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[ICE]: expected region for .. but found Type ..

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C-bug F-inherent_associated_types F-min_generic_const_args 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
#![feature(inherent_associated_types)]
#![feature(min_generic_const_args)]
struct OnDiskDirEntry<'a> {
    ,
}
impl<'a> OnDiskDirEntry<'a> {
    type const LFN_FRAGMENT_LEN: usize = 3;
    fn lfn_contents() -> [char; Self::LFN_FRAGMENT_LEN] {
        2
    }
}

original:

#![feature(inherent_associated_types)]
#![feature(min_generic_const_args)]
struct OnDiskDirEntry<'a> {
    a: &'a str,
}
impl<'a> OnDiskDirEntry<'a> {
    type const LFN_FRAGMENT_LEN: usize = 3;
    fn lfn_contents() -> [char; Self::LFN_FRAGMENT_LEN] {
        2
    }
}

fn main() {}

Version information

rustc 1.97.0-nightly (345a975e7 2026-04-27)
binary: rustc
commit-hash: 345a975e76de74f090c55d5b56f5f6dd41b655a2
commit-date: 2026-04-27
host: x86_64-unknown-linux-gnu
release: 1.97.0-nightly
LLVM version: 22.1.2

Possibly related line of code:
https://github.com/rust-lang/rust/blob/345a975e76de74f090c55d5b56f5f6dd41b655a2/compiler/rustc_type_ir/src/binder.rs#L846-L858

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

Program output 

error: expected identifier, found `,`
 --> /tmp/icemaker_global_tempdir.1DKMRuVQsV2v/rustc_testrunner_tmpdir_reporting.8PmYhQSsMCBd/mvce.rs:4:5
  |
3 | struct OnDiskDirEntry<'a> {
  |        -------------- while parsing this struct
4 |     ,
  |     ^ expected identifier

warning: the feature `inherent_associated_types` is incomplete and may not be safe to use and/or cause compiler crashes
 --> /tmp/icemaker_global_tempdir.1DKMRuVQsV2v/rustc_testrunner_tmpdir_reporting.8PmYhQSsMCBd/mvce.rs: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

warning: the feature `min_generic_const_args` is incomplete and may not be safe to use and/or cause compiler crashes
 --> /tmp/icemaker_global_tempdir.1DKMRuVQsV2v/rustc_testrunner_tmpdir_reporting.8PmYhQSsMCBd/mvce.rs:2:12
  |
2 | #![feature(min_generic_const_args)]
  |            ^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #132980 <https://github.com/rust-lang/rust/issues/132980> for more information

error[E0601]: `main` function not found in crate `mvce`
  --> /tmp/icemaker_global_tempdir.1DKMRuVQsV2v/rustc_testrunner_tmpdir_reporting.8PmYhQSsMCBd/mvce.rs:11:2
   |
11 | }
   |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.1DKMRuVQsV2v/rustc_testrunner_tmpdir_reporting.8PmYhQSsMCBd/mvce.rs`


thread 'rustc' (1111333) panicked at /rustc-dev/345a975e76de74f090c55d5b56f5f6dd41b655a2/compiler/rustc_type_ir/src/binder.rs:852:9:
expected region for `'a/#0` ('a/#0/0) but found Type(OnDiskDirEntry<'a/#0>) when instantiating args=[OnDiskDirEntry<'a/#0>]
stack backtrace:
   0:     0x7f3cf5cfae99 - <<std[a9eb3f39b69ad3ff]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[b04a2f1476229c25]::fmt::Display>::fmt
   1:     0x7f3cf6415e48 - core[b04a2f1476229c25]::fmt::write
   2:     0x7f3cf5d11986 - <std[a9eb3f39b69ad3ff]::sys::stdio::unix::Stderr as std[a9eb3f39b69ad3ff]::io::Write>::write_fmt
   3:     0x7f3cf5cd109e - std[a9eb3f39b69ad3ff]::panicking::default_hook::{closure#0}
   4:     0x7f3cf5cee583 - std[a9eb3f39b69ad3ff]::panicking::default_hook
   5:     0x7f3cf4cd9a71 - std[a9eb3f39b69ad3ff]::panicking::update_hook::<alloc[f5407cb677668734]::boxed::Box<rustc_driver_impl[4eedd31b7089c8cf]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7f3cf5cee862 - std[a9eb3f39b69ad3ff]::panicking::panic_with_hook
   7:     0x7f3cf5cd1162 - std[a9eb3f39b69ad3ff]::panicking::panic_handler::{closure#0}
   8:     0x7f3cf5cc5659 - std[a9eb3f39b69ad3ff]::sys::backtrace::__rust_end_short_backtrace::<std[a9eb3f39b69ad3ff]::panicking::panic_handler::{closure#0}, !>
   9:     0x7f3cf5cd2bcd - __rustc[929c9a63fff72118]::rust_begin_unwind
  10:     0x7f3cf29183cc - core[b04a2f1476229c25]::panicking::panic_fmt
  11:     0x7f3cf2958271 - <rustc_type_ir[aefac294ccc7744]::binder::ArgFolder<rustc_middle[dbb0bf4bafb0ab2f]::ty::context::TyCtxt>>::region_param_expected
  12:     0x7f3cf6443e29 - <rustc_type_ir[aefac294ccc7744]::binder::ArgFolder<rustc_middle[dbb0bf4bafb0ab2f]::ty::context::TyCtxt> as rustc_type_ir[aefac294ccc7744]::fold::TypeFolder<rustc_middle[dbb0bf4bafb0ab2f]::ty::context::TyCtxt>>::fold_ty
  13:     0x7f3cf52d97cf - <rustc_type_ir[aefac294ccc7744]::binder::EarlyBinder<rustc_middle[dbb0bf4bafb0ab2f]::ty::context::TyCtxt, rustc_middle[dbb0bf4bafb0ab2f]::ty::Ty>>::instantiate::<&[rustc_middle[dbb0bf4bafb0ab2f]::ty::generic_args::GenericArg]>
  14:     0x7f3cf7792d98 - <rustc_middle[dbb0bf4bafb0ab2f]::ty::print::pretty::FmtPrinter as rustc_middle[dbb0bf4bafb0ab2f]::ty::print::Printer>::print_def_path
  15:     0x7f3cf7792974 - <rustc_middle[dbb0bf4bafb0ab2f]::ty::print::pretty::FmtPrinter as rustc_middle[dbb0bf4bafb0ab2f]::ty::print::Printer>::print_def_path
  16:     0x7f3cf538c851 - <rustc_middle[dbb0bf4bafb0ab2f]::ty::print::pretty::FmtPrinter as rustc_middle[dbb0bf4bafb0ab2f]::ty::print::pretty::PrettyPrinter>::pretty_print_const
  17:     0x7f3cf7794774 - <rustc_middle[dbb0bf4bafb0ab2f]::ty::print::pretty::FmtPrinter as rustc_middle[dbb0bf4bafb0ab2f]::ty::print::pretty::PrettyPrinter>::pretty_print_type
  18:     0x7f3cf533706f - <rustc_middle[dbb0bf4bafb0ab2f]::ty::Ty as rustc_error_messages[caeaf8dd9fec5fd]::IntoDiagArg>::into_diag_arg
  19:     0x7f3cf4f6ce8f - <rustc_hir_typeck[4f770d89427b513d]::errors::ExpectedReturnTypeLabel as rustc_errors[5d147c2ea826cfcc]::diagnostic::Subdiagnostic>::add_to_diag::<rustc_span[bbe1d656f07b00ee]::ErrorGuaranteed>
  20:     0x7f3cf4fc9e8e - <rustc_hir_typeck[4f770d89427b513d]::fn_ctxt::FnCtxt>::suggest_missing_return_type
  21:     0x7f3cf4fcca5e - <rustc_hir_typeck[4f770d89427b513d]::fn_ctxt::FnCtxt>::suggest_mismatched_types_on_tail
  22:     0x7f3cf447ff6c - <rustc_hir_typeck[4f770d89427b513d]::coercion::CoerceMany>::report_return_mismatched_types
  23:     0x7f3cf68a83fa - <rustc_hir_typeck[4f770d89427b513d]::fn_ctxt::FnCtxt>::check_expr_block
  24:     0x7f3cf689086f - <rustc_hir_typeck[4f770d89427b513d]::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
  25:     0x7f3cf689bf05 - rustc_hir_typeck[4f770d89427b513d]::check::check_fn
  26:     0x7f3cf6d9a506 - rustc_hir_typeck[4f770d89427b513d]::typeck_with_inspect::{closure#0}
  27:     0x7f3cf6d97404 - rustc_query_impl[2be19ef8da6108aa]::query_impl::typeck_root::invoke_provider_fn::__rust_begin_short_backtrace
  28:     0x7f3cf6518166 - rustc_query_impl[2be19ef8da6108aa]::execution::try_execute_query::<rustc_data_structures[fb6e4346a2abdb30]::vec_cache::VecCache<rustc_span[bbe1d656f07b00ee]::def_id::LocalDefId, rustc_middle[dbb0bf4bafb0ab2f]::query::erase::ErasedData<[u8; 8usize]>, rustc_middle[dbb0bf4bafb0ab2f]::dep_graph::graph::DepNodeIndex>, false>
  29:     0x7f3cf6517a43 - rustc_query_impl[2be19ef8da6108aa]::query_impl::typeck_root::execute_query_non_incr::__rust_end_short_backtrace
  30:     0x7f3cf6ca8a54 - <rustc_middle[dbb0bf4bafb0ab2f]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_hir_analysis[54c6f91f6d421f22]::check_crate::{closure#1}>::{closure#0}
  31:     0x7f3cf6ca8426 - rustc_hir_analysis[54c6f91f6d421f22]::check_crate
  32:     0x7f3cf7343821 - rustc_interface[fb0ae15303da83fc]::passes::analysis
  33:     0x7f3cf757e7c9 - rustc_query_impl[2be19ef8da6108aa]::execution::try_execute_query::<rustc_middle[dbb0bf4bafb0ab2f]::query::caches::SingleCache<rustc_middle[dbb0bf4bafb0ab2f]::query::erase::ErasedData<[u8; 0usize]>>, false>
  34:     0x7f3cf757e429 - rustc_query_impl[2be19ef8da6108aa]::query_impl::analysis::execute_query_non_incr::__rust_end_short_backtrace
  35:     0x7f3cf759b742 - rustc_interface[fb0ae15303da83fc]::interface::run_compiler::<(), rustc_driver_impl[4eedd31b7089c8cf]::run_compiler::{closure#0}>::{closure#1}
  36:     0x7f3cf75731fe - std[a9eb3f39b69ad3ff]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[fb0ae15303da83fc]::util::run_in_thread_with_globals<rustc_interface[fb0ae15303da83fc]::util::run_in_thread_pool_with_globals<rustc_interface[fb0ae15303da83fc]::interface::run_compiler<(), rustc_driver_impl[4eedd31b7089c8cf]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  37:     0x7f3cf75738ed - <std[a9eb3f39b69ad3ff]::thread::lifecycle::spawn_unchecked<rustc_interface[fb0ae15303da83fc]::util::run_in_thread_with_globals<rustc_interface[fb0ae15303da83fc]::util::run_in_thread_pool_with_globals<rustc_interface[fb0ae15303da83fc]::interface::run_compiler<(), rustc_driver_impl[4eedd31b7089c8cf]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[b04a2f1476229c25]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  38:     0x7f3cf757472c - <std[a9eb3f39b69ad3ff]::sys::thread::unix::Thread>::new::thread_start
  39:     0x7f3cf10a597a - <unknown>
  40:     0x7f3cf11292bc - <unknown>
  41:                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 1.97.0-nightly (345a975e7 2026-04-27) running on x86_64-unknown-linux-gnu

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

query stack during panic:
#0 [typeck_root] type-checking `<impl at /tmp/icemaker_global_tempdir.1DKMRuVQsV2v/rustc_testrunner_tmpdir_reporting.8PmYhQSsMCBd/mvce.rs:6:1: 6:28>::lfn_contents`
#1 [analysis] running analysis passes on crate `mvce`
end of query stack
error: aborting due to 2 previous errors; 2 warnings emitted

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

@rustbot label +F-inherent_associated_types +F-min_generic_const_args +WG-trait-system-refactor

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First steps

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  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Reproduce the ICE with the reduced a.rs example and -Znext-solver=globally, then inspect compiler/rustc_type_ir/src/binder.rs around region_param_expected at lines 846-858. Determine why the reported generic arguments reach this path with a type where a region is expected, and add a regression test showing compilation no longer panics.

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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