ICE: `expected a const, but found a type`

[matthiaskrgr]

auto-reduced (treereduce-rust):

#![feature(min_generic_const_args)]

pub trait Tr<A> {
    const SIZE: usize;
}

fn mk_array(_x: T) -> [(); <T as Tr<bool>>::SIZE] {}

original:

//@ check-pass

#![feature(min_generic_const_args)]
#![allow(incomplete_features)]

pub trait Tr<A> {
    #[type_const]
    const SIZE: usize;
}

fn mk_array<T: Tr<{ Tr::N }>>(_x: T) -> [(); <T as Tr<bool>>::SIZE] {
    [(); T::SIZE]
}

fn main() {}

Version information

rustc 1.88.0-nightly (62c5f58f5 2025-05-04)
binary: rustc
commit-hash: 62c5f58f57670ce65e7fec34f8c4ba00c27da2d9
commit-date: 2025-05-04
host: x86_64-unknown-linux-gnu
release: 1.88.0-nightly
LLVM version: 20.1.4

Possibly related line of code:

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

Lines 602 to 614 in 62c5f58

	pub fn as_const(&self) -> Option<Const<'tcx>> {
	if let TermKind::Const(c) = self.unpack() { Some(c) } else { None }
	}
	pub fn expect_const(&self) -> Const<'tcx> {
	self.as_const().expect("expected a const, but found a type")
	}
	pub fn into_arg(self) -> GenericArg<'tcx> {
	match self.unpack() {
	TermKind::Ty(ty) => ty.into(),
	TermKind::Const(c) => c.into(),

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc -Zincremental-verify-ich=yes -Cincremental=<dir> -Cdebuginfo=2 -Clink-dead-code=true -Zvalidate-mir --edition=2024

Program output

error[E0412]: cannot find type `T` in this scope
 --> /tmp/icemaker_global_tempdir.Oua5GnodnFsV/rustc_testrunner_tmpdir_reporting.oxonLTTvXSSr/mvce.rs:7:17
  |
3 | pub trait Tr<A> {
  | --------------- similarly named trait `Tr` defined here
...
7 | fn mk_array(_x: T) -> [(); <T as Tr<bool>>::SIZE] {}
  |                 ^
  |
help: a trait with a similar name exists
  |
7 | fn mk_array(_x: Tr) -> [(); <T as Tr<bool>>::SIZE] {}
  |                  +
help: you might be missing a type parameter
  |
7 | fn mk_array<T>(_x: T) -> [(); <T as Tr<bool>>::SIZE] {}
  |            +++

error[E0412]: cannot find type `T` in this scope
 --> /tmp/icemaker_global_tempdir.Oua5GnodnFsV/rustc_testrunner_tmpdir_reporting.oxonLTTvXSSr/mvce.rs:7:29
  |
3 | pub trait Tr<A> {
  | --------------- similarly named trait `Tr` defined here
...
7 | fn mk_array(_x: T) -> [(); <T as Tr<bool>>::SIZE] {}
  |                             ^
  |
help: a trait with a similar name exists
  |
7 | fn mk_array(_x: T) -> [(); <Tr as Tr<bool>>::SIZE] {}
  |                              +
help: you might be missing a type parameter
  |
7 | fn mk_array<T>(_x: T) -> [(); <T as Tr<bool>>::SIZE] {}
  |            +++

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.Oua5GnodnFsV/rustc_testrunner_tmpdir_reporting.oxonLTTvXSSr/mvce.rs:1:12
  |
1 | #![feature(min_generic_const_args)]
  |            ^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #132980 <https://github.com/rust-lang/rust/issues/132980> for more information
  = note: `#[warn(incomplete_features)]` on by default

error[E0601]: `main` function not found in crate `mvce`
 --> /tmp/icemaker_global_tempdir.Oua5GnodnFsV/rustc_testrunner_tmpdir_reporting.oxonLTTvXSSr/mvce.rs:7:53
  |
7 | fn mk_array(_x: T) -> [(); <T as Tr<bool>>::SIZE] {}
  |                                                     ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.Oua5GnodnFsV/rustc_testrunner_tmpdir_reporting.oxonLTTvXSSr/mvce.rs`


thread 'rustc' panicked at compiler/rustc_middle/src/ty/mod.rs:608:25:
expected a const, but found a type
stack backtrace:
   0:     0x7324f2712223 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h6b332601350bb5c6
   1:     0x7324f2e059c7 - core::fmt::write::hb5fe01a95abc29e1
   2:     0x7324f3e98f91 - std::io::Write::write_fmt::h1a9a2af5dd87e4f4
   3:     0x7324f2712082 - std::sys::backtrace::BacktraceLock::print::hac93dfc7377e4e4f
   4:     0x7324f2715eca - std::panicking::default_hook::{{closure}}::h18accd01c87ea8a4
   5:     0x7324f2715a4f - std::panicking::default_hook::h6459f689707c1046
   6:     0x7324f175ea33 - std[e3983e566f061714]::panicking::update_hook::<alloc[b6e091bb297dae1f]::boxed::Box<rustc_driver_impl[5d6200e4aea60082]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x7324f2716743 - std::panicking::rust_panic_with_hook::h70d11df0afe220d6
   8:     0x7324f271643a - std::panicking::begin_panic_handler::{{closure}}::h2419c2a21ca71abf
   9:     0x7324f27126e9 - std::sys::backtrace::__rust_end_short_backtrace::hbf27e53c84a7889f
  10:     0x7324f27160fd - __rustc[69bf9ee49405c344]::rust_begin_unwind
  11:     0x7324ef07abe0 - core::panicking::panic_fmt::h20040fc1a566cc6e
  12:     0x7324f041d76b - core::option::expect_failed::h21ba9be59cf7b568
  13:     0x7324f1df8dac - <rustc_middle[6b8e0ac128082c0d]::ty::Term>::expect_const
  14:     0x7324f39c0bc2 - <rustc_trait_selection[75470f24c5ad7214]::traits::normalize::AssocTypeNormalizer as rustc_type_ir[6c8d148d64442fb8]::fold::TypeFolder<rustc_middle[6b8e0ac128082c0d]::ty::context::TyCtxt>>::fold_const
  15:     0x7324f2fbe1d1 - <rustc_middle[6b8e0ac128082c0d]::ty::Ty as rustc_type_ir[6c8d148d64442fb8]::fold::TypeSuperFoldable<rustc_middle[6b8e0ac128082c0d]::ty::context::TyCtxt>>::super_fold_with::<rustc_trait_selection[75470f24c5ad7214]::traits::normalize::AssocTypeNormalizer>
  16:     0x7324f33e822e - <rustc_hir_analysis[aac2cbd98712a32f]::check::wfcheck::WfCheckingCtxt>::normalize::<rustc_middle[6b8e0ac128082c0d]::ty::Ty>
  17:     0x7324f33f850e - rustc_hir_analysis[aac2cbd98712a32f]::check::wfcheck::check_fn_or_method
  18:     0x7324f33e9879 - rustc_hir_analysis[aac2cbd98712a32f]::check::wfcheck::check_item_fn
  19:     0x7324f36a4091 - rustc_hir_analysis[aac2cbd98712a32f]::check::wfcheck::check_well_formed
  20:     0x7324f36a21e5 - rustc_query_impl[bb33f36fe50a9360]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[bb33f36fe50a9360]::query_impl::check_well_formed::dynamic_query::{closure#2}::{closure#0}, rustc_middle[6b8e0ac128082c0d]::query::erase::Erased<[u8; 1usize]>>
  21:     0x7324f3136c77 - rustc_query_system[54ce3036e317418]::query::plumbing::try_execute_query::<rustc_query_impl[bb33f36fe50a9360]::DynamicConfig<rustc_data_structures[79c7d893e4c18eef]::vec_cache::VecCache<rustc_span[79c1c5c8104510f4]::def_id::LocalDefId, rustc_middle[6b8e0ac128082c0d]::query::erase::Erased<[u8; 1usize]>, rustc_query_system[54ce3036e317418]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[bb33f36fe50a9360]::plumbing::QueryCtxt, true>
  22:     0x7324f313642f - rustc_query_impl[bb33f36fe50a9360]::query_impl::check_well_formed::get_query_incr::__rust_end_short_backtrace
  23:     0x7324f369e7e5 - rustc_hir_analysis[aac2cbd98712a32f]::check::wfcheck::check_mod_type_wf
  24:     0x7324f369e5df - rustc_query_impl[bb33f36fe50a9360]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[bb33f36fe50a9360]::query_impl::check_mod_type_wf::dynamic_query::{closure#2}::{closure#0}, rustc_middle[6b8e0ac128082c0d]::query::erase::Erased<[u8; 1usize]>>
  25:     0x7324f3da8bd5 - rustc_query_system[54ce3036e317418]::query::plumbing::try_execute_query::<rustc_query_impl[bb33f36fe50a9360]::DynamicConfig<rustc_query_system[54ce3036e317418]::query::caches::DefaultCache<rustc_span[79c1c5c8104510f4]::def_id::LocalModDefId, rustc_middle[6b8e0ac128082c0d]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[bb33f36fe50a9360]::plumbing::QueryCtxt, true>
  26:     0x7324f3da96ba - rustc_query_impl[bb33f36fe50a9360]::query_impl::check_mod_type_wf::get_query_incr::__rust_end_short_backtrace
  27:     0x7324f30de972 - rustc_hir_analysis[aac2cbd98712a32f]::check_crate
  28:     0x7324f312635f - rustc_interface[25c3c47d512a17f3]::passes::run_required_analyses
  29:     0x7324f3d9df9e - rustc_interface[25c3c47d512a17f3]::passes::analysis
  30:     0x7324f3d9df6d - rustc_query_impl[bb33f36fe50a9360]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[bb33f36fe50a9360]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[6b8e0ac128082c0d]::query::erase::Erased<[u8; 0usize]>>
  31:     0x7324f3d9a9c0 - rustc_query_system[54ce3036e317418]::query::plumbing::try_execute_query::<rustc_query_impl[bb33f36fe50a9360]::DynamicConfig<rustc_query_system[54ce3036e317418]::query::caches::SingleCache<rustc_middle[6b8e0ac128082c0d]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[bb33f36fe50a9360]::plumbing::QueryCtxt, true>
  32:     0x7324f3d9a3f5 - rustc_query_impl[bb33f36fe50a9360]::query_impl::analysis::get_query_incr::__rust_end_short_backtrace
  33:     0x7324f3d1e0be - rustc_interface[25c3c47d512a17f3]::passes::create_and_enter_global_ctxt::<core[7fd0d062384da063]::option::Option<rustc_interface[25c3c47d512a17f3]::queries::Linker>, rustc_driver_impl[5d6200e4aea60082]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  34:     0x7324f3e6c7dc - rustc_interface[25c3c47d512a17f3]::interface::run_compiler::<(), rustc_driver_impl[5d6200e4aea60082]::run_compiler::{closure#0}>::{closure#1}
  35:     0x7324f3e8fa3e - std[e3983e566f061714]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[25c3c47d512a17f3]::util::run_in_thread_with_globals<rustc_interface[25c3c47d512a17f3]::util::run_in_thread_pool_with_globals<rustc_interface[25c3c47d512a17f3]::interface::run_compiler<(), rustc_driver_impl[5d6200e4aea60082]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  36:     0x7324f3e8feb4 - <<std[e3983e566f061714]::thread::Builder>::spawn_unchecked_<rustc_interface[25c3c47d512a17f3]::util::run_in_thread_with_globals<rustc_interface[25c3c47d512a17f3]::util::run_in_thread_pool_with_globals<rustc_interface[25c3c47d512a17f3]::interface::run_compiler<(), rustc_driver_impl[5d6200e4aea60082]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[7fd0d062384da063]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  37:     0x7324f3e912ab - std::sys::pal::unix::thread::Thread::new::thread_start::h18c419b796c40416
  38:     0x7324edc9e70a - <unknown>
  39:     0x7324edd22aac - <unknown>
  40:                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 1.88.0-nightly (62c5f58f5 2025-05-04) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z incremental-verify-ich=yes -C incremental=[REDACTED] -C debuginfo=2 -C link-dead-code=true -Z validate-mir

query stack during panic:
#0 [check_well_formed] checking that `mk_array` is well-formed
#1 [check_mod_type_wf] checking that types are well-formed in top-level module
#2 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 3 previous errors; 1 warning emitted

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

@rustbot label +F-min_generic_const_args +-Zvalidate-mir

[moxian]

bisects to #140549 cc @BoxyUwU
@rustbot label: +S-has-mcve +S-has-bisection +A-const-generics