ICE: `expected len of array pat to be definite`

[matthiaskrgr]

auto-reduced (treereduce-rust):

#![feature(generic_const_exprs)]
fn is_123<const N: usize>(
    x: [u32; {
        N + 1;
        5
    }],
) -> bool {
    match x {
        [1, 2] => true,
        _ => false,
    }
}

original:

fn is_123<const N: usize>(x: [u32; { N + 1; 5 }]) -> bool {
    match x {
        [1, 2] => true, //~ ERROR mismatched types
        _ => false
    }
}

fn main() {}

Version information

rustc 1.88.0-nightly (c580c498a 2025-04-14)
binary: rustc
commit-hash: c580c498a1fe144d7c5b2dfc7faab1a229aa288b
commit-date: 2025-04-14
host: x86_64-unknown-linux-gnu
release: 1.88.0-nightly
LLVM version: 20.1.2

Possibly related line of code:

rust/compiler/rustc_mir_build/src/builder/matches/match_pair.rs

Lines 43 to 55 in c580c49

	let tcx = self.tcx;
	let (min_length, exact_size) = if let Some(place_resolved) = place.try_to_place(self) {
	match place_resolved.ty(&self.local_decls, tcx).ty.kind() {
	ty::Array(_, length) => (
	length
	.try_to_target_usize(tcx)
	.expect("expected len of array pat to be definite"),
	true,
	),
	_ => ((prefix.len() + suffix.len()).try_into().unwrap(), false),
	}
	} else {
	((prefix.len() + suffix.len()).try_into().unwrap(), false)

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

Program output

warning: the feature `generic_const_exprs` is incomplete and may not be safe to use and/or cause compiler crashes
 --> <crate attribute>:1:12
  |
1 | #![feature(generic_const_exprs)]
  |            ^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #76560 <https://github.com/rust-lang/rust/issues/76560> for more information
  = note: `#[warn(incomplete_features)]` on by default

error[E0601]: `main` function not found in crate `mvce`
  --> /tmp/icemaker_global_tempdir.fj15fX8ubODB/rustc_testrunner_tmpdir_reporting.s7AyRYc5iQ5Z/mvce.rs:11:2
   |
11 | }
   |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.fj15fX8ubODB/rustc_testrunner_tmpdir_reporting.s7AyRYc5iQ5Z/mvce.rs`

error: overly complex generic constant
 --> /tmp/icemaker_global_tempdir.fj15fX8ubODB/rustc_testrunner_tmpdir_reporting.s7AyRYc5iQ5Z/mvce.rs:2:14
  |
2 |       x: [u32; {
  |  ______________^
3 | |         N + 1;
4 | |         5
5 | |     }],
  | |_____^ blocks are not supported in generic constants
  |
  = help: consider moving this anonymous constant into a `const` function
  = note: this operation may be supported in the future


thread 'rustc' panicked at compiler/rustc_mir_build/src/builder/matches/match_pair.rs:49:26:
expected len of array pat to be definite
stack backtrace:
   0:     0x76a70d8f5803 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::he137faf1a18df1db
   1:     0x76a70e006051 - core::fmt::write::h91755fbfaddf7ffb
   2:     0x76a70f4ae011 - std::io::Write::write_fmt::hc96efb08aa08a02e
   3:     0x76a70d8f5662 - std::sys::backtrace::BacktraceLock::print::ha70abd97698330ed
   4:     0x76a70d8f914a - std::panicking::default_hook::{{closure}}::h8caf203b8fc4b95a
   5:     0x76a70d8f8ccf - std::panicking::default_hook::h18654ff1fe4ed24b
   6:     0x76a70c963603 - std[8cc451807e0e7beb]::panicking::update_hook::<alloc[73f72a203e12369d]::boxed::Box<rustc_driver_impl[c4ad34755bb049a3]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x76a70d8f99c3 - std::panicking::rust_panic_with_hook::h76d5e98d413ceeff
   8:     0x76a70d8f96ba - std::panicking::begin_panic_handler::{{closure}}::hb0a99bab0ad16d13
   9:     0x76a70d8f5cc9 - std::sys::backtrace::__rust_end_short_backtrace::h5393ec4087a5e093
  10:     0x76a70d8f937d - __rustc[b1dbf4b513c8357d]::rust_begin_unwind
  11:     0x76a70a28acc0 - core::panicking::panic_fmt::h7c4cd96a8d4dc575
  12:     0x76a70b18277b - core::option::expect_failed::h66a4d167aeb7206d
  13:     0x76a70fb5dc8e - <rustc_mir_build[c533163d3a765291]::builder::Builder>::prefix_slice_suffix.cold
  14:     0x76a70eb7f712 - <rustc_mir_build[c533163d3a765291]::builder::matches::MatchPairTree>::for_pattern
  15:     0x76a70eb7ac30 - <rustc_mir_build[c533163d3a765291]::builder::Builder>::lower_match_tree
  16:     0x76a70e410cf8 - <rustc_mir_build[c533163d3a765291]::builder::Builder>::expr_into_dest
  17:     0x76a70e412019 - <rustc_mir_build[c533163d3a765291]::builder::Builder>::expr_into_dest
  18:     0x76a70ed8ecf7 - <rustc_mir_build[c533163d3a765291]::builder::Builder>::ast_block_stmts
  19:     0x76a70ed8d80c - <rustc_mir_build[c533163d3a765291]::builder::Builder>::ast_block
  20:     0x76a70e40fa84 - <rustc_mir_build[c533163d3a765291]::builder::Builder>::expr_into_dest
  21:     0x76a70e412019 - <rustc_mir_build[c533163d3a765291]::builder::Builder>::expr_into_dest
  22:     0x76a70e3dad86 - rustc_mir_build[c533163d3a765291]::builder::build_mir
  23:     0x76a70e00a0c5 - rustc_mir_transform[a20fd6afc3bb3ae5]::mir_built
  24:     0x76a70e00a097 - rustc_query_impl[994d8d0f232e6f76]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[994d8d0f232e6f76]::query_impl::mir_built::dynamic_query::{closure#2}::{closure#0}, rustc_middle[cdeaf14df3177256]::query::erase::Erased<[u8; 8usize]>>
  25:     0x76a70e25c22c - rustc_query_system[fdd22cd2b18f3d20]::query::plumbing::try_execute_query::<rustc_query_impl[994d8d0f232e6f76]::DynamicConfig<rustc_data_structures[bb20cc737854270d]::vec_cache::VecCache<rustc_span[d423fa63ad2163fb]::def_id::LocalDefId, rustc_middle[cdeaf14df3177256]::query::erase::Erased<[u8; 8usize]>, rustc_query_system[fdd22cd2b18f3d20]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[994d8d0f232e6f76]::plumbing::QueryCtxt, false>
  26:     0x76a70e25bd4b - rustc_query_impl[994d8d0f232e6f76]::query_impl::mir_built::get_query_non_incr::__rust_end_short_backtrace
  27:     0x76a70e9759c3 - rustc_mir_build[c533163d3a765291]::check_unsafety::check_unsafety
  28:     0x76a70e975799 - rustc_query_impl[994d8d0f232e6f76]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[994d8d0f232e6f76]::query_impl::check_unsafety::dynamic_query::{closure#2}::{closure#0}, rustc_middle[cdeaf14df3177256]::query::erase::Erased<[u8; 0usize]>>
  29:     0x76a70e974d9d - rustc_query_system[fdd22cd2b18f3d20]::query::plumbing::try_execute_query::<rustc_query_impl[994d8d0f232e6f76]::DynamicConfig<rustc_data_structures[bb20cc737854270d]::vec_cache::VecCache<rustc_span[d423fa63ad2163fb]::def_id::LocalDefId, rustc_middle[cdeaf14df3177256]::query::erase::Erased<[u8; 0usize]>, rustc_query_system[fdd22cd2b18f3d20]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[994d8d0f232e6f76]::plumbing::QueryCtxt, false>
  30:     0x76a70e974a7e - rustc_query_impl[994d8d0f232e6f76]::query_impl::check_unsafety::get_query_non_incr::__rust_end_short_backtrace
  31:     0x76a70e97139d - rustc_interface[89d8dc017f235532]::passes::run_required_analyses
  32:     0x76a70ef46d1e - rustc_interface[89d8dc017f235532]::passes::analysis
  33:     0x76a70ef46ced - rustc_query_impl[994d8d0f232e6f76]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[994d8d0f232e6f76]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[cdeaf14df3177256]::query::erase::Erased<[u8; 0usize]>>
  34:     0x76a70ef4c7bd - rustc_query_system[fdd22cd2b18f3d20]::query::plumbing::try_execute_query::<rustc_query_impl[994d8d0f232e6f76]::DynamicConfig<rustc_query_system[fdd22cd2b18f3d20]::query::caches::SingleCache<rustc_middle[cdeaf14df3177256]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[994d8d0f232e6f76]::plumbing::QueryCtxt, false>
  35:     0x76a70ef4c4b8 - rustc_query_impl[994d8d0f232e6f76]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  36:     0x76a70f33ccbe - rustc_interface[89d8dc017f235532]::passes::create_and_enter_global_ctxt::<core[9797c9d635bbf607]::option::Option<rustc_interface[89d8dc017f235532]::queries::Linker>, rustc_driver_impl[c4ad34755bb049a3]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  37:     0x76a70f13ddc4 - rustc_interface[89d8dc017f235532]::interface::run_compiler::<(), rustc_driver_impl[c4ad34755bb049a3]::run_compiler::{closure#0}>::{closure#1}
  38:     0x76a70f1633b2 - std[8cc451807e0e7beb]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[89d8dc017f235532]::util::run_in_thread_with_globals<rustc_interface[89d8dc017f235532]::util::run_in_thread_pool_with_globals<rustc_interface[89d8dc017f235532]::interface::run_compiler<(), rustc_driver_impl[c4ad34755bb049a3]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  39:     0x76a70f163cab - <<std[8cc451807e0e7beb]::thread::Builder>::spawn_unchecked_<rustc_interface[89d8dc017f235532]::util::run_in_thread_with_globals<rustc_interface[89d8dc017f235532]::util::run_in_thread_pool_with_globals<rustc_interface[89d8dc017f235532]::interface::run_compiler<(), rustc_driver_impl[c4ad34755bb049a3]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[9797c9d635bbf607]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  40:     0x76a70f1650ab - std::sys::pal::unix::thread::Thread::new::thread_start::heb5f87066979571c
  41:     0x76a708ea370a - <unknown>
  42:     0x76a708f27aac - <unknown>
  43:                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 (c580c498a 2025-04-14) running on x86_64-unknown-linux-gnu

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

query stack during panic:
#0 [mir_built] building MIR for `is_123`
#1 [check_unsafety] unsafety-checking `is_123`
#2 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 2 previous errors; 1 warning emitted

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

@rustbot label +F-generic_const_exprs

[matthiaskrgr]

bit smaller

#![feature(generic_const_exprs)]
fn a<const b: usize>(
    c: [u32; {
        b;
        5
    }],
) -> bool {
    match c {
        [] => true,
        _ => d,
    }
}