[ICE]: cannot find `N/#0` in param-env: ParamEnv

[matthiaskrgr]

auto-reduced (treereduce-rust):

struct S<const N: usize>([(); N]);

impl<const N: usize> S<N> {
    fn foo() -> [(); const {
           let _: Self = loop {};
           1
       }] {
        todo!()
    }
}

original:

struct S<const N: usize>([(); N]);

impl<const N: usize> S<N> {
    fn foo() -> [(); const {
           let _: Self = loop {};
           1
       }] {
        todo!()
    }
}

fn main() {}

Version information

rustc 1.96.0-nightly (f66622c7e 2026-03-23)
binary: rustc
commit-hash: f66622c7eca7fc48ccc4dac848ff911b09a4d566
commit-date: 2026-03-23
host: x86_64-unknown-linux-gnu
release: 1.96.0-nightly
LLVM version: 22.1.0

Possibly related line of code:

rust/compiler/rustc_middle/src/ty/sty.rs

Lines 346 to 358 in f66622c

	// `Option<Ty<'tcx>>` instead of `Ty<'tcx>`; however, this is generally
	// considered to be a bandaid solution, since it hides more important
	// underlying issues with how we construct generics and predicates of
	// items. It's advised to fix the underlying issue rather than trying
	// to modify this function.
	let ty = candidates.next().unwrap_or_else(|| {
	bug!("cannot find `{self:?}` in param-env: {env:#?}");
	});
	assert!(
	candidates.next().is_none(),
	"did not expect duplicate `ConstParamHasTy` for `{self:?}` in param-env: {env:#?}"
	);
	ty

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc

Program output

error[E0601]: `main` function not found in crate `mvce`
  --> /tmp/icemaker_global_tempdir.V83zeUDD57C1/rustc_testrunner_tmpdir_reporting.T7QySKNN0ONt/mvce.rs:10:2
   |
10 | }
   |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.V83zeUDD57C1/rustc_testrunner_tmpdir_reporting.T7QySKNN0ONt/mvce.rs`

warning: unreachable expression
 --> /tmp/icemaker_global_tempdir.V83zeUDD57C1/rustc_testrunner_tmpdir_reporting.T7QySKNN0ONt/mvce.rs:6:12
  |
5 |            let _: Self = loop {};
  |                          ------- any code following this expression is unreachable
6 |            1
  |            ^ unreachable expression
  |
  = note: `#[warn(unreachable_code)]` (part of `#[warn(unused)]`) on by default

error: internal compiler error: /rustc-dev/f66622c7eca7fc48ccc4dac848ff911b09a4d566/compiler/rustc_middle/src/ty/sty.rs:352:13: cannot find `N/#0` in param-env: ParamEnv {
                                    caller_bounds: [],
                                }


thread 'rustc' (2782697) panicked at /rustc-dev/f66622c7eca7fc48ccc4dac848ff911b09a4d566/compiler/rustc_middle/src/ty/sty.rs:352:13:
Box<dyn Any>
stack backtrace:
   0:     0x7f4c665ae7eb - <<std[dbd160d25717361c]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[af451d9ab418ad9]::fmt::Display>::fmt
   1:     0x7f4c66c2f708 - core[af451d9ab418ad9]::fmt::write
   2:     0x7f4c665c57f6 - <std[dbd160d25717361c]::sys::stdio::unix::Stderr as std[dbd160d25717361c]::io::Write>::write_fmt
   3:     0x7f4c665847f8 - std[dbd160d25717361c]::panicking::default_hook::{closure#0}
   4:     0x7f4c665a1bf3 - std[dbd160d25717361c]::panicking::default_hook
   5:     0x7f4c6559e53c - std[dbd160d25717361c]::panicking::update_hook::<alloc[fbe1da7a96f3d929]::boxed::Box<rustc_driver_impl[11714a0f3389eb15]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7f4c665a1ed2 - std[dbd160d25717361c]::panicking::panic_with_hook
   7:     0x7f4c655cdbc1 - std[dbd160d25717361c]::panicking::begin_panic::<rustc_errors[e98ce2b093ebe336]::ExplicitBug>::{closure#0}
   8:     0x7f4c655c6626 - std[dbd160d25717361c]::sys::backtrace::__rust_end_short_backtrace::<std[dbd160d25717361c]::panicking::begin_panic<rustc_errors[e98ce2b093ebe336]::ExplicitBug>::{closure#0}, !>
   9:     0x7f4c655c660b - std[dbd160d25717361c]::panicking::begin_panic::<rustc_errors[e98ce2b093ebe336]::ExplicitBug>
  10:     0x7f4c655d9231 - <rustc_errors[e98ce2b093ebe336]::diagnostic::BugAbort as rustc_errors[e98ce2b093ebe336]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  11:     0x7f4c65bb84e9 - rustc_middle[b076dae3d4f5c3b1]::util::bug::opt_span_bug_fmt::<rustc_span[6d146ac66dea4702]::span_encoding::Span>::{closure#0}
  12:     0x7f4c65bb8672 - rustc_middle[b076dae3d4f5c3b1]::ty::context::tls::with_opt::<rustc_middle[b076dae3d4f5c3b1]::util::bug::opt_span_bug_fmt<rustc_span[6d146ac66dea4702]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  13:     0x7f4c65ba914b - rustc_middle[b076dae3d4f5c3b1]::ty::context::tls::with_context_opt::<rustc_middle[b076dae3d4f5c3b1]::ty::context::tls::with_opt<rustc_middle[b076dae3d4f5c3b1]::util::bug::opt_span_bug_fmt<rustc_span[6d146ac66dea4702]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  14:     0x7f4c6331f9c4 - rustc_middle[b076dae3d4f5c3b1]::util::bug::bug_fmt
  15:     0x7f4c670df8de - <rustc_trait_selection[56daeb37355a4237]::traits::fulfill::FulfillProcessor as rustc_data_structures[f30a6dcac191e75f]::obligation_forest::ObligationProcessor>::process_obligation
  16:     0x7f4c66c069c1 - <rustc_data_structures[f30a6dcac191e75f]::obligation_forest::ObligationForest<rustc_trait_selection[56daeb37355a4237]::traits::fulfill::PendingPredicateObligation>>::process_obligations::<rustc_trait_selection[56daeb37355a4237]::traits::fulfill::FulfillProcessor>
  17:     0x7f4c6724021b - rustc_hir_typeck[85596fe57aff474]::typeck_with_inspect::{closure#0}
  18:     0x7f4c6723d2b0 - rustc_query_impl[d8b9270f442bd7e9]::query_impl::typeck::invoke_provider_fn::__rust_begin_short_backtrace
  19:     0x7f4c66ce7926 - rustc_query_impl[d8b9270f442bd7e9]::execution::try_execute_query::<rustc_data_structures[f30a6dcac191e75f]::vec_cache::VecCache<rustc_span[6d146ac66dea4702]::def_id::LocalDefId, rustc_middle[b076dae3d4f5c3b1]::query::erase::ErasedData<[u8; 8usize]>, rustc_middle[b076dae3d4f5c3b1]::dep_graph::graph::DepNodeIndex>, false>
  20:     0x7f4c66ce7383 - rustc_query_impl[d8b9270f442bd7e9]::query_impl::typeck::execute_query_non_incr::__rust_end_short_backtrace
  21:     0x7f4c6765e7a5 - rustc_mir_build[d007e31154e585d0]::thir::pattern::check_match::check_match
  22:     0x7f4c6765dc07 - rustc_query_impl[d8b9270f442bd7e9]::query_impl::check_match::invoke_provider_fn::__rust_begin_short_backtrace
  23:     0x7f4c678bbe24 - rustc_query_impl[d8b9270f442bd7e9]::execution::try_execute_query::<rustc_data_structures[f30a6dcac191e75f]::vec_cache::VecCache<rustc_span[6d146ac66dea4702]::def_id::LocalDefId, rustc_middle[b076dae3d4f5c3b1]::query::erase::ErasedData<[u8; 1usize]>, rustc_middle[b076dae3d4f5c3b1]::dep_graph::graph::DepNodeIndex>, false>
  24:     0x7f4c678bba82 - rustc_query_impl[d8b9270f442bd7e9]::query_impl::check_match::execute_query_non_incr::__rust_end_short_backtrace
  25:     0x7f4c67665b4a - rustc_mir_build[d007e31154e585d0]::builder::build_mir_inner_impl
  26:     0x7f4c66c0b434 - rustc_mir_transform[60db7b1400619a9]::mir_built
  27:     0x7f4c66ce7926 - rustc_query_impl[d8b9270f442bd7e9]::execution::try_execute_query::<rustc_data_structures[f30a6dcac191e75f]::vec_cache::VecCache<rustc_span[6d146ac66dea4702]::def_id::LocalDefId, rustc_middle[b076dae3d4f5c3b1]::query::erase::ErasedData<[u8; 8usize]>, rustc_middle[b076dae3d4f5c3b1]::dep_graph::graph::DepNodeIndex>, false>
  28:     0x7f4c66ce7443 - rustc_query_impl[d8b9270f442bd7e9]::query_impl::mir_built::execute_query_non_incr::__rust_end_short_backtrace
  29:     0x7f4c66f92e43 - rustc_query_impl[d8b9270f442bd7e9]::query_impl::trivial_const::invoke_provider_fn::__rust_begin_short_backtrace
  30:     0x7f4c66f922ee - rustc_query_impl[d8b9270f442bd7e9]::execution::try_execute_query::<rustc_middle[b076dae3d4f5c3b1]::query::caches::DefIdCache<rustc_middle[b076dae3d4f5c3b1]::query::erase::ErasedData<[u8; 32usize]>>, false>
  31:     0x7f4c66f91950 - rustc_query_impl[d8b9270f442bd7e9]::query_impl::trivial_const::execute_query_non_incr::__rust_end_short_backtrace
  32:     0x7f4c67cd597a - rustc_const_eval[7dbf906d1701551b]::const_eval::eval_queries::eval_to_allocation_raw_provider
  33:     0x7f4c67cd29d6 - rustc_query_impl[d8b9270f442bd7e9]::query_impl::eval_to_allocation_raw::invoke_provider_fn::__rust_begin_short_backtrace
  34:     0x7f4c675af357 - rustc_query_impl[d8b9270f442bd7e9]::execution::try_execute_query::<rustc_middle[b076dae3d4f5c3b1]::query::caches::DefaultCache<rustc_middle[b076dae3d4f5c3b1]::ty::PseudoCanonicalInput<rustc_middle[b076dae3d4f5c3b1]::mir::interpret::GlobalId>, rustc_middle[b076dae3d4f5c3b1]::query::erase::ErasedData<[u8; 24usize]>>, false>
  35:     0x7f4c675aed4f - rustc_query_impl[d8b9270f442bd7e9]::query_impl::eval_to_allocation_raw::execute_query_non_incr::__rust_end_short_backtrace
  36:     0x7f4c67121013 - rustc_const_eval[7dbf906d1701551b]::const_eval::valtrees::eval_to_valtree
  37:     0x7f4c67120de6 - <rustc_const_eval[7dbf906d1701551b]::provide::{closure#0} as core[af451d9ab418ad9]::ops::function::FnOnce<(rustc_middle[b076dae3d4f5c3b1]::ty::context::TyCtxt, rustc_middle[b076dae3d4f5c3b1]::ty::PseudoCanonicalInput<rustc_middle[b076dae3d4f5c3b1]::mir::interpret::GlobalId>)>>::call_once
  38:     0x7f4c67120d9c - rustc_query_impl[d8b9270f442bd7e9]::query_impl::eval_to_valtree::invoke_provider_fn::__rust_begin_short_backtrace
  39:     0x7f4c6748d72a - rustc_query_impl[d8b9270f442bd7e9]::execution::try_execute_query::<rustc_middle[b076dae3d4f5c3b1]::query::caches::DefaultCache<rustc_middle[b076dae3d4f5c3b1]::ty::PseudoCanonicalInput<rustc_middle[b076dae3d4f5c3b1]::mir::interpret::GlobalId>, rustc_middle[b076dae3d4f5c3b1]::query::erase::ErasedData<[u8; 16usize]>>, false>
  40:     0x7f4c6748d358 - rustc_query_impl[d8b9270f442bd7e9]::query_impl::eval_to_valtree::execute_query_non_incr::__rust_end_short_backtrace
  41:     0x7f4c63ab43c9 - <rustc_middle[b076dae3d4f5c3b1]::ty::context::TyCtxt>::const_eval_resolve_for_typeck
  42:     0x7f4c66d509ca - rustc_trait_selection[56daeb37355a4237]::traits::try_evaluate_const
  43:     0x7f4c66d54301 - <rustc_trait_selection[56daeb37355a4237]::traits::normalize::AssocTypeNormalizer as rustc_type_ir[51689a520c4c106f]::fold::TypeFolder<rustc_middle[b076dae3d4f5c3b1]::ty::context::TyCtxt>>::fold_const
  44:     0x7f4c66d55db6 - <rustc_middle[b076dae3d4f5c3b1]::ty::Ty as rustc_type_ir[51689a520c4c106f]::fold::TypeSuperFoldable<rustc_middle[b076dae3d4f5c3b1]::ty::context::TyCtxt>>::super_fold_with::<rustc_trait_selection[56daeb37355a4237]::traits::normalize::AssocTypeNormalizer>
  45:     0x7f4c674fde23 - <rustc_hir_analysis[1ead07aac7240e30]::check::wfcheck::WfCheckingCtxt>::deeply_normalize::<rustc_middle[b076dae3d4f5c3b1]::ty::Ty>
  46:     0x7f4c674ff4e9 - rustc_hir_analysis[1ead07aac7240e30]::check::wfcheck::check_fn_or_method
  47:     0x7f4c67507f0a - rustc_hir_analysis[1ead07aac7240e30]::check::wfcheck::check_associated_item
  48:     0x7f4c678be244 - rustc_hir_analysis[1ead07aac7240e30]::check::check::check_item_type
  49:     0x7f4c678bc826 - rustc_hir_analysis[1ead07aac7240e30]::check::wfcheck::check_well_formed
  50:     0x7f4c678bc807 - rustc_query_impl[d8b9270f442bd7e9]::query_impl::check_well_formed::invoke_provider_fn::__rust_begin_short_backtrace
  51:     0x7f4c678bbe24 - rustc_query_impl[d8b9270f442bd7e9]::execution::try_execute_query::<rustc_data_structures[f30a6dcac191e75f]::vec_cache::VecCache<rustc_span[6d146ac66dea4702]::def_id::LocalDefId, rustc_middle[b076dae3d4f5c3b1]::query::erase::ErasedData<[u8; 1usize]>, rustc_middle[b076dae3d4f5c3b1]::dep_graph::graph::DepNodeIndex>, false>
  52:     0x7f4c678bbbbb - rustc_query_impl[d8b9270f442bd7e9]::query_impl::check_well_formed::execute_query_non_incr::__rust_end_short_backtrace
  53:     0x7f4c678b8328 - rustc_hir_analysis[1ead07aac7240e30]::check::wfcheck::check_type_wf
  54:     0x7f4c678b8213 - rustc_query_impl[d8b9270f442bd7e9]::query_impl::check_type_wf::invoke_provider_fn::__rust_begin_short_backtrace
  55:     0x7f4c67e35690 - rustc_query_impl[d8b9270f442bd7e9]::execution::try_execute_query::<rustc_middle[b076dae3d4f5c3b1]::query::caches::SingleCache<rustc_middle[b076dae3d4f5c3b1]::query::erase::ErasedData<[u8; 1usize]>>, false>
  56:     0x7f4c67e35469 - rustc_query_impl[d8b9270f442bd7e9]::query_impl::check_type_wf::execute_query_non_incr::__rust_end_short_backtrace
  57:     0x7f4c66dbf897 - rustc_hir_analysis[1ead07aac7240e30]::check_crate
  58:     0x7f4c66ce40da - rustc_interface[7600c9e4d7bd5078]::passes::analysis
  59:     0x7f4c67e37e09 - rustc_query_impl[d8b9270f442bd7e9]::execution::try_execute_query::<rustc_middle[b076dae3d4f5c3b1]::query::caches::SingleCache<rustc_middle[b076dae3d4f5c3b1]::query::erase::ErasedData<[u8; 0usize]>>, false>
  60:     0x7f4c67e37a75 - rustc_query_impl[d8b9270f442bd7e9]::query_impl::analysis::execute_query_non_incr::__rust_end_short_backtrace
  61:     0x7f4c67e559d4 - rustc_interface[7600c9e4d7bd5078]::interface::run_compiler::<(), rustc_driver_impl[11714a0f3389eb15]::run_compiler::{closure#0}>::{closure#1}
  62:     0x7f4c67e2297e - std[dbd160d25717361c]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[7600c9e4d7bd5078]::util::run_in_thread_with_globals<rustc_interface[7600c9e4d7bd5078]::util::run_in_thread_pool_with_globals<rustc_interface[7600c9e4d7bd5078]::interface::run_compiler<(), rustc_driver_impl[11714a0f3389eb15]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  63:     0x7f4c67e23220 - <std[dbd160d25717361c]::thread::lifecycle::spawn_unchecked<rustc_interface[7600c9e4d7bd5078]::util::run_in_thread_with_globals<rustc_interface[7600c9e4d7bd5078]::util::run_in_thread_pool_with_globals<rustc_interface[7600c9e4d7bd5078]::interface::run_compiler<(), rustc_driver_impl[11714a0f3389eb15]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[af451d9ab418ad9]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  64:     0x7f4c67e240ac - <std[dbd160d25717361c]::sys::thread::unix::Thread>::new::thread_start
  65:     0x7f4c618a597a - <unknown>
  66:     0x7f4c619292bc - <unknown>
  67:                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 1.96.0-nightly (f66622c7e 2026-03-23) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [typeck] type-checking `<impl at /tmp/icemaker_global_tempdir.V83zeUDD57C1/rustc_testrunner_tmpdir_reporting.T7QySKNN0ONt/mvce.rs:3:1: 3:26>::foo::{constant#0}`
#1 [check_match] match-checking `<impl at /tmp/icemaker_global_tempdir.V83zeUDD57C1/rustc_testrunner_tmpdir_reporting.T7QySKNN0ONt/mvce.rs:3:1: 3:26>::foo::{constant#0}`
#2 [mir_built] building MIR for `<impl at /tmp/icemaker_global_tempdir.V83zeUDD57C1/rustc_testrunner_tmpdir_reporting.T7QySKNN0ONt/mvce.rs:3:1: 3:26>::foo::{constant#0}`
#3 [trivial_const] checking if `<impl at /tmp/icemaker_global_tempdir.V83zeUDD57C1/rustc_testrunner_tmpdir_reporting.T7QySKNN0ONt/mvce.rs:3:1: 3:26>::foo::{constant#0}` is a trivial const
#4 [eval_to_allocation_raw] const-evaluating + checking `<impl at /tmp/icemaker_global_tempdir.V83zeUDD57C1/rustc_testrunner_tmpdir_reporting.T7QySKNN0ONt/mvce.rs:3:1: 3:26>::foo::{constant#0}`
#5 [eval_to_valtree] evaluating type-level constant
#6 [check_well_formed] checking that `<impl at /tmp/icemaker_global_tempdir.V83zeUDD57C1/rustc_testrunner_tmpdir_reporting.T7QySKNN0ONt/mvce.rs:3:1: 3:26>::foo` is well-formed
#7 [check_type_wf] checking that types are well-formed
#8 [analysis] running analysis passes on crate `mvce`
end of query stack
error: aborting due to 2 previous errors; 1 warning emitted

For more information about this error, try `rustc --explain E0601`.

[matthiaskrgr]

before the ICE; this would

error: generic `Self` types are currently not permitted in anonymous constants
 --> a.rs:5:19
  |
5 |            let _: Self = loop {};
  |                   ^^^^
  |
note: not a concrete type
 --> a.rs:3:22
  |
3 | impl<const N: usize> S<N> {
  |                      ^^^^

warning: unreachable expression
 --> a.rs:6:12
  |
5 |            let _: Self = loop {};
  |                          ------- any code following this expression is unreachable
6 |            1
  |            ^ unreachable expression
  |
  = note: `#[warn(unreachable_code)]` (part of `#[warn(unused)]`) on by default

error: aborting due to 1 previous error; 1 warning emitted

[matthiaskrgr]

bisects to #152234

[BoxyUwU]

do you know if it ICEd at some point before #152234? iirc we had some stable behaviour, and then we changed things for mGCA which then effectively got reverted (by that PR) back to the stable behaviour. It's unclear to me whether that #152234 broke things or if it reverted things to a previously broken state (which had temporarily been fixed for a few weeks)

[matthiaskrgr]

hmm
2024-01-01: err
2025-01-01: err
2025-06-01: err
2025-08-01: err
2025-11-01: err
2026-01-01: err
2026-02-02: err
2026-02-07: err
2026-02-08: ice 🧊

[BoxyUwU]

ok yeah that's a pretty clear signal 😆 thanks for checking

[BoxyUwU]

A similar ICE for closures:

struct S<const N: usize>([(); N]);

impl<const N: usize> S<N> {
    fn foo() -> [(); { || {
           let _: Self = loop {};
           1
       }; 1 }] {
        todo!()
    }
}