ICE: `Found discriminant with invalid size, has 0 bytes`

[matthiaskrgr]

auto-reduced (treereduce-rust):

//@compile-flags: -Zvalidate-mir
enum Single {
    Both,
    Front,
    Back,
}

fn main() {
    let _val: Single = unsafe { std::mem::transmute::<(), Single>(()) };
}

original:

#[allow(dead_code)]
enum Single {
    Both,
    Front,
    Back
}

fn main() {
    let _val: Single = unsafe { std::mem::transmute::<(), Single>(()) };
}

Version information

rustc 1.91.0-nightly (8df154bff 2025-08-23)
binary: rustc
commit-hash: 8df154bffddcb6bbb543ad69aff971795c5adbc2
commit-date: 2025-08-23
host: x86_64-unknown-linux-gnu
release: 1.91.0-nightly
LLVM version: 21.1.0

Possibly related line of code:

rust/compiler/rustc_mir_transform/src/check_enums.rs

Lines 255 to 267 in 8df154b

	match size.bytes() {
	1 => tcx.types.u8,
	2 => tcx.types.u16,
	4 => tcx.types.u32,
	8 => tcx.types.u64,
	16 => tcx.types.u128,
	invalid => bug!("Found discriminant with invalid size, has {} bytes", invalid),
	}
	};
	let (cast_kind, discr_ty_bits) = if discr.size.bytes() < op_size.bytes() {
	// The discriminant is less wide than the operand, cast the operand into
	// [MaybeUninit; N] and then index into it.

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc -Zvalidate-mir

Program output

error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
 --> /tmp/icemaker_global_tempdir.04D9S6j216F0/rustc_testrunner_tmpdir_reporting.N3eUhcnKj0fr/mvce.rs:8:33
  |
8 |     let _val: Single = unsafe { std::mem::transmute::<(), Single>(()) };
  |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: source type: `()` (0 bits)
  = note: target type: `Single` (8 bits)

error: internal compiler error: compiler/rustc_mir_transform/src/check_enums.rs:261:24: Found discriminant with invalid size, has 0 bytes


thread 'rustc' (1218568) panicked at compiler/rustc_mir_transform/src/check_enums.rs:261:24:
Box<dyn Any>
stack backtrace:
   0:     0x7fc647a65de3 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h8ee9afd01992c0ca
   1:     0x7fc648201a98 - core::fmt::write::hfc0f7d2ec3afcf13
   2:     0x7fc647a1a463 - std::io::Write::write_fmt::h1808bd8f2dd858a5
   3:     0x7fc647a2b4a2 - std::sys::backtrace::BacktraceLock::print::h9d7baa74bc3f6b8a
   4:     0x7fc647a31329 - std::panicking::default_hook::{{closure}}::he40de9e29b072647
   5:     0x7fc647a30e53 - std::panicking::default_hook::h8e4b76f299931446
   6:     0x7fc646a89b55 - std[213c558d849d7d91]::panicking::update_hook::<alloc[8c4e956626d831dd]::boxed::Box<rustc_driver_impl[11d3a6ec691753b1]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x7fc647a317de - std::panicking::panic_with_hook::h549e3fb761993e77
   8:     0x7fc646ac6a81 - std[213c558d849d7d91]::panicking::begin_panic::<rustc_errors[cc6c12e856929c31]::ExplicitBug>::{closure#0}
   9:     0x7fc646aba316 - std[213c558d849d7d91]::sys::backtrace::__rust_end_short_backtrace::<std[213c558d849d7d91]::panicking::begin_panic<rustc_errors[cc6c12e856929c31]::ExplicitBug>::{closure#0}, !>
  10:     0x7fc646ab7e29 - std[213c558d849d7d91]::panicking::begin_panic::<rustc_errors[cc6c12e856929c31]::ExplicitBug>
  11:     0x7fc646ad21b1 - <rustc_errors[cc6c12e856929c31]::diagnostic::BugAbort as rustc_errors[cc6c12e856929c31]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  12:     0x7fc64707c7da - rustc_middle[659b03de131b3a39]::util::bug::opt_span_bug_fmt::<rustc_span[779f303ab57b8d65]::span_encoding::Span>::{closure#0}
  13:     0x7fc64707c83a - rustc_middle[659b03de131b3a39]::ty::context::tls::with_opt::<rustc_middle[659b03de131b3a39]::util::bug::opt_span_bug_fmt<rustc_span[779f303ab57b8d65]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  14:     0x7fc64706b45b - rustc_middle[659b03de131b3a39]::ty::context::tls::with_context_opt::<rustc_middle[659b03de131b3a39]::ty::context::tls::with_opt<rustc_middle[659b03de131b3a39]::util::bug::opt_span_bug_fmt<rustc_span[779f303ab57b8d65]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  15:     0x7fc6440e1340 - rustc_middle[659b03de131b3a39]::util::bug::bug_fmt
  16:     0x7fc6495a7739 - rustc_mir_transform[d3b79acc3196c9f4]::check_enums::insert_discr_cast_to_u128
  17:     0x7fc648f99d61 - <rustc_mir_transform[d3b79acc3196c9f4]::check_enums::CheckEnums as rustc_mir_transform[d3b79acc3196c9f4]::pass_manager::MirPass>::run_pass
  18:     0x7fc648209436 - rustc_mir_transform[d3b79acc3196c9f4]::run_optimization_passes
  19:     0x7fc648de75e6 - rustc_mir_transform[d3b79acc3196c9f4]::optimized_mir
  20:     0x7fc648de7239 - rustc_query_impl[d1909fe17814b3af]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[d1909fe17814b3af]::query_impl::optimized_mir::dynamic_query::{closure#2}::{closure#0}, rustc_middle[659b03de131b3a39]::query::erase::Erased<[u8; 8usize]>>
  21:     0x7fc648227ab9 - rustc_query_system[672d1867d8a3a753]::query::plumbing::try_execute_query::<rustc_query_impl[d1909fe17814b3af]::DynamicConfig<rustc_query_system[672d1867d8a3a753]::query::caches::DefIdCache<rustc_middle[659b03de131b3a39]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[d1909fe17814b3af]::plumbing::QueryCtxt, false>
  22:     0x7fc648226f9b - rustc_query_impl[d1909fe17814b3af]::query_impl::optimized_mir::get_query_non_incr::__rust_end_short_backtrace
  23:     0x7fc6452ce84b - <rustc_middle[659b03de131b3a39]::ty::context::TyCtxt>::instance_mir
  24:     0x7fc6484067b1 - rustc_interface[ac200cc1ac94b679]::passes::analysis
  25:     0x7fc64840538b - rustc_query_impl[d1909fe17814b3af]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[d1909fe17814b3af]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[659b03de131b3a39]::query::erase::Erased<[u8; 0usize]>>
  26:     0x7fc6492bda20 - rustc_query_system[672d1867d8a3a753]::query::plumbing::try_execute_query::<rustc_query_impl[d1909fe17814b3af]::DynamicConfig<rustc_query_system[672d1867d8a3a753]::query::caches::SingleCache<rustc_middle[659b03de131b3a39]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[d1909fe17814b3af]::plumbing::QueryCtxt, false>
  27:     0x7fc6492bd676 - rustc_query_impl[d1909fe17814b3af]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  28:     0x7fc64949b94b - rustc_interface[ac200cc1ac94b679]::passes::create_and_enter_global_ctxt::<core[7df29245c2304c2]::option::Option<rustc_interface[ac200cc1ac94b679]::queries::Linker>, rustc_driver_impl[11d3a6ec691753b1]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  29:     0x7fc6493e9adf - rustc_interface[ac200cc1ac94b679]::interface::run_compiler::<(), rustc_driver_impl[11d3a6ec691753b1]::run_compiler::{closure#0}>::{closure#1}
  30:     0x7fc649365e38 - std[213c558d849d7d91]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[ac200cc1ac94b679]::util::run_in_thread_with_globals<rustc_interface[ac200cc1ac94b679]::util::run_in_thread_pool_with_globals<rustc_interface[ac200cc1ac94b679]::interface::run_compiler<(), rustc_driver_impl[11d3a6ec691753b1]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  31:     0x7fc649365b1c - <<std[213c558d849d7d91]::thread::Builder>::spawn_unchecked_<rustc_interface[ac200cc1ac94b679]::util::run_in_thread_with_globals<rustc_interface[ac200cc1ac94b679]::util::run_in_thread_pool_with_globals<rustc_interface[ac200cc1ac94b679]::interface::run_compiler<(), rustc_driver_impl[11d3a6ec691753b1]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[7df29245c2304c2]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  32:     0x7fc64936bacd - std::sys::pal::unix::thread::Thread::new::thread_start::h543657e01047c55a
  33:     0x7fc642a969cb - <unknown>
  34:     0x7fc642b1aa4c - <unknown>
  35:                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 1.91.0-nightly (8df154bff 2025-08-23) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z validate-mir -Z dump-mir-dir=dir

query stack during panic:
#0 [optimized_mir] optimizing MIR for `main`
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0512`.

[matthiaskrgr]

smaller

enum A {
    B,
    C,
}

fn main() {
    let _: A = unsafe { std::mem::transmute(()) };
}

[samueltardieu]

@rustbot claim