rust-lang / rust-lang/rust

[ICE]: cannot find `..` in param-env: ParamEnv

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C-bug I-ICE needs-triage T-compiler WG-trait-system-refactor
Dominant language
Rust
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Description

auto-reduced (treereduce-rust):

//@compile-flags: -Znext-solver=globally
trait Processor {
    type Ctx<'a, const N: usize>;
}
pub struct Phasor;
impl Processor for Phasor {
    type Ctx<'a, const N: usize> = Self::Ctx</*'a,*/ N>;
}

original:

trait Processor {
    type Ctx<'a, const N: usize>;
}
pub struct Phasor;
impl Processor for Phasor {
    type Ctx<'a, const N: usize> = Self::Ctx</*'a,*/ N>;
}

fn main() {}

Version information

rustc 1.96.0-nightly (0e95a0f4c 2026-03-31)
binary: rustc
commit-hash: 0e95a0f4c677002a5d4ac5bc59d97885e6f51f71
commit-date: 2026-03-31
host: x86_64-unknown-linux-gnu
release: 1.96.0-nightly
LLVM version: 22.1.2

Possibly related line of code:
https://github.com/rust-lang/rust/blob/0e95a0f4c677002a5d4ac5bc59d97885e6f51f71/compiler/rustc_type_ir/src/binder.rs#L1299-L1311

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc -Znext-solver=globally

Program output 

error[E0601]: `main` function not found in crate `mvce`
 --> /tmp/icemaker_global_tempdir.RLRUDhZuCleE/rustc_testrunner_tmpdir_reporting.JM00f4mTzkQh/mvce.rs:7:2
  |
7 | }
  |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.RLRUDhZuCleE/rustc_testrunner_tmpdir_reporting.JM00f4mTzkQh/mvce.rs`

error[E0107]: associated type takes 1 lifetime argument but 0 lifetime arguments were supplied
 --> /tmp/icemaker_global_tempdir.RLRUDhZuCleE/rustc_testrunner_tmpdir_reporting.JM00f4mTzkQh/mvce.rs:6:42
  |
6 |     type Ctx<'a, const N: usize> = Self::Ctx</*'a,*/ N>;
  |                                          ^^^ expected 1 lifetime argument
  |
note: associated type defined here, with 1 lifetime parameter: `'a`
 --> /tmp/icemaker_global_tempdir.RLRUDhZuCleE/rustc_testrunner_tmpdir_reporting.JM00f4mTzkQh/mvce.rs:2:10
  |
2 |     type Ctx<'a, const N: usize>;
  |          ^^^ --
help: add missing lifetime argument
  |
6 |     type Ctx<'a, const N: usize> = Self::Ctx<'a, /*'a,*/ N>;
  |                                              +++


thread 'rustc' (4127042) panicked at /rustc-dev/0e95a0f4c677002a5d4ac5bc59d97885e6f51f71/compiler/rustc_type_ir/src/binder.rs:1305:13:
cannot find `!BoundConst { var: 2, .. }` in param-env: ParamEnv {
    caller_bounds: [
        Binder { value: ConstArgHasType(!BoundConst { var: 0, .. }, usize), bound_vars: [] },
        Binder { value: ProjectionPredicate(AliasTerm { args: [Phasor, '^0.Named(DefId(0:11 ~ mvce[3aca]::{impl#0}::Ctx::'a)), ^BoundConst { var: 1, .. }], def_id: DefId(0:4 ~ mvce[3aca]::Processor::Ctx), .. }, Term::Ty(Alias(Projection, AliasTy { args: [Phasor, '?0, ^BoundConst { var: 1, .. }], def_id: DefId(0:4 ~ mvce[3aca]::Processor::Ctx), .. }))), bound_vars: [Region(BrNamed(DefId(0:11 ~ mvce[3aca]::{impl#0}::Ctx::'a))), Const] },
    ],
}
stack backtrace:
   0:     0x7fb28fd9328b - <<std[3a154748be1cf171]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[6dbc07b3c623ddea]::fmt::Display>::fmt
   1:     0x7fb29042fa48 - core[6dbc07b3c623ddea]::fmt::write
   2:     0x7fb28fdaa296 - <std[3a154748be1cf171]::sys::stdio::unix::Stderr as std[3a154748be1cf171]::io::Write>::write_fmt
   3:     0x7fb28fd69298 - std[3a154748be1cf171]::panicking::default_hook::{closure#0}
   4:     0x7fb28fd86693 - std[3a154748be1cf171]::panicking::default_hook
   5:     0x7fb28ed8ae5c - std[3a154748be1cf171]::panicking::update_hook::<alloc[19c55003204b46b1]::boxed::Box<rustc_driver_impl[c52c1912fce43640]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7fb28fd86972 - std[3a154748be1cf171]::panicking::panic_with_hook
   7:     0x7fb28fd69358 - std[3a154748be1cf171]::panicking::panic_handler::{closure#0}
   8:     0x7fb28fd60089 - std[3a154748be1cf171]::sys::backtrace::__rust_end_short_backtrace::<std[3a154748be1cf171]::panicking::panic_handler::{closure#0}, !>
   9:     0x7fb28fd6adbd - __rustc[3228c5cf9e31aa5]::rust_begin_unwind
  10:     0x7fb28ca8c98c - core[6dbc07b3c623ddea]::panicking::panic_fmt
  11:     0x7fb290f0d949 - <rustc_next_trait_solver[b318c687bb70fcf4]::solve::search_graph::SearchGraphDelegate<rustc_trait_selection[d2702b535d15bdf3]::solve::delegate::SolverDelegate> as rustc_type_ir[c0667d30c5973e36]::search_graph::Delegate>::compute_goal::{closure#0}
  12:     0x7fb290f226f5 - <rustc_next_trait_solver[b318c687bb70fcf4]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[d2702b535d15bdf3]::solve::delegate::SolverDelegate, rustc_middle[779b3639bfd63c5a]::ty::context::TyCtxt>>::evaluate_goal_raw
  13:     0x7fb290f11ad2 - <rustc_next_trait_solver[b318c687bb70fcf4]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[d2702b535d15bdf3]::solve::delegate::SolverDelegate, rustc_middle[779b3639bfd63c5a]::ty::context::TyCtxt>>::try_evaluate_added_goals
  14:     0x7fb290f04b9c - <rustc_next_trait_solver[b318c687bb70fcf4]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[d2702b535d15bdf3]::solve::delegate::SolverDelegate, rustc_middle[779b3639bfd63c5a]::ty::context::TyCtxt>>::evaluate_added_goals_and_make_canonical_response
  15:     0x7fb291226d92 - <rustc_next_trait_solver[b318c687bb70fcf4]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[d2702b535d15bdf3]::solve::delegate::SolverDelegate, rustc_middle[779b3639bfd63c5a]::ty::context::TyCtxt>>::assemble_impl_candidates::<rustc_type_ir[c0667d30c5973e36]::predicate::NormalizesTo<rustc_middle[779b3639bfd63c5a]::ty::context::TyCtxt>>::{closure#0}
  16:     0x7fb29122ad0a - <rustc_next_trait_solver[b318c687bb70fcf4]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[d2702b535d15bdf3]::solve::delegate::SolverDelegate, rustc_middle[779b3639bfd63c5a]::ty::context::TyCtxt>>::assemble_and_evaluate_candidates::<rustc_type_ir[c0667d30c5973e36]::predicate::NormalizesTo<rustc_middle[779b3639bfd63c5a]::ty::context::TyCtxt>>
  17:     0x7fb290f0be7d - <rustc_next_trait_solver[b318c687bb70fcf4]::solve::search_graph::SearchGraphDelegate<rustc_trait_selection[d2702b535d15bdf3]::solve::delegate::SolverDelegate> as rustc_type_ir[c0667d30c5973e36]::search_graph::Delegate>::compute_goal::{closure#0}
  18:     0x7fb290f226f5 - <rustc_next_trait_solver[b318c687bb70fcf4]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[d2702b535d15bdf3]::solve::delegate::SolverDelegate, rustc_middle[779b3639bfd63c5a]::ty::context::TyCtxt>>::evaluate_goal_raw
  19:     0x7fb290f11f2a - <rustc_next_trait_solver[b318c687bb70fcf4]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[d2702b535d15bdf3]::solve::delegate::SolverDelegate, rustc_middle[779b3639bfd63c5a]::ty::context::TyCtxt>>::try_evaluate_added_goals
  20:     0x7fb290f0b455 - <rustc_next_trait_solver[b318c687bb70fcf4]::solve::search_graph::SearchGraphDelegate<rustc_trait_selection[d2702b535d15bdf3]::solve::delegate::SolverDelegate> as rustc_type_ir[c0667d30c5973e36]::search_graph::Delegate>::compute_goal::{closure#0}
  21:     0x7fb290f226f5 - <rustc_next_trait_solver[b318c687bb70fcf4]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[d2702b535d15bdf3]::solve::delegate::SolverDelegate, rustc_middle[779b3639bfd63c5a]::ty::context::TyCtxt>>::evaluate_goal_raw
  22:     0x7fb290f1097a - <rustc_trait_selection[d2702b535d15bdf3]::solve::fulfill::FulfillmentCtxt<rustc_infer[411e9d562e286661]::traits::engine::ScrubbedTraitError> as rustc_infer[411e9d562e286661]::traits::engine::TraitEngine<rustc_infer[411e9d562e286661]::traits::engine::ScrubbedTraitError>>::try_evaluate_obligations
  23:     0x7fb290c8aa9d - <rustc_trait_selection[d2702b535d15bdf3]::solve::normalize::NormalizationFolder<rustc_infer[411e9d562e286661]::traits::engine::ScrubbedTraitError>>::normalize_alias_term
  24:     0x7fb290f16987 - rustc_trait_selection[d2702b535d15bdf3]::solve::normalize::deeply_normalize_with_skipped_universes_and_ambiguous_coroutine_goals::<rustc_middle[779b3639bfd63c5a]::ty::Ty, rustc_infer[411e9d562e286661]::traits::engine::ScrubbedTraitError>
  25:     0x7fb290f161b0 - <rustc_trait_selection[d2702b535d15bdf3]::solve::normalize::DeeplyNormalizeForDiagnosticsFolder as rustc_type_ir[c0667d30c5973e36]::fold::TypeFolder<rustc_middle[779b3639bfd63c5a]::ty::context::TyCtxt>>::fold_ty
  26:     0x7fb291c64f9f - <rustc_trait_selection[d2702b535d15bdf3]::solve::normalize::DeeplyNormalizeForDiagnosticsFolder as rustc_type_ir[c0667d30c5973e36]::fold::TypeFolder<rustc_middle[779b3639bfd63c5a]::ty::context::TyCtxt>>::fold_predicate.warm
  27:     0x7fb291593661 - rustc_trait_selection[d2702b535d15bdf3]::solve::fulfill::derive_errors::find_best_leaf_obligation
  28:     0x7fb29159236b - <rustc_trait_selection[d2702b535d15bdf3]::traits::FulfillmentError as rustc_infer[411e9d562e286661]::traits::engine::FromSolverError<rustc_trait_selection[d2702b535d15bdf3]::solve::fulfill::NextSolverError>>::from_solver_error
  29:     0x7fb290f1c59d - <rustc_trait_selection[d2702b535d15bdf3]::solve::fulfill::FulfillmentCtxt<rustc_trait_selection[d2702b535d15bdf3]::traits::FulfillmentError> as rustc_infer[411e9d562e286661]::traits::engine::TraitEngine<rustc_trait_selection[d2702b535d15bdf3]::traits::FulfillmentError>>::try_evaluate_obligations
  30:     0x7fb291355d8d - <rustc_trait_selection[d2702b535d15bdf3]::solve::normalize::NormalizationFolder<rustc_trait_selection[d2702b535d15bdf3]::traits::FulfillmentError>>::normalize_alias_term
  31:     0x7fb291356ff2 - <rustc_trait_selection[d2702b535d15bdf3]::solve::normalize::NormalizationFolder<rustc_trait_selection[d2702b535d15bdf3]::traits::FulfillmentError> as rustc_type_ir[c0667d30c5973e36]::fold::FallibleTypeFolder<rustc_middle[779b3639bfd63c5a]::ty::context::TyCtxt>>::try_fold_ty::{closure#0}
  32:     0x7fb291357445 - <&rustc_middle[779b3639bfd63c5a]::ty::list::RawList<(), rustc_middle[779b3639bfd63c5a]::ty::generic_args::GenericArg> as rustc_type_ir[c0667d30c5973e36]::fold::TypeFoldable<rustc_middle[779b3639bfd63c5a]::ty::context::TyCtxt>>::try_fold_with::<rustc_trait_selection[d2702b535d15bdf3]::solve::normalize::NormalizationFolder<rustc_trait_selection[d2702b535d15bdf3]::traits::FulfillmentError>>
  33:     0x7fb290d14574 - rustc_hir_analysis[7c08228f19d0137f]::check::compare_impl_item::check_type_bounds
  34:     0x7fb290d437c3 - rustc_hir_analysis[7c08228f19d0137f]::check::compare_impl_item::compare_impl_item
  35:     0x7fb290d40cc7 - rustc_query_impl[c37ce76b3c963ce0]::query_impl::compare_impl_item::invoke_provider_fn::__rust_begin_short_backtrace
  36:     0x7fb290a6bb24 - rustc_query_impl[c37ce76b3c963ce0]::execution::try_execute_query::<rustc_data_structures[bea30f1e39587d4b]::vec_cache::VecCache<rustc_span[b2b4aa46a618de45]::def_id::LocalDefId, rustc_middle[779b3639bfd63c5a]::query::erase::ErasedData<[u8; 1usize]>, rustc_middle[779b3639bfd63c5a]::dep_graph::graph::DepNodeIndex>, false>
  37:     0x7fb290a6b64e - rustc_query_impl[c37ce76b3c963ce0]::query_impl::compare_impl_item::execute_query_non_incr::__rust_end_short_backtrace
  38:     0x7fb290a6eb49 - rustc_hir_analysis[7c08228f19d0137f]::check::check::check_item_type
  39:     0x7fb290a6c526 - rustc_hir_analysis[7c08228f19d0137f]::check::wfcheck::check_well_formed
  40:     0x7fb290a6c507 - rustc_query_impl[c37ce76b3c963ce0]::query_impl::check_well_formed::invoke_provider_fn::__rust_begin_short_backtrace
  41:     0x7fb290a6bb24 - rustc_query_impl[c37ce76b3c963ce0]::execution::try_execute_query::<rustc_data_structures[bea30f1e39587d4b]::vec_cache::VecCache<rustc_span[b2b4aa46a618de45]::def_id::LocalDefId, rustc_middle[779b3639bfd63c5a]::query::erase::ErasedData<[u8; 1usize]>, rustc_middle[779b3639bfd63c5a]::dep_graph::graph::DepNodeIndex>, false>
  42:     0x7fb290a6b8bb - rustc_query_impl[c37ce76b3c963ce0]::query_impl::check_well_formed::execute_query_non_incr::__rust_end_short_backtrace
  43:     0x7fb290a67e4b - rustc_hir_analysis[7c08228f19d0137f]::check::wfcheck::check_type_wf
  44:     0x7fb290a67d55 - rustc_query_impl[c37ce76b3c963ce0]::query_impl::check_type_wf::invoke_provider_fn::__rust_begin_short_backtrace
  45:     0x7fb29160a050 - rustc_query_impl[c37ce76b3c963ce0]::execution::try_execute_query::<rustc_middle[779b3639bfd63c5a]::query::caches::SingleCache<rustc_middle[779b3639bfd63c5a]::query::erase::ErasedData<[u8; 1usize]>>, false>
  46:     0x7fb291609e29 - rustc_query_impl[c37ce76b3c963ce0]::query_impl::check_type_wf::execute_query_non_incr::__rust_end_short_backtrace
  47:     0x7fb290a8bd1b - rustc_hir_analysis[7c08228f19d0137f]::check_crate
  48:     0x7fb290531eb5 - rustc_interface[2621fa1c67a1630]::passes::analysis
  49:     0x7fb29160c7c9 - rustc_query_impl[c37ce76b3c963ce0]::execution::try_execute_query::<rustc_middle[779b3639bfd63c5a]::query::caches::SingleCache<rustc_middle[779b3639bfd63c5a]::query::erase::ErasedData<[u8; 0usize]>>, false>
  50:     0x7fb29160c435 - rustc_query_impl[c37ce76b3c963ce0]::query_impl::analysis::execute_query_non_incr::__rust_end_short_backtrace
  51:     0x7fb2915ba5e2 - rustc_interface[2621fa1c67a1630]::interface::run_compiler::<(), rustc_driver_impl[c52c1912fce43640]::run_compiler::{closure#0}>::{closure#1}
  52:     0x7fb2915e48fe - std[3a154748be1cf171]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[2621fa1c67a1630]::util::run_in_thread_with_globals<rustc_interface[2621fa1c67a1630]::util::run_in_thread_pool_with_globals<rustc_interface[2621fa1c67a1630]::interface::run_compiler<(), rustc_driver_impl[c52c1912fce43640]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  53:     0x7fb2915e51a0 - <std[3a154748be1cf171]::thread::lifecycle::spawn_unchecked<rustc_interface[2621fa1c67a1630]::util::run_in_thread_with_globals<rustc_interface[2621fa1c67a1630]::util::run_in_thread_pool_with_globals<rustc_interface[2621fa1c67a1630]::interface::run_compiler<(), rustc_driver_impl[c52c1912fce43640]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[6dbc07b3c623ddea]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  54:     0x7fb2915e602c - <std[3a154748be1cf171]::sys::thread::unix::Thread>::new::thread_start
  55:     0x7fb28b0a097a - <unknown>
  56:     0x7fb28b1242bc - <unknown>
  57:                0x0 - <unknown>

error: the compiler unexpectedly panicked. This is a bug

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 (0e95a0f4c 2026-03-31) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z next-solver=globally -Z dump-mir-dir=dir

query stack during panic:
#0 [compare_impl_item] checking assoc item `<impl at /tmp/icemaker_global_tempdir.RLRUDhZuCleE/rustc_testrunner_tmpdir_reporting.JM00f4mTzkQh/mvce.rs:5:1: 5:26>::Ctx` is compatible with trait definition
#1 [check_well_formed] checking that `<impl at /tmp/icemaker_global_tempdir.RLRUDhZuCleE/rustc_testrunner_tmpdir_reporting.JM00f4mTzkQh/mvce.rs:5:1: 5:26>` is well-formed
#2 [check_type_wf] checking that types are well-formed
#3 [analysis] running analysis passes on crate `mvce`
end of query stack
error: aborting due to 2 previous errors

Some errors have detailed explanations: E0107, E0601.
For more information about an error, try `rustc --explain E0107`.

@rustbot label +WG-trait-system-refactor

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the reduced reproducer and the next-solver command shown in the issue, then inspect compiler/rustc_type_ir/src/binder.rs around line 1305 and the related stack frames. Reproduce the ICE before investigating the ParamEnv handling; done means the example no longer causes rustc to panic and instead reports a diagnostic.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Quiet
Clarity
Needs clarification
Newbie friendliness
35/100

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