marker traits can result in ambiguity during monomorphization

#![feature(marker_trait_attr)]

#[marker]
trait Trait {}
impl<'a, T: 'static> Trait for (T, &'a u32) {}
impl<'a, T: 'static> Trait for (&'a u32, T) {}
trait Other {
    fn method() {}
}
impl<T: Trait> Other for T {}

fn foo<T: 'static>() {
    <(T, &u32)>::method();
}

fn main() {
    foo::<&u32>();
}

while (T, &u32): Trait has an unambiguous choice, (&u32, &u32): Trait has two choices, each of which results in different lifetime constraints. This results in ambiguity when monomorphizing foo, resulting in

error: internal compiler error: compiler/rustc_middle/src/ty/instance.rs:579:21: failed to resolve instance for <(&u32, &u32) as Other>::method
  --> src/main.rs:16:5
   |
16 |     <(T, &u32)>::method();
   |     ^^^^^^^^^^^^^^^^^^^^^


thread 'rustc' (2044881) panicked at compiler/rustc_middle/src/ty/instance.rs:579:21:
Box<dyn Any>
stack backtrace:
   0:     0x7f311f870583 - <<std[f73caa43acb03b8c]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[1f5c4c040b19307]::fmt::Display>::fmt
   1:     0x7f311fe11bc8 - core[1f5c4c040b19307]::fmt::write
   2:     0x7f311f886ef6 - <std[f73caa43acb03b8c]::sys::stdio::unix::Stderr as std[f73caa43acb03b8c]::io::Write>::write_fmt
   3:     0x7f311f846fa3 - std[f73caa43acb03b8c]::panicking::default_hook::{closure#0}
   4:     0x7f311f8666b0 - std[f73caa43acb03b8c]::panicking::default_hook
   5:     0x7f311e8cd527 - std[f73caa43acb03b8c]::panicking::update_hook::<alloc[31570ea252afb3e]::boxed::Box<rustc_driver_impl[a62c041d93dad5ac]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7f311f866992 - std[f73caa43acb03b8c]::panicking::panic_with_hook
   7:     0x7f311e9094e1 - std[f73caa43acb03b8c]::panicking::begin_panic::<rustc_errors[e47345eadeff1cc0]::ExplicitBug>::{closure#0}
   8:     0x7f311e8f9e06 - std[f73caa43acb03b8c]::sys::backtrace::__rust_end_short_backtrace::<std[f73caa43acb03b8c]::panicking::begin_panic<rustc_errors[e47345eadeff1cc0]::ExplicitBug>::{closure#0}, !>
   9:     0x7f311e8f9a4a - std[f73caa43acb03b8c]::panicking::begin_panic::<rustc_errors[e47345eadeff1cc0]::ExplicitBug>
  10:     0x7f311e926df1 - <rustc_errors[e47345eadeff1cc0]::diagnostic::BugAbort as rustc_errors[e47345eadeff1cc0]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  11:     0x7f311ee670bc - <rustc_errors[e47345eadeff1cc0]::DiagCtxtHandle>::span_bug::<rustc_span[4312f27e8c783d4a]::span_encoding::Span, alloc[31570ea252afb3e]::string::String>
  12:     0x7f311ee88486 - rustc_middle[55db46a3a9e6fd01]::util::bug::opt_span_bug_fmt::<rustc_span[4312f27e8c783d4a]::span_encoding::Span>::{closure#0}
  13:     0x7f311ee88632 - rustc_middle[55db46a3a9e6fd01]::ty::context::tls::with_opt::<rustc_middle[55db46a3a9e6fd01]::util::bug::opt_span_bug_fmt<rustc_span[4312f27e8c783d4a]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  14:     0x7f311ee7979b - rustc_middle[55db46a3a9e6fd01]::ty::context::tls::with_context_opt::<rustc_middle[55db46a3a9e6fd01]::ty::context::tls::with_opt<rustc_middle[55db46a3a9e6fd01]::util::bug::opt_span_bug_fmt<rustc_span[4312f27e8c783d4a]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  15:     0x7f311cf14878 - rustc_middle[55db46a3a9e6fd01]::util::bug::span_bug_fmt::<rustc_span[4312f27e8c783d4a]::span_encoding::Span>
  16:     0x7f312004476d - <rustc_middle[55db46a3a9e6fd01]::ty::instance::Instance>::expect_resolve
  17:     0x7f311d058c56 - rustc_monomorphize[6bc2048e015f6b8e]::collector::items_of_instance
  18:     0x7f311ffdb946 - rustc_query_impl[dd57941d79eef654]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[dd57941d79eef654]::query_impl::items_of_instance::dynamic_query::{closure#2}::{closure#0}, rustc_middle[55db46a3a9e6fd01]::query::erase::Erased<[u8; 32usize]>>
  19:     0x7f311ffda417 - rustc_query_system[fbdf9c15bf535051]::query::plumbing::try_execute_query::<rustc_query_impl[dd57941d79eef654]::DynamicConfig<rustc_query_system[fbdf9c15bf535051]::query::caches::DefaultCache<(rustc_middle[55db46a3a9e6fd01]::ty::instance::Instance, rustc_middle[55db46a3a9e6fd01]::mir::mono::CollectionMode), rustc_middle[55db46a3a9e6fd01]::query::erase::Erased<[u8; 32usize]>>, false, false, false>, rustc_query_impl[dd57941d79eef654]::plumbing::QueryCtxt, true>
  20:     0x7f311ffd8ce7 - rustc_query_impl[dd57941d79eef654]::query_impl::items_of_instance::get_query_incr::__rust_end_short_backtrace
  21:     0x7f312085ff99 - rustc_monomorphize[6bc2048e015f6b8e]::collector::collect_items_rec::{closure#0}
  22:     0x7f31211d2417 - rustc_monomorphize[6bc2048e015f6b8e]::collector::collect_items_rec
  23:     0x7f31211d55d4 - rustc_monomorphize[6bc2048e015f6b8e]::collector::collect_items_rec
  24:     0x7f31201851e3 - rustc_monomorphize[6bc2048e015f6b8e]::collector::collect_crate_mono_items::{closure#1}::{closure#0}
  25:     0x7f312082c4ba - rustc_monomorphize[6bc2048e015f6b8e]::partitioning::collect_and_partition_mono_items
  26:     0x7f31211ef616 - rustc_query_impl[dd57941d79eef654]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[dd57941d79eef654]::query_impl::collect_and_partition_mono_items::dynamic_query::{closure#2}::{closure#0}, rustc_middle[55db46a3a9e6fd01]::query::erase::Erased<[u8; 24usize]>>
  27:     0x7f31211ef5dd - <rustc_query_impl[dd57941d79eef654]::query_impl::collect_and_partition_mono_items::dynamic_query::{closure#2} as core[1f5c4c040b19307]::ops::function::FnOnce<(rustc_middle[55db46a3a9e6fd01]::ty::context::TyCtxt, ())>>::call_once
  28:     0x7f31211ee45c - rustc_query_system[fbdf9c15bf535051]::query::plumbing::try_execute_query::<rustc_query_impl[dd57941d79eef654]::DynamicConfig<rustc_query_system[fbdf9c15bf535051]::query::caches::SingleCache<rustc_middle[55db46a3a9e6fd01]::query::erase::Erased<[u8; 24usize]>>, false, false, false>, rustc_query_impl[dd57941d79eef654]::plumbing::QueryCtxt, true>
  29:     0x7f31211edf88 - rustc_query_impl[dd57941d79eef654]::query_impl::collect_and_partition_mono_items::get_query_incr::__rust_end_short_backtrace
  30:     0x7f312106ce29 - rustc_codegen_ssa[8f1f60f4942dc47f]::base::codegen_crate::<rustc_codegen_llvm[b9ffd0f94873fbe0]::LlvmCodegenBackend>
  31:     0x7f312106a8b3 - <rustc_codegen_llvm[b9ffd0f94873fbe0]::LlvmCodegenBackend as rustc_codegen_ssa[8f1f60f4942dc47f]::traits::backend::CodegenBackend>::codegen_crate
  32:     0x7f3120e6aa0c - <rustc_interface[414c0bf7369858c6]::queries::Linker>::codegen_and_build_linker
  33:     0x7f31210d9b1b - <rustc_interface[414c0bf7369858c6]::passes::create_and_enter_global_ctxt<core[1f5c4c040b19307]::option::Option<rustc_interface[414c0bf7369858c6]::queries::Linker>, rustc_driver_impl[a62c041d93dad5ac]::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core[1f5c4c040b19307]::ops::function::FnOnce<(&rustc_session[349a198c03ceb088]::session::Session, rustc_middle[55db46a3a9e6fd01]::ty::context::CurrentGcx, alloc[31570ea252afb3e]::sync::Arc<rustc_data_structures[734b602c71f49520]::jobserver::Proxy>, &std[f73caa43acb03b8c]::sync::once_lock::OnceLock<rustc_middle[55db46a3a9e6fd01]::ty::context::GlobalCtxt>, &rustc_data_structures[734b602c71f49520]::sync::worker_local::WorkerLocal<rustc_middle[55db46a3a9e6fd01]::arena::Arena>, &rustc_data_structures[734b602c71f49520]::sync::worker_local::WorkerLocal<rustc_hir[81959f50af77ce73]::Arena>, rustc_driver_impl[a62c041d93dad5ac]::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
  34:     0x7f3120f22e05 - rustc_interface[414c0bf7369858c6]::interface::run_compiler::<(), rustc_driver_impl[a62c041d93dad5ac]::run_compiler::{closure#0}>::{closure#1}
  35:     0x7f3120edc914 - std[f73caa43acb03b8c]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[414c0bf7369858c6]::util::run_in_thread_with_globals<rustc_interface[414c0bf7369858c6]::util::run_in_thread_pool_with_globals<rustc_interface[414c0bf7369858c6]::interface::run_compiler<(), rustc_driver_impl[a62c041d93dad5ac]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  36:     0x7f3120edc61e - <<std[f73caa43acb03b8c]::thread::Builder>::spawn_unchecked_<rustc_interface[414c0bf7369858c6]::util::run_in_thread_with_globals<rustc_interface[414c0bf7369858c6]::util::run_in_thread_pool_with_globals<rustc_interface[414c0bf7369858c6]::interface::run_compiler<(), rustc_driver_impl[a62c041d93dad5ac]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[1f5c4c040b19307]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  37:     0x7f3120ee242f - <std[f73caa43acb03b8c]::sys::thread::unix::Thread>::new::thread_start
  38:     0x7f311aa9caa4 - start_thread
                               at ./nptl/pthread_create.c:447:8
  39:     0x7f311ab29c6c - clone3
                               at ./misc/../sysdeps/unix/sysv/linux/x86_64/clone3.S:78:0
  40:                0x0 - <unknown>

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 `/home/lcnr/test1/rustc-ice-2025-12-01T12_13_29-2044879.txt` to your bug report

note: compiler flags: --crate-type bin -C embed-bitcode=no -C debuginfo=2 -C incremental=[REDACTED]

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

query stack during panic:
#0 [items_of_instance] collecting items used by `foo::<&u32>`
#1 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack

Start by compiling the provided reproducer and inspect compiler/rustc_middle/src/ty/instance.rs around line 579, then follow the items_of_instance query into rustc_monomorphize. Confirm the ambiguity during monomorphization and the resulting ICE. Done means the example no longer panics and receives an appropriate compiler result or diagnostic.