_backtrace::<std[eaa43e369792efb9]::panicking::begin_panic<rustc_errors[3cfa44cd230a06eb]::ExplicitBug>::{closure#0}, !>
   6:     0x7ff8e943e11c - std[eaa43e369792efb9]::panicking::begin_panic::<rustc_errors[3cfa44cd230a06eb]::ExplicitBug>
   7:     0x7ff8e946bd21 - <rustc_errors[3cfa44cd230a06eb]::diagnostic::BugAbort as rustc_errors[3cfa44cd230a06eb]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
   8:     0x7ff8e99f729c - <rustc_errors[3cfa44cd230a06eb]::DiagCtxtHandle>::span_bug::<rustc_span[14443e5baae9ab13]::span_encoding::Span, alloc[83e7320d2ee5a6b1]::string::String>
   9:     0x7ff8e9a19e86 - rustc_middle[829e5443847148d]::util::bug::opt_span_bug_fmt::<rustc_span[14443e5baae9ab13]::span_encoding::Span>::{closure#0}
  10:     0x7ff8e9a1a032 - rustc_middle[829e5443847148d]::ty::context::tls::with_opt::<rustc_middle[829e5443847148d]::util::bug::opt_span_bug_fmt<rustc_span[14443e5baae9ab13]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  11:     0x7ff8e9a0b65b - rustc_middle[829e5443847148d]::ty::context::tls::with_context_opt::<rustc_middle[829e5443847148d]::ty::context::tls::with_opt<rustc_middle[829e5443847148d]::util::bug::opt_span_bug_fmt<rustc_span[14443e5baae9ab13]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  12:     0x7ff8e7d29388 - rustc_middle[829e5443847148d]::util::bug::span_bug_fmt::<rustc_span[14443e5baae9ab13]::span_encoding::Span>
  13:     0x7ff8eb236895 - <rustc_monomorphize[bf80cbd22413be96]::collector::MirUsedCollector as rustc_middle[829e5443847148d]::mir::visit::Visitor>::visit_operand
  14:     0x7ff8eb23ed02 - <rustc_monomorphize[bf80cbd22413be96]::collector::MirUsedCollector as rustc_middle[829e5443847148d]::mir::visit::Visitor>::visit_terminator
  15:     0x7ff8eb237d89 - rustc_monomorphize[bf80cbd22413be96]::collector::items_of_instance
  16:     0x7ff8eb237782 - rustc_query_impl[1b717c0605046cd0]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[1b717c0605046cd0]::query_impl::items_of_instance::dynamic_query::{closure#2}::{closure#0}, rustc_middle[829e5443847148d]::query::erase::Erased<[u8; 32usize]>>
  17:     0x7ff8eb6b7894 - rustc_query_system[5c35911b3dd767e9]::query::plumbing::try_execute_query::<rustc_query_impl[1b717c0605046cd0]::DynamicConfig<rustc_query_system[5c35911b3dd767e9]::query::caches::DefaultCache<(rustc_middle[829e5443847148d]::ty::instance::Instance, rustc_middle[829e5443847148d]::mir::mono::CollectionMode), rustc_middle[829e5443847148d]::query::erase::Erased<[u8; 32usize]>>, false, false, false>, rustc_query_impl[1b717c0605046cd0]::plumbing::QueryCtxt, false>
  18:     0x7ff8eb6b74b3 - rustc_query_impl[1b717c0605046cd0]::query_impl::items_of_instance::get_query_non_incr::__rust_end_short_backtrace
  19:     0x7ff8ebef07ae - rustc_monomorphize[bf80cbd22413be96]::collector::collect_items_rec
  20:     0x7ff8ebef4981 - rustc_monomorphize[bf80cbd22413be96]::collector::collect_items_rec
  21:     0x7ff8ead5d6a1 - rustc_monomorphize[bf80cbd22413be96]::collector::collect_crate_mono_items::{closure#1}::{closure#0}
  22:     0x7ff8eafc83ba - rustc_monomorphize[bf80cbd22413be96]::partitioning::collect_and_partition_mono_items
  23:     0x7ff8ebdedda4 - rustc_query_impl[1b717c0605046cd0]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[1b717c0605046cd0]::query_impl::collect_and_partition_mono_items::dynamic_query::{closure#2}::{closure#0}, rustc_middle[829e5443847148d]::query::erase::Erased<[u8; 24usize]>>
  24:     0x7ff8ebdedd89 - <rustc_query_impl[1b717c0605046cd0]::query_impl::collect_and_partition_mono_items::dynamic_query::{closure#2} as core[4141bff0bf9ca9dd]::ops::function::FnOnce<(rustc_middle[829e5443847148d]::ty::context::TyCtxt, ())>>::call_once
  25:     0x7ff8ebded8de - rustc_query_system[5c35911b3dd767e9]::query::plumbing::try_execute_query::<rustc_query_impl[1b717c0605046cd0]::DynamicConfig<rustc_query_system[5c35911b3dd767e9]::query::caches::SingleCache<rustc_middle[829e5443847148d]::query::erase::Erased<[u8; 24usize]>>, false, false, false>, rustc_query_impl[1b717c0605046cd0]::plumbing::QueryCtxt, false>
  26:     0x7ff8ebded683 - rustc_query_impl[1b717c0605046cd0]::query_impl::collect_and_partition_mono_items::get_query_non_incr::__rust_end_short_backtrace
  27:     0x7ff8ebdfa7e5 - rustc_codegen_ssa[99944605b17ba303]::base::codegen_crate::<rustc_codegen_llvm[33f85da736ede4fd]::LlvmCodegenBackend>
  28:     0x7ff8ebdf82bd - <rustc_codegen_llvm[33f85da736ede4fd]::LlvmCodegenBackend as rustc_codegen_ssa[99944605b17ba303]::traits::backend::CodegenBackend>::codegen_crate
  29:     0x7ff8ebd9878f - <rustc_interface[eaf9095e0de5b742]::queries::Linker>::codegen_and_build_linker
  30:     0x7ff8ebd95d99 - <rustc_interface[eaf9095e0de5b742]::passes::create_and_enter_global_ctxt<core[4141bff0bf9ca9dd]::option::Option<rustc_interface[eaf9095e0de5b742]::queries::Linker>, rustc_driver_impl[5af842e777d449e6]::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core[4141bff0bf9ca9dd]::ops::function::FnOnce<(&rustc_session[ea1eec3ed26e42d5]::session::Session, rustc_middle[829e5443847148d]::ty::context::CurrentGcx, alloc[83e7320d2ee5a6b1]::sync::Arc<rustc_data_structures[4e9b1f6e439fd3ef]::jobserver::Proxy>, &std[eaa43e369792efb9]::sync::once_lock::OnceLock<rustc_middle[829e5443847148d]::ty::context::GlobalCtxt>, &rustc_data_structures[4e9b1f6e439fd3ef]::sync::worker_local::WorkerLocal<rustc_middle[829e5443847148d]::arena::Arena>, &rustc_data_structures[4e9b1f6e439fd3ef]::sync::worker_local::WorkerLocal<rustc_hir[67c6535075053fb5]::Arena>, rustc_driver_impl[5af842e777d449e6]::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
  31:     0x7ff8ebc6e398 - rustc_interface[eaf9095e0de5b742]::interface::run_compiler::<(), rustc_driver_impl[5af842e777d449e6]::run_compiler::{closure#0}>::{closure#1}
  32:     0x7ff8ebaf7e8e - std[eaa43e369792efb9]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[eaf9095e0de5b742]::util::run_in_thread_with_globals<rustc_interface[eaf9095e0de5b742]::util::run_in_thread_pool_with_globals<rustc_interface[eaf9095e0de5b742]::interface::run_compiler<(), rustc_driver_impl[5af842e777d449e6]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  33:     0x7ff8ebaf7c60 - <std[eaa43e369792efb9]::thread::lifecycle::spawn_unchecked<rustc_interface[eaf9095e0de5b742]::util::run_in_thread_with_globals<rustc_interface[eaf9095e0de5b742]::util::run_in_thread_pool_with_globals<rustc_interface[eaf9095e0de5b742]::interface::run_compiler<(), rustc_driver_impl[5af842e777d449e6]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[4141bff0bf9ca9dd]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  34:     0x7ff8ebaf6038 - <std[eaa43e369792efb9]::sys::thread::unix::Thread>::new::thread_start
  35:     0x7ff8e5494ac3 - start_thread
                               at ./nptl/pthread_create.c:442:8
  36:     0x7ff8e55268c0 - __GI___clone3
                               at ./misc/../sysdeps/unix/sysv/linux/x86_64/clone3.S:81:0
  37:                0x0 - <unknown>


rustc version: 1.94.0-nightly (e29fcf45e 2026-01-04)
platform: x86_64-unknown-linux-gnu

query stack during panic:
#0 [items_of_instance] collecting items used by `const_bool::<TypeA>`
#1 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack

[SpriteOvO]

@rustbot label -needs-triage +S-has-mcve

Duplicate of #147507.

More minimal

#![feature(specialization)]

trait Marker {}

struct Ty;
impl Marker for Ty {}

trait Tr {
    fn method();
}

impl<T> Tr for T {
    default fn method() {}
}

impl<T: Marker> Tr for T
where
    T: Tr, // Cycle
{
    fn method() {}
}

fn call<T>() {
    <T as Tr>::method();
}

fn main() {
    call::<Ty>();
}

Backtrace

thread 'rustc' (68) panicked at /rustc-dev/31cd367b9ca1ce359268e7adf4ea540408c0ad85/compiler/rustc_middle/src/ty/instance.rs:579:21:
Box<dyn Any>
stack backtrace:
   0:     0x76aa9f632483 - <<std[5e3fd61f923da6c8]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[63159e14536656d8]::fmt::Display>::fmt
   1:     0x76aa9fc0fb48 - core[63159e14536656d8]::fmt::write
   2:     0x76aa9f648706 - <std[5e3fd61f923da6c8]::sys::stdio::unix::Stderr as std[5e3fd61f923da6c8]::io::Write>::write_fmt
   3:     0x76aa9f608078 - std[5e3fd61f923da6c8]::panicking::default_hook::{closure#0}
   4:     0x76aa9f625d13 - std[5e3fd61f923da6c8]::panicking::default_hook
   5:     0x76aa9e60532a - std[5e3fd61f923da6c8]::panicking::update_hook::<alloc[4cad7486372ceb04]::boxed::Box<rustc_driver_impl[f8637fc8d48c8951]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x76aa9f625ff2 - std[5e3fd61f923da6c8]::panicking::panic_with_hook
   7:     0x76aa9e6426c1 - std[5e3fd61f923da6c8]::panicking::begin_panic::<rustc_errors[52ac9412f380f3dc]::ExplicitBug>::{closure#0}
   8:     0x76aa9e632bf6 - std[5e3fd61f923da6c8]::sys::backtrace::__rust_end_short_backtrace::<std[5e3fd61f923da6c8]::panicking::begin_panic<rustc_errors[52ac9412f380f3dc]::ExplicitBug>::{closure#0}, !>
   9:     0x76aa9e63018c - std[5e3fd61f923da6c8]::panicking::begin_panic::<rustc_errors[52ac9412f380f3dc]::ExplicitBug>
  10:     0x76aa9e660401 - <rustc_errors[52ac9412f380f3dc]::diagnostic::BugAbort as rustc_errors[52ac9412f380f3dc]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  11:     0x76aa9ebedbec - <rustc_errors[52ac9412f380f3dc]::DiagCtxtHandle>::span_bug::<rustc_span[645e6eaf91144e9d]::span_encoding::Span, alloc[4cad7486372ceb04]::string::String>
  12:     0x76aa9ec106d6 - rustc_middle[67562721e1b05fd6]::util::bug::opt_span_bug_fmt::<rustc_span[645e6eaf91144e9d]::span_encoding::Span>::{closure#0}
  13:     0x76aa9ec10882 - rustc_middle[67562721e1b05fd6]::ty::context::tls::with_opt::<rustc_middle[67562721e1b05fd6]::util::bug::opt_span_bug_fmt<rustc_span[645e6eaf91144e9d]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  14:     0x76aa9ec0199b - rustc_middle[67562721e1b05fd6]::ty::context::tls::with_context_opt::<rustc_middle[67562721e1b05fd6]::ty::context::tls::with_opt<rustc_middle[67562721e1b05fd6]::util::bug::opt_span_bug_fmt<rustc_span[645e6eaf91144e9d]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  15:     0x76aa9ccf0cb8 - rustc_middle[67562721e1b05fd6]::util::bug::span_bug_fmt::<rustc_span[645e6eaf91144e9d]::span_encoding::Span>
  16:     0x76aaa0338637 - <rustc_middle[67562721e1b05fd6]::ty::instance::Instance>::expect_resolve
  17:     0x76aaa0646afd - <rustc_monomorphize[790bc95bcdb39fc2]::collector::MirUsedCollector as rustc_middle[67562721e1b05fd6]::mir::visit::Visitor>::visit_terminator
  18:     0x76aaa0640379 - rustc_monomorphize[790bc95bcdb39fc2]::collector::items_of_instance
  19:     0x76aaa063fd72 - rustc_query_impl[c81645f40ad5ab79]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[c81645f40ad5ab79]::query_impl::items_of_instance::dynamic_query::{closure#2}::{closure#0}, rustc_middle[67562721e1b05fd6]::query::erase::Erased<[u8; 32usize]>>
  20:     0x76aaa08d2c54 - rustc_query_system[1476958347158890]::query::plumbing::try_execute_query::<rustc_query_impl[c81645f40ad5ab79]::DynamicConfig<rustc_query_system[1476958347158890]::query::caches::DefaultCache<(rustc_middle[67562721e1b05fd6]::ty::instance::Instance, rustc_middle[67562721e1b05fd6]::mir::mono::CollectionMode), rustc_middle[67562721e1b05fd6]::query::erase::Erased<[u8; 32usize]>>, false, false, false>, rustc_query_impl[c81645f40ad5ab79]::plumbing::QueryCtxt, false>
  21:     0x76aaa08d2873 - rustc_query_impl[c81645f40ad5ab79]::query_impl::items_of_instance::get_query_non_incr::__rust_end_short_backtrace
  22:     0x76aaa10301ee - rustc_monomorphize[790bc95bcdb39fc2]::collector::collect_items_rec
  23:     0x76aaa1032141 - rustc_monomorphize[790bc95bcdb39fc2]::collector::collect_items_rec
  24:     0x76aaa0012ba1 - rustc_monomorphize[790bc95bcdb39fc2]::collector::collect_crate_mono_items::{closure#1}::{closure#0}
  25:     0x76aaa02fd414 - rustc_monomorphize[790bc95bcdb39fc2]::partitioning::collect_and_partition_mono_items
  26:     0x76aaa10ce516 - rustc_query_impl[c81645f40ad5ab79]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[c81645f40ad5ab79]::query_impl::collect_and_partition_mono_items::dynamic_query::{closure#2}::{closure#0}, rustc_middle[67562721e1b05fd6]::query::erase::Erased<[u8; 24usize]>>
  27:     0x76aaa10ce4db - <rustc_query_impl[c81645f40ad5ab79]::query_impl::collect_and_partition_mono_items::dynamic_query::{closure#2} as core[63159e14536656d8]::ops::function::FnOnce<(rustc_middle[67562721e1b05fd6]::ty::context::TyCtxt, ())>>::call_once
  28:     0x76aaa10ce036 - rustc_query_system[1476958347158890]::query::plumbing::try_execute_query::<rustc_query_impl[c81645f40ad5ab79]::DynamicConfig<rustc_query_system[1476958347158890]::query::caches::SingleCache<rustc_middle[67562721e1b05fd6]::query::erase::Erased<[u8; 24usize]>>, false, false, false>, rustc_query_impl[c81645f40ad5ab79]::plumbing::QueryCtxt, false>
  29:     0x76aaa10cdddb - rustc_query_impl[c81645f40ad5ab79]::query_impl::collect_and_partition_mono_items::get_query_non_incr::__rust_end_short_backtrace
  30:     0x76aaa0fab79c - rustc_codegen_ssa[423cc1a0b32ad6d6]::base::codegen_crate::<rustc_codegen_llvm[4607a12f9ea27dae]::LlvmCodegenBackend>
  31:     0x76aaa0fa927d - <rustc_codegen_llvm[4607a12f9ea27dae]::LlvmCodegenBackend as rustc_codegen_ssa[423cc1a0b32ad6d6]::traits::backend::CodegenBackend>::codegen_crate
  32:     0x76aaa0f6358f - <rustc_interface[512530dc4d7ac68]::queries::Linker>::codegen_and_build_linker
  33:     0x76aaa0f60b9c - <rustc_interface[512530dc4d7ac68]::passes::create_and_enter_global_ctxt<core[63159e14536656d8]::option::Option<rustc_interface[512530dc4d7ac68]::queries::Linker>, rustc_driver_impl[f8637fc8d48c8951]::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core[63159e14536656d8]::ops::function::FnOnce<(&rustc_session[4eb1ea45e01dbc75]::session::Session, rustc_middle[67562721e1b05fd6]::ty::context::CurrentGcx, alloc[4cad7486372ceb04]::sync::Arc<rustc_data_structures[242ff0c0bfa75374]::jobserver::Proxy>, &std[5e3fd61f923da6c8]::sync::once_lock::OnceLock<rustc_middle[67562721e1b05fd6]::ty::context::GlobalCtxt>, &rustc_data_structures[242ff0c0bfa75374]::sync::worker_local::WorkerLocal<rustc_middle[67562721e1b05fd6]::arena::Arena>, &rustc_data_structures[242ff0c0bfa75374]::sync::worker_local::WorkerLocal<rustc_hir[4cefefb946bd3673]::Arena>, rustc_driver_impl[f8637fc8d48c8951]::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
  34:     0x76aaa0eb68d8 - rustc_interface[512530dc4d7ac68]::interface::run_compiler::<(), rustc_driver_impl[f8637fc8d48c8951]::run_compiler::{closure#0}>::{closure#1}
  35:     0x76aaa0d10ace - std[5e3fd61f923da6c8]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[512530dc4d7ac68]::util::run_in_thread_with_globals<rustc_interface[512530dc4d7ac68]::util::run_in_thread_pool_with_globals<rustc_interface[512530dc4d7ac68]::interface::run_compiler<(), rustc_driver_impl[f8637fc8d48c8951]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  36:     0x76aaa0d108a0 - <std[5e3fd61f923da6c8]::thread::lifecycle::spawn_unchecked<rustc_interface[512530dc4d7ac68]::util::run_in_thread_with_globals<rustc_interface[512530dc4d7ac68]::util::run_in_thread_pool_with_globals<rustc_interface[512530dc4d7ac68]::interface::run_compiler<(), rustc_driver_impl[f8637fc8d48c8951]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[63159e14536656d8]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  37:     0x76aaa0d0ec78 - <std[5e3fd61f923da6c8]::sys::thread::unix::Thread>::new::thread_start
  38:     0x76aa9a88aaa4 - <unknown>
  39:     0x76aa9a917a64 - clone
  40:                0x0 - <unknown>