ICE: `.. was equal to .. during selection but now it is not`

[matthiaskrgr]

snippet:

#![feature(inherent_associated_types)]
struct Foo<T>(T);

impl<'a> Foo<fn(&'a ())> {
    type Assoc = &mut Peekable<I>;
}

fn bar(_: for<'a> fn(Foo<fn(Foo<fn(&'a ())>::Assoc)>::Assoc)) {}

pub fn main() {}

Version information

rustc 1.89.0-nightly (8a65ee082 2025-06-19)
binary: rustc
commit-hash: 8a65ee08296b36342bf7c3cdc15312ccbc357227
commit-date: 2025-06-19
host: x86_64-unknown-linux-gnu
release: 1.89.0-nightly
LLVM version: 20.1.7

Possibly related line of code:

rust/compiler/rustc_trait_selection/src/traits/project.rs

Lines 599 to 611 in 8a65ee0

	);
	}
	match selcx.infcx.at(&cause, param_env).eq(DefineOpaqueTypes::Yes, impl_ty, self_ty) {
	Ok(mut ok) => obligations.append(&mut ok.obligations),
	Err(_) => {
	tcx.dcx().span_bug(
	cause.span,
	format!("{self_ty:?} was equal to {impl_ty:?} during selection but now it is not"),
	);
	}
	}

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc -Zcrate-attr=feature(inherent_associated_types)

Program output

error[E0637]: `&` without an explicit lifetime name cannot be used here
 --> /tmp/icemaker_global_tempdir.i7gCAV0RIrd2/rustc_testrunner_tmpdir_reporting.UHeEfyVeQdiK/mvce.rs:4:18
  |
4 |     type Assoc = &mut Peekable<I>;
  |                  ^ explicit lifetime name needed here

error[E0412]: cannot find type `Peekable` in this scope
 --> /tmp/icemaker_global_tempdir.i7gCAV0RIrd2/rustc_testrunner_tmpdir_reporting.UHeEfyVeQdiK/mvce.rs:4:23
  |
4 |     type Assoc = &mut Peekable<I>;
  |                       ^^^^^^^^ not found in this scope
  |
help: consider importing this struct
  |
1 + use std::iter::Peekable;
  |

error[E0412]: cannot find type `I` in this scope
 --> /tmp/icemaker_global_tempdir.i7gCAV0RIrd2/rustc_testrunner_tmpdir_reporting.UHeEfyVeQdiK/mvce.rs:4:32
  |
4 |     type Assoc = &mut Peekable<I>;
  |                                ^ not found in this scope
  |
help: you might be missing a type parameter
  |
3 | impl<'a, I> Foo<fn(&'a ())> {
  |        +++

warning: the feature `inherent_associated_types` is incomplete and may not be safe to use and/or cause compiler crashes
 --> <crate attribute>:1:12
  |
1 | #![feature(inherent_associated_types)]
  |            ^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #8995 <https://github.com/rust-lang/rust/issues/8995> for more information
  = note: `#[warn(incomplete_features)]` on by default

error[E0601]: `main` function not found in crate `mvce`
 --> /tmp/icemaker_global_tempdir.i7gCAV0RIrd2/rustc_testrunner_tmpdir_reporting.UHeEfyVeQdiK/mvce.rs:7:65
  |
7 | fn bar(_: for<'a> fn(Foo<fn(Foo<fn(&'a ())>::Assoc)>::Assoc)) {}
  |                                                                 ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.i7gCAV0RIrd2/rustc_testrunner_tmpdir_reporting.UHeEfyVeQdiK/mvce.rs`

error: internal compiler error: Foo<Binder { value: fn(&'{region error} mut {type error}), bound_vars: [] }> was equal to Foo<Binder { value: fn(&'?1 ()), bound_vars: [] }> during selection but now it is not
 --> /tmp/icemaker_global_tempdir.i7gCAV0RIrd2/rustc_testrunner_tmpdir_reporting.UHeEfyVeQdiK/mvce.rs:7:11
  |
7 | fn bar(_: for<'a> fn(Foo<fn(Foo<fn(&'a ())>::Assoc)>::Assoc)) {}
  |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


thread 'rustc' panicked at compiler/rustc_trait_selection/src/traits/project.rs:605:23:
Box<dyn Any>
stack backtrace:
   0:     0x758dc0ae7d63 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::hddb5e99560778aed
   1:     0x758dc1202c7f - core::fmt::write::h12c234f0edc7b706
   2:     0x758dc0add8e3 - std::io::Write::write_fmt::h1c65d113e08eaaff
   3:     0x758dc0ae7bc2 - std::sys::backtrace::BacktraceLock::print::h9652e925cb317248
   4:     0x758dc0aeb6ba - std::panicking::default_hook::{{closure}}::h58c1e754bf819be3
   5:     0x758dc0aeb23f - std::panicking::default_hook::hc5c32bd16ea0a105
   6:     0x758dbfbdecc7 - std[cbbb60a5cfd7ff5b]::panicking::update_hook::<alloc[e39721c0a904e78d]::boxed::Box<rustc_driver_impl[8279e16dc5823618]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x758dc0aebf13 - std::panicking::rust_panic_with_hook::h87286a060ae3c579
   8:     0x758dbfc1bf01 - std[cbbb60a5cfd7ff5b]::panicking::begin_panic::<rustc_errors[9915c3893ed9e801]::ExplicitBug>::{closure#0}
   9:     0x758dbfc103a6 - std[cbbb60a5cfd7ff5b]::sys::backtrace::__rust_end_short_backtrace::<std[cbbb60a5cfd7ff5b]::panicking::begin_panic<rustc_errors[9915c3893ed9e801]::ExplicitBug>::{closure#0}, !>
  10:     0x758dbfc10097 - std[cbbb60a5cfd7ff5b]::panicking::begin_panic::<rustc_errors[9915c3893ed9e801]::ExplicitBug>
  11:     0x758dbfc25c51 - <rustc_errors[9915c3893ed9e801]::diagnostic::BugAbort as rustc_errors[9915c3893ed9e801]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  12:     0x758dc0820320 - <rustc_errors[9915c3893ed9e801]::DiagCtxtHandle>::span_bug::<rustc_span[6b6fbcae0de9ce85]::span_encoding::Span, alloc[e39721c0a904e78d]::string::String>
  13:     0x758dc0984aa8 - rustc_trait_selection[afbea2bae8dda0a8]::traits::project::compute_inherent_assoc_term_args
  14:     0x758dc09a7397 - rustc_trait_selection[afbea2bae8dda0a8]::traits::project::normalize_inherent_projection
  15:     0x758dc09a7249 - <rustc_trait_selection[afbea2bae8dda0a8]::traits::normalize::AssocTypeNormalizer>::normalize_inherent_projection::{closure#0}
  16:     0x758dc14c1ff1 - <rustc_trait_selection[afbea2bae8dda0a8]::traits::normalize::AssocTypeNormalizer as rustc_type_ir[f425777cba5ad9d]::fold::TypeFolder<rustc_middle[93a0193236758a7b]::ty::context::TyCtxt>>::fold_ty
  17:     0x758dc14bb714 - <&rustc_middle[93a0193236758a7b]::ty::list::RawList<(), rustc_middle[93a0193236758a7b]::ty::Ty> as rustc_type_ir[f425777cba5ad9d]::fold::TypeFoldable<rustc_middle[93a0193236758a7b]::ty::context::TyCtxt>>::fold_with::<rustc_trait_selection[afbea2bae8dda0a8]::traits::normalize::AssocTypeNormalizer>
  18:     0x758dc14c0262 - <rustc_middle[93a0193236758a7b]::ty::Ty as rustc_type_ir[f425777cba5ad9d]::fold::TypeSuperFoldable<rustc_middle[93a0193236758a7b]::ty::context::TyCtxt>>::super_fold_with::<rustc_trait_selection[afbea2bae8dda0a8]::traits::normalize::AssocTypeNormalizer>
  19:     0x758dc1780e81 - <rustc_hir_analysis[1a09115eb8831c21]::check::wfcheck::WfCheckingCtxt>::deeply_normalize::<rustc_middle[93a0193236758a7b]::ty::Ty>
  20:     0x758dc17824f3 - rustc_hir_analysis[1a09115eb8831c21]::check::wfcheck::check_fn_or_method
  21:     0x758dc19d0dce - rustc_hir_analysis[1a09115eb8831c21]::check::wfcheck::check_item_fn
  22:     0x758dc19d4928 - rustc_hir_analysis[1a09115eb8831c21]::check::wfcheck::check_item
  23:     0x758dc19cbcd9 - rustc_hir_analysis[1a09115eb8831c21]::check::wfcheck::check_well_formed
  24:     0x758dc19cbb59 - rustc_query_impl[fbc74060fac8c140]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[fbc74060fac8c140]::query_impl::check_well_formed::dynamic_query::{closure#2}::{closure#0}, rustc_middle[93a0193236758a7b]::query::erase::Erased<[u8; 1usize]>>
  25:     0x758dc19cb38d - rustc_query_system[f33ca17fd4fddce8]::query::plumbing::try_execute_query::<rustc_query_impl[fbc74060fac8c140]::DynamicConfig<rustc_data_structures[4ddca31ccfde7627]::vec_cache::VecCache<rustc_span[6b6fbcae0de9ce85]::def_id::LocalDefId, rustc_middle[93a0193236758a7b]::query::erase::Erased<[u8; 1usize]>, rustc_query_system[f33ca17fd4fddce8]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[fbc74060fac8c140]::plumbing::QueryCtxt, false>
  26:     0x758dc19caec2 - rustc_query_impl[fbc74060fac8c140]::query_impl::check_well_formed::get_query_non_incr::__rust_end_short_backtrace
  27:     0x758dc19c865f - rustc_hir_analysis[1a09115eb8831c21]::check::wfcheck::check_type_wf
  28:     0x758dc19c856f - rustc_query_impl[fbc74060fac8c140]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[fbc74060fac8c140]::query_impl::check_type_wf::dynamic_query::{closure#2}::{closure#0}, rustc_middle[93a0193236758a7b]::query::erase::Erased<[u8; 1usize]>>
  29:     0x758dc226fe88 - rustc_query_system[f33ca17fd4fddce8]::query::plumbing::try_execute_query::<rustc_query_impl[fbc74060fac8c140]::DynamicConfig<rustc_query_system[f33ca17fd4fddce8]::query::caches::SingleCache<rustc_middle[93a0193236758a7b]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[fbc74060fac8c140]::plumbing::QueryCtxt, false>
  30:     0x758dc226fc82 - rustc_query_impl[fbc74060fac8c140]::query_impl::check_type_wf::get_query_non_incr::__rust_end_short_backtrace
  31:     0x758dc144c0de - rustc_hir_analysis[1a09115eb8831c21]::check_crate
  32:     0x758dc1c6fb59 - rustc_interface[c880869786bb6952]::passes::analysis
  33:     0x758dc1c6f76d - rustc_query_impl[fbc74060fac8c140]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[fbc74060fac8c140]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[93a0193236758a7b]::query::erase::Erased<[u8; 0usize]>>
  34:     0x758dc226994e - rustc_query_system[f33ca17fd4fddce8]::query::plumbing::try_execute_query::<rustc_query_impl[fbc74060fac8c140]::DynamicConfig<rustc_query_system[f33ca17fd4fddce8]::query::caches::SingleCache<rustc_middle[93a0193236758a7b]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[fbc74060fac8c140]::plumbing::QueryCtxt, false>
  35:     0x758dc2269536 - rustc_query_impl[fbc74060fac8c140]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  36:     0x758dc24a5b56 - rustc_interface[c880869786bb6952]::passes::create_and_enter_global_ctxt::<core[26657d77fe39fc47]::option::Option<rustc_interface[c880869786bb6952]::queries::Linker>, rustc_driver_impl[8279e16dc5823618]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  37:     0x758dc2466aa3 - rustc_interface[c880869786bb6952]::interface::run_compiler::<(), rustc_driver_impl[8279e16dc5823618]::run_compiler::{closure#0}>::{closure#1}
  38:     0x758dc2444278 - std[cbbb60a5cfd7ff5b]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[c880869786bb6952]::util::run_in_thread_with_globals<rustc_interface[c880869786bb6952]::util::run_in_thread_pool_with_globals<rustc_interface[c880869786bb6952]::interface::run_compiler<(), rustc_driver_impl[8279e16dc5823618]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  39:     0x758dc2443f56 - <<std[cbbb60a5cfd7ff5b]::thread::Builder>::spawn_unchecked_<rustc_interface[c880869786bb6952]::util::run_in_thread_with_globals<rustc_interface[c880869786bb6952]::util::run_in_thread_pool_with_globals<rustc_interface[c880869786bb6952]::interface::run_compiler<(), rustc_driver_impl[8279e16dc5823618]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[26657d77fe39fc47]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  40:     0x758dc244253d - std::sys::pal::unix::thread::Thread::new::thread_start::h62db728285f9da4a
  41:     0x758dbbea57eb - <unknown>
  42:     0x758dbbf2918c - <unknown>
  43:                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: rustc 1.89.0-nightly (8a65ee082 2025-06-19) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z crate-attr=feature(inherent_associated_types) -Z dump-mir-dir=dir

query stack during panic:
#0 [check_well_formed] checking that `bar` is well-formed
#1 [check_type_wf] checking that types are well-formed
#2 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 5 previous errors; 1 warning emitted

Some errors have detailed explanations: E0412, E0601, E0637.
For more information about an error, try `rustc --explain E0412`.

@rustbot label +F-inherent_associated_types

[matthiaskrgr]

bisects to #140247

[fmease]

cc @BoxyUwU