`repr(transparent)` wrapper with uninhabited ZST has different return ABI than wrapped field.

[zachs18]

This code ICEs Miri, the normal compiler compiles this without complaint, but might be miscompiling it¹, though a const version using -Zunleash-the-miri-inside-of-you does ICE the compiler.

Code

type Field = u8; // any non-1ZST probably
#[repr(transparent)]
struct Foo {
  _zst: std::convert::Infallible,
  _field: Field,
}
fn foo() -> Foo {
  panic!()
}
fn main() {
  let f: fn() -> Foo = foo;
  let f: fn() -> Field = unsafe { std::mem::transmute(f) };
  f();
}  

cargo +nightly miri run

modified version using `-Zunleash-the-miri-inside-of-you` that ICEs rustc

type Field = u8; // any non-1ZST probably
#[repr(transparent)]
struct Foo {
  _zst: std::convert::Infallible,
  _field: Field,
}
const fn foo() -> Foo {
    panic!()
}
fn main() {
    const {
        let f: fn() -> Foo = foo;
        let f: fn() -> Field = unsafe { std::mem::transmute(f) };
        f();
    }
}

RUSTFLAGS="-Zunleash-the-miri-inside-of-you" cargo +nightly run

Backtrace:

thread 'rustc' panicked at compiler/rustc_const_eval/src/interpret/call.rs:253:13:
assertion failed: caller_abi.eq_abi(callee_abi)
stack backtrace:
   0:     0x7e3cb9b652a5 - std::backtrace::Backtrace::create::h8985a2f6ef2eb84f
   1:     0x7e3cb80e39f5 - std::backtrace::Backtrace::force_capture::h9a611efb2a942dc9
   2:     0x7e3cb725a4e0 - std[4faddc71fb6fa0e5]::panicking::update_hook::<alloc[c55780341d73682]::boxed::Box<rustc_driver_impl[80a9ca323d3e3890]::install_ice_hook::{closure#1}>>::{closure#0}
   3:     0x7e3cb80fc333 - std::panicking::rust_panic_with_hook::hf47d9e178e038701
   4:     0x7e3cb80fbff6 - std::panicking::begin_panic_handler::{{closure}}::hd9b9f2d3dd54e86c
   5:     0x7e3cb80f99e9 - std::sys::backtrace::__rust_end_short_backtrace::h342bb668fc58e319
   6:     0x7e3cb80fbced - rust_begin_unwind
   7:     0x7e3cb4da8db0 - core::panicking::panic_fmt::hb3cd74fdfc019257
   8:     0x7e3cb5897b9c - core::panicking::panic::h92e08174b9fe42d1
   9:     0x7e3cb92c11bb - <rustc_const_eval[e3d2a21e14ef9d90]::interpret::eval_context::InterpCx<rustc_const_eval[e3d2a21e14ef9d90]::const_eval::machine::CompileTimeMachine>>::init_stack_frame
  10:     0x7e3cb92bde11 - <rustc_const_eval[e3d2a21e14ef9d90]::interpret::eval_context::InterpCx<rustc_const_eval[e3d2a21e14ef9d90]::const_eval::machine::CompileTimeMachine>>::init_fn_call
  11:     0x7e3cb6314e8f - rustc_const_eval[e3d2a21e14ef9d90]::const_eval::eval_queries::eval_to_allocation_raw_provider
  12:     0x7e3cb9041f5c - rustc_query_impl[283282ac12660651]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[283282ac12660651]::query_impl::eval_to_allocation_raw::dynamic_query::{closure#2}::{closure#0}, rustc_middle[d099a8b084c78b18]::query::erase::Erased<[u8; 24usize]>>
  13:     0x7e3cb9041fb7 - <rustc_query_impl[283282ac12660651]::query_impl::eval_to_allocation_raw::dynamic_query::{closure#2} as core[b8d8efd06dd057cd]::ops::function::FnOnce<(rustc_middle[d099a8b084c78b18]::ty::context::TyCtxt, rustc_middle[d099a8b084c78b18]::ty::PseudoCanonicalInput<rustc_middle[d099a8b084c78b18]::mir::interpret::GlobalId>)>>::call_once
  14:     0x7e3cb90406be - rustc_query_system[57efd1c10fae1b2]::query::plumbing::try_execute_query::<rustc_query_impl[283282ac12660651]::DynamicConfig<rustc_query_system[57efd1c10fae1b2]::query::caches::DefaultCache<rustc_middle[d099a8b084c78b18]::ty::PseudoCanonicalInput<rustc_middle[d099a8b084c78b18]::mir::interpret::GlobalId>, rustc_middle[d099a8b084c78b18]::query::erase::Erased<[u8; 24usize]>>, false, false, false>, rustc_query_impl[283282ac12660651]::plumbing::QueryCtxt, true>
  15:     0x7e3cb903fffd - rustc_query_impl[283282ac12660651]::query_impl::eval_to_allocation_raw::get_query_incr::__rust_end_short_backtrace
  16:     0x7e3cb904475f - rustc_const_eval[e3d2a21e14ef9d90]::const_eval::eval_queries::eval_to_const_value_raw_provider
  17:     0x7e3cb9044554 - rustc_query_impl[283282ac12660651]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[283282ac12660651]::query_impl::eval_to_const_value_raw::dynamic_query::{closure#2}::{closure#0}, rustc_middle[d099a8b084c78b18]::query::erase::Erased<[u8; 24usize]>>
  18:     0x7e3cb904450d - <rustc_query_impl[283282ac12660651]::query_impl::eval_to_const_value_raw::dynamic_query::{closure#2} as core[b8d8efd06dd057cd]::ops::function::FnOnce<(rustc_middle[d099a8b084c78b18]::ty::context::TyCtxt, rustc_middle[d099a8b084c78b18]::ty::PseudoCanonicalInput<rustc_middle[d099a8b084c78b18]::mir::interpret::GlobalId>)>>::call_once
  19:     0x7e3cb903f2e6 - rustc_query_system[57efd1c10fae1b2]::query::plumbing::try_execute_query::<rustc_query_impl[283282ac12660651]::DynamicConfig<rustc_query_system[57efd1c10fae1b2]::query::caches::DefaultCache<rustc_middle[d099a8b084c78b18]::ty::PseudoCanonicalInput<rustc_middle[d099a8b084c78b18]::mir::interpret::GlobalId>, rustc_middle[d099a8b084c78b18]::query::erase::Erased<[u8; 24usize]>>, false, true, false>, rustc_query_impl[283282ac12660651]::plumbing::QueryCtxt, true>
  20:     0x7e3cb903ebc7 - rustc_query_impl[283282ac12660651]::query_impl::eval_to_const_value_raw::get_query_incr::__rust_end_short_backtrace
  21:     0x7e3cb95e110b - <rustc_middle[d099a8b084c78b18]::ty::context::TyCtxt>::const_eval_resolve
  22:     0x7e3cb5825b39 - <rustc_mir_transform[e7af90bdb15a3f20]::known_panics_lint::ConstPropagator as rustc_middle[d099a8b084c78b18]::mir::visit::Visitor>::visit_operand
  23:     0x7e3cb5829080 - <rustc_mir_transform[e7af90bdb15a3f20]::known_panics_lint::ConstPropagator as rustc_middle[d099a8b084c78b18]::mir::visit::Visitor>::visit_assign
  24:     0x7e3cb582799b - <rustc_mir_transform[e7af90bdb15a3f20]::known_panics_lint::ConstPropagator as rustc_middle[d099a8b084c78b18]::mir::visit::Visitor>::visit_body
  25:     0x7e3cb95f79f9 - <rustc_mir_transform[e7af90bdb15a3f20]::known_panics_lint::KnownPanicsLint as rustc_mir_transform[e7af90bdb15a3f20]::pass_manager::MirLint>::run_lint
  26:     0x7e3cb880b6f3 - rustc_mir_transform[e7af90bdb15a3f20]::run_analysis_to_runtime_passes
  27:     0x7e3cb8c28174 - rustc_mir_transform[e7af90bdb15a3f20]::mir_drops_elaborated_and_const_checked
  28:     0x7e3cb8c27da5 - rustc_query_impl[283282ac12660651]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[283282ac12660651]::query_impl::mir_drops_elaborated_and_const_checked::dynamic_query::{closure#2}::{closure#0}, rustc_middle[d099a8b084c78b18]::query::erase::Erased<[u8; 8usize]>>
  29:     0x7e3cb8c2bcd9 - rustc_query_system[57efd1c10fae1b2]::query::plumbing::try_execute_query::<rustc_query_impl[283282ac12660651]::DynamicConfig<rustc_data_structures[ff6b6eb237e26604]::vec_cache::VecCache<rustc_span[6fdbe82b95f770dc]::def_id::LocalDefId, rustc_middle[d099a8b084c78b18]::query::erase::Erased<[u8; 8usize]>, rustc_query_system[57efd1c10fae1b2]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[283282ac12660651]::plumbing::QueryCtxt, true>
  30:     0x7e3cb8c24dae - rustc_query_impl[283282ac12660651]::query_impl::mir_drops_elaborated_and_const_checked::get_query_incr::__rust_end_short_backtrace
  31:     0x7e3cb8c248f3 - <rustc_middle[d099a8b084c78b18]::hir::map::Map>::par_body_owners::<rustc_interface[5facaafd3cd44e6f]::passes::run_required_analyses::{closure#3}::{closure#0}>::{closure#0}
  32:     0x7e3cb8c21df4 - rustc_interface[5facaafd3cd44e6f]::passes::run_required_analyses
  33:     0x7e3cb976ca9e - rustc_interface[5facaafd3cd44e6f]::passes::analysis
  34:     0x7e3cb976ca6f - rustc_query_impl[283282ac12660651]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[283282ac12660651]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[d099a8b084c78b18]::query::erase::Erased<[u8; 0usize]>>
  35:     0x7e3cb97683cf - rustc_query_system[57efd1c10fae1b2]::query::plumbing::try_execute_query::<rustc_query_impl[283282ac12660651]::DynamicConfig<rustc_query_system[57efd1c10fae1b2]::query::caches::SingleCache<rustc_middle[d099a8b084c78b18]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[283282ac12660651]::plumbing::QueryCtxt, true>
  36:     0x7e3cb9767d47 - rustc_query_impl[283282ac12660651]::query_impl::analysis::get_query_incr::__rust_end_short_backtrace
  37:     0x7e3cb9820669 - rustc_interface[5facaafd3cd44e6f]::passes::create_and_enter_global_ctxt::<core[b8d8efd06dd057cd]::option::Option<rustc_interface[5facaafd3cd44e6f]::queries::Linker>, rustc_driver_impl[80a9ca323d3e3890]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  38:     0x7e3cb9813896 - rustc_interface[5facaafd3cd44e6f]::interface::run_compiler::<(), rustc_driver_impl[80a9ca323d3e3890]::run_compiler::{closure#0}>::{closure#1}
  39:     0x7e3cb9670807 - std[4faddc71fb6fa0e5]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[5facaafd3cd44e6f]::util::run_in_thread_with_globals<rustc_interface[5facaafd3cd44e6f]::util::run_in_thread_pool_with_globals<rustc_interface[5facaafd3cd44e6f]::interface::run_compiler<(), rustc_driver_impl[80a9ca323d3e3890]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  40:     0x7e3cb96704d9 - <<std[4faddc71fb6fa0e5]::thread::Builder>::spawn_unchecked_<rustc_interface[5facaafd3cd44e6f]::util::run_in_thread_with_globals<rustc_interface[5facaafd3cd44e6f]::util::run_in_thread_pool_with_globals<rustc_interface[5facaafd3cd44e6f]::interface::run_compiler<(), rustc_driver_impl[80a9ca323d3e3890]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[b8d8efd06dd057cd]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  41:     0x7e3cb966fc6b - std::sys::pal::unix::thread::Thread::new::thread_start::h07063fc11dcd9adf
  42:     0x7e3cb389ca94 - start_thread
                               at ./nptl/pthread_create.c:447:8
  43:     0x7e3cb3929c3c - clone3
                               at ./misc/../sysdeps/unix/sysv/linux/x86_64/clone3.S:78:0
  44:                0x0 - <unknown>


rustc version: 1.86.0-nightly (9a1d156f3 2025-01-19)
platform: x86_64-unknown-linux-gnu

query stack during panic:
#0 [eval_to_allocation_raw] const-evaluating + checking `main::{constant#0}`
#1 [eval_to_const_value_raw] simplifying constant for the type system `main::{constant#0}`
#2 [mir_drops_elaborated_and_const_checked] elaborating drops for `main`
#3 [analysis] running analysis passes on this crate
end of query stack

Meta

rustc --version --verbose:

rustc 1.86.0-nightly (9a1d156f3 2025-01-19)
binary: rustc
commit-hash: 9a1d156f38c51441ee51e5a068f1d0caf4bb0f27
commit-date: 2025-01-19
host: x86_64-unknown-linux-gnu
release: 1.86.0-nightly
LLVM version: 19.1.7

Error output

Error output

thread 'rustc' panicked at /rustc/9a1d156f38c51441ee51e5a068f1d0caf4bb0f27/compiler/rustc_const_eval/src/interpret/call.rs:253:13:
assertion failed: caller_abi.eq_abi(callee_abi)
stack backtrace:
   0:     0x78aed8af951a - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h70d5098cfd90b01a
   1:     0x78aed9212da6 - core::fmt::write::h854c563aac02d08a
   2:     0x78aeda171291 - std::io::Write::write_fmt::hac4174e389a0e8e6
   3:     0x78aed8af9372 - std::sys::backtrace::BacktraceLock::print::hc6aba0d6b3a91e1f
   4:     0x78aed8afb7f2 - std::panicking::default_hook::{{closure}}::h9628a97795328020
   5:     0x78aed8afb67a - std::panicking::default_hook::h0dc7e102448ae736
   6:     0x78aed7c59eeb - std[4faddc71fb6fa0e5]::panicking::update_hook::<alloc[c55780341d73682]::boxed::Box<rustc_driver_impl[80a9ca323d3e3890]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x78aed8afc333 - std::panicking::rust_panic_with_hook::hf47d9e178e038701
   8:     0x78aed8afbff6 - std::panicking::begin_panic_handler::{{closure}}::hd9b9f2d3dd54e86c
   9:     0x78aed8af99e9 - std::sys::backtrace::__rust_end_short_backtrace::h342bb668fc58e319
  10:     0x78aed8afbced - rust_begin_unwind
  11:     0x78aed57a8db0 - core::panicking::panic_fmt::hb3cd74fdfc019257
  12:     0x78aed6297b9c - core::panicking::panic::h92e08174b9fe42d1
  13:     0x592a4689f7ae - <rustc_const_eval[e3d2a21e14ef9d90]::interpret::eval_context::InterpCx<miri[d18ac4a41561b88d]::machine::MiriMachine>>::check_argument_compat
  14:     0x592a4689ec5a - <rustc_const_eval[e3d2a21e14ef9d90]::interpret::eval_context::InterpCx<miri[d18ac4a41561b88d]::machine::MiriMachine>>::init_stack_frame
  15:     0x592a4689c98b - <rustc_const_eval[e3d2a21e14ef9d90]::interpret::eval_context::InterpCx<miri[d18ac4a41561b88d]::machine::MiriMachine>>::init_fn_call
  16:     0x592a46915db4 - miri[d18ac4a41561b88d]::eval::eval_entry::{closure#0}
  17:     0x592a46911e9b - miri[d18ac4a41561b88d]::eval::eval_entry
  18:     0x592a467c4226 - <miri[e9b5d2e94aebaf6c]::MiriCompilerCalls as rustc_driver_impl[80a9ca323d3e3890]::Callbacks>::after_analysis
  19:     0x78aeda220676 - rustc_interface[5facaafd3cd44e6f]::passes::create_and_enter_global_ctxt::<core[b8d8efd06dd057cd]::option::Option<rustc_interface[5facaafd3cd44e6f]::queries::Linker>, rustc_driver_impl[80a9ca323d3e3890]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  20:     0x78aeda213896 - rustc_interface[5facaafd3cd44e6f]::interface::run_compiler::<(), rustc_driver_impl[80a9ca323d3e3890]::run_compiler::{closure#0}>::{closure#1}
  21:     0x78aeda070807 - std[4faddc71fb6fa0e5]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[5facaafd3cd44e6f]::util::run_in_thread_with_globals<rustc_interface[5facaafd3cd44e6f]::util::run_in_thread_pool_with_globals<rustc_interface[5facaafd3cd44e6f]::interface::run_compiler<(), rustc_driver_impl[80a9ca323d3e3890]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  22:     0x78aeda0704d9 - <<std[4faddc71fb6fa0e5]::thread::Builder>::spawn_unchecked_<rustc_interface[5facaafd3cd44e6f]::util::run_in_thread_with_globals<rustc_interface[5facaafd3cd44e6f]::util::run_in_thread_pool_with_globals<rustc_interface[5facaafd3cd44e6f]::interface::run_compiler<(), rustc_driver_impl[80a9ca323d3e3890]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[b8d8efd06dd057cd]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  23:     0x78aeda06fc6b - std::sys::pal::unix::thread::Thread::new::thread_start::h07063fc11dcd9adf
  24:     0x78aed429ca94 - start_thread
                               at ./nptl/pthread_create.c:447:8
  25:     0x78aed4329c3c - clone3
                               at ./misc/../sysdeps/unix/sysv/linux/x86_64/clone3.S:78:0
  26:                0x0 - <unknown>

error: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/miri/issues/new

note: please make sure that you have updated to the latest nightly

note: please attach the file at `/tmp/sadkjlhfsakdf/rustc-ice-2025-01-21T00_17_45-3693735.txt` to your bug report

note: compiler flags: --crate-type bin -C embed-bitcode=no -C debuginfo=2 -C incremental=[REDACTED]

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack

Miri caused an ICE during evaluation. Here's the interpreter backtrace at the time of the panic:
note: the place in the program where the ICE was triggered
  --> src/main.rs:7:5
   |
7  |     panic!()
   |     ^^^^^^^^
   |
   = note: BACKTRACE:
   = note: inside `foo` at /home/zachary/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/panic.rs:90:9: 90:30
note: inside `main`
  --> src/main.rs:12:5
   |
12 |     f();
   |     ^^^
   = note: inside `<fn() as std::ops::FnOnce<()>>::call_once - shim(fn())` at /home/zachary/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5: 250:71
   = note: inside `std::sys::backtrace::__rust_begin_short_backtrace::<fn(), ()>` at /home/zachary/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys/backtrace.rs:152:18: 152:21
   = note: inside closure at /home/zachary/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:194:18: 194:75
   = note: inside `std::ops::function::impls::<impl std::ops::FnOnce<()> for &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>::call_once` at /home/zachary/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:284:13: 284:31
   = note: inside `std::panicking::r#try::do_call::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>` at /home/zachary/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:587:40: 587:43
   = note: inside `std::panicking::r#try::<i32, &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>` at /home/zachary/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:550:19: 550:88
   = note: inside `std::panic::catch_unwind::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>` at /home/zachary/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panic.rs:358:14: 358:33
   = note: inside closure at /home/zachary/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:163:24: 163:49
   = note: inside `std::panicking::r#try::do_call::<{closure@std::rt::lang_start_internal::{closure#0}}, isize>` at /home/zachary/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:587:40: 587:43
   = note: inside `std::panicking::r#try::<isize, {closure@std::rt::lang_start_internal::{closure#0}}>` at /home/zachary/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:550:19: 550:88
   = note: inside `std::panic::catch_unwind::<{closure@std::rt::lang_start_internal::{closure#0}}, isize>` at /home/zachary/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panic.rs:358:14: 358:33
   = note: inside `std::rt::lang_start_internal` at /home/zachary/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:159:5: 181:7
   = note: inside `std::rt::lang_start::<()>` at /home/zachary/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:193:5: 198:6
   = note: this note originates in the macro `$crate::panic::panic_2021` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)

ICE file

thread 'rustc' panicked at /rustc/9a1d156f38c51441ee51e5a068f1d0caf4bb0f27/compiler/rustc_const_eval/src/interpret/call.rs:253:13:
assertion failed: caller_abi.eq_abi(callee_abi)
stack backtrace:
   0:     0x78aeda5652a5 - std::backtrace::Backtrace::create::h8985a2f6ef2eb84f
   1:     0x78aed8ae39f5 - std::backtrace::Backtrace::force_capture::h9a611efb2a942dc9
   2:     0x78aed7c5a4e0 - std[4faddc71fb6fa0e5]::panicking::update_hook::<alloc[c55780341d73682]::boxed::Box<rustc_driver_impl[80a9ca323d3e3890]::install_ice_hook::{closure#1}>>::{closure#0}
   3:     0x78aed8afc333 - std::panicking::rust_panic_with_hook::hf47d9e178e038701
   4:     0x78aed8afbff6 - std::panicking::begin_panic_handler::{{closure}}::hd9b9f2d3dd54e86c
   5:     0x78aed8af99e9 - std::sys::backtrace::__rust_end_short_backtrace::h342bb668fc58e319
   6:     0x78aed8afbced - rust_begin_unwind
   7:     0x78aed57a8db0 - core::panicking::panic_fmt::hb3cd74fdfc019257
   8:     0x78aed6297b9c - core::panicking::panic::h92e08174b9fe42d1
   9:     0x592a4689f7ae - <rustc_const_eval[e3d2a21e14ef9d90]::interpret::eval_context::InterpCx<miri[d18ac4a41561b88d]::machine::MiriMachine>>::check_argument_compat
  10:     0x592a4689ec5a - <rustc_const_eval[e3d2a21e14ef9d90]::interpret::eval_context::InterpCx<miri[d18ac4a41561b88d]::machine::MiriMachine>>::init_stack_frame
  11:     0x592a4689c98b - <rustc_const_eval[e3d2a21e14ef9d90]::interpret::eval_context::InterpCx<miri[d18ac4a41561b88d]::machine::MiriMachine>>::init_fn_call
  12:     0x592a46915db4 - miri[d18ac4a41561b88d]::eval::eval_entry::{closure#0}
  13:     0x592a46911e9b - miri[d18ac4a41561b88d]::eval::eval_entry
  14:     0x592a467c4226 - <miri[e9b5d2e94aebaf6c]::MiriCompilerCalls as rustc_driver_impl[80a9ca323d3e3890]::Callbacks>::after_analysis
  15:     0x78aeda220676 - rustc_interface[5facaafd3cd44e6f]::passes::create_and_enter_global_ctxt::<core[b8d8efd06dd057cd]::option::Option<rustc_interface[5facaafd3cd44e6f]::queries::Linker>, rustc_driver_impl[80a9ca323d3e3890]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  16:     0x78aeda213896 - rustc_interface[5facaafd3cd44e6f]::interface::run_compiler::<(), rustc_driver_impl[80a9ca323d3e3890]::run_compiler::{closure#0}>::{closure#1}
  17:     0x78aeda070807 - std[4faddc71fb6fa0e5]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[5facaafd3cd44e6f]::util::run_in_thread_with_globals<rustc_interface[5facaafd3cd44e6f]::util::run_in_thread_pool_with_globals<rustc_interface[5facaafd3cd44e6f]::interface::run_compiler<(), rustc_driver_impl[80a9ca323d3e3890]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  18:     0x78aeda0704d9 - <<std[4faddc71fb6fa0e5]::thread::Builder>::spawn_unchecked_<rustc_interface[5facaafd3cd44e6f]::util::run_in_thread_with_globals<rustc_interface[5facaafd3cd44e6f]::util::run_in_thread_pool_with_globals<rustc_interface[5facaafd3cd44e6f]::interface::run_compiler<(), rustc_driver_impl[80a9ca323d3e3890]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[b8d8efd06dd057cd]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  19:     0x78aeda06fc6b - std::sys::pal::unix::thread::Thread::new::thread_start::h07063fc11dcd9adf
  20:     0x78aed429ca94 - start_thread
                               at ./nptl/pthread_create.c:447:8
  21:     0x78aed4329c3c - clone3
                               at ./misc/../sysdeps/unix/sysv/linux/x86_64/clone3.S:78:0
  22:                0x0 - <unknown>


rustc version: 1.86.0-nightly (9a1d156f3 2025-01-19)
platform: x86_64-unknown-linux-gnu

query stack during panic:
end of query stack

@rustbot label +A-miri +A-ABI

---

This shouldn't ICE regardless, but whether there is a miscompilation depends on if the original code is sound:

CC rust-lang/unsafe-code-guidelines#485

Possible Miscompilation

Does #[repr(transparent)]s ABI-compatility guarantee apply in the prescence of uninhabited 1-ZST fields?

A #[repr(transparent)] wrapper that contains an uninhabited 1-ZST field gets Uninhabited ABI IIUC, which may contradict the guarantees about #[repr(transparent)] wrappers being ABI-compatible.

Consider Foo from above:

type Field = u8; // any non-1ZST probably
#[repr(transparent)]
struct Foo {
  _zst: std::convert::Infallible,
  _field: Field,
}

If #[repr(transparent)]'s ABI-compatility guarantees do apply, then the above code should be sound:

1. If a function returns Foo, it must diverge (e.g. by panicking).
2. If Foo is ABI-compatible with Field, then it should be sound to transmute a function pointer to a diverging fn() -> Foo into a fn() -> Field that also diverges.

rustc_layout(debug)

Using #![feature(rustc_attrs)] and #[rustc_layout(debug)] we see that the ABI it has is

error: layout_of(Foo) = Layout {
          // snip
           abi: Uninhabited,
          // snip
       }

whereas Field has

error: layout_of(u8) = Layout {
           // snip
           abi: Scalar(
               Initialized {
                   value: Int(
                       I8,
                       false,
                   ),
                   valid_range: 0..=255,
               },
           ),
           // snip
        }

Possible Miscompilation

Assuming this is supposed to be sound, then there is an observable miscompilation here too. Where Field is returned by invisible reference, fn(u32) -> Foo does not return Foo by invisible reference, so transmuting fn(u32) -> Foo into fn(u32) -> Field will pass a different number of arguments.

Specifically on x86_64-unknown-linux-gnu: If Field is returned on the stack, then fn(..) -> Foo does not reserve stack space or pass a hidden reference parameter, but fn(..) -> Field does. This can be made into an observable miscompilation if the function takes parameters: (playground link, IIUC: the invisible reference is being passed, but the callee isn't expecting it, so it thinks it's the first "real" parameter instead (caller: ret by-ref in rdi, x in rsi; callee: ret uninhabited, x in rdi)).

Footnotes

1. see "Possible Miscompilation" section ↩

[ia0]

I would argue the original code is unsound. The intent (coming from the name) of repr(transparent) is to have the same runtime behavior as the "wrapped" field. Using an uninhabited 1-ZST breaks this intent. I believe this is an oversight of layout.repr.transparent.constraint-field. In particular, I would expect the first bullet point to be (added term in bold):

• any number of inhabited fields with size 0 and alignment 1 (e.g. PhantomData<T>), and

[RalfJung]

That would be a breaking change though since we currently accept code like this on stable:

enum Void {}

#[rept(transparent)]
struct Wrap<T>(Void, T);

@rust-lang/lang turns out accepting this code is incoherent; we cannot make this a transparent wrapper around T as Wrap<T> and T can behave differently when used as return type for a function. Would you be open to making this a hard error (likely with a transition period)?

Cc @rust-lang/opsem

[hanna-kruppe]

I’m not sure it’s incoherent from a language perspective. Clearly it clashes with the current lowering chosen by the compiler, but this may just be a compiler bug.

There’s no problem when the 1-ZST field is inhabited from the compiler’s perspective, even if it has a custom safety invariant that makes it effectively impossible to construct, right? It’s just that the compiler currently applies the “return type is known to be uninhabited -> function can’t return at all” reasoning to functions where this contradicts our general function ABI compatibility rules. Similarly, if there was a target where NonZeroU32 could be passed in a different set of registers than regular u32, it would also be incorrect for rustc to pass NonZeroU32 arguments and results in those registers because someone might transmute fn(u32) -> NonZeroU32 to fn(NonZeroU32) -> u32 and call it and that would be fine according to https://doc.rust-lang.org/std/primitive.fn.html#abi-compatibility — even sound w.r.t. the safety invariant of NonZeroU32.

Changing this would be a slight loss of optimization power in theory but it doesn’t seem so bad since any call site that calls the function as fn() -> Foo is still assured that the call won’t return, even if it may need to reserve stack space that could otherwise be omitted. But this is already the case for functions returning e.g., (String, !, bool) since that’s not a ZST either - so it’s not specific to repr(transparent) or ABI compatibility rules.

[ia0]

I think the problem goes a bit further than ABI issues. It's the essence of what repr(transparent) means. Where is the UB in the following program? (You can assume that the modules are different crates.)

pub mod crate_foo {
    enum False {}
    #[repr(transparent)]
    pub struct Foo {
        proof: False,
        pub value: u32,
    }
    impl Foo {
        pub fn open(self) -> ! {
            match self.proof {}
        }
    }
}

pub mod crate_bar {
    use crate::crate_foo::Foo;
    pub fn convert(foo: u32) -> Foo {
        // SAFETY: crate_foo::Foo is a repr(transparent) u32.
        unsafe { std::mem::transmute(foo) }
    }
}

fn main() {
    crate_bar::convert(42).open();
    println!("unreachable");
}

I can see at least a few options:

• The #[repr(transparent)] on Foo because proof is not a ghost value (aka a unit type that does not contribute to the operational semantics).
• Making the #[repr(transparent)] part of the public API of Foo by making the value field public.
• Exposing the function Foo::open().
• The transmute in convert() because a public repr(transparent) doesn't mean you can always convert (in any direction, but in this case wrap).

Miri complains on the last point, because that's where a language UB occurs. But is this really what we want to forbid?

[hanna-kruppe]

It's the fault of crate_bar for trying to construct a Foo value via transmute without knowing the full safety and validity invariants of Foo (I'm pretty sure a transmute in the other direction is always sound, it should be equivalent to just reading the public field). This is not only about whether the compiler considers something uninhabited. For example, suppose the private field in the newtype wrapper is an inhabited ZST that serves as witness of std::is_*_feature_detected!("some_specific_target_feature"), with a safe constructor that checks this condition. Then crate_foo may use existence of this witness to justify invoking std::arch intrinsics that are UB to call if the corresponding target_feature is not actually available. In this case crate_bar's transmute is unsound for exactly the same reason, but without any language-level invariants being involved.

(This is not a fictional example, I've written such code before except that I did not yet put the witness ZST in a field of a transparent wrapper.)

[RalfJung]

It’s just that the compiler currently applies the “return type is known to be uninhabited -> function can’t return at all” reasoning to functions where this contradicts our general function ABI compatibility rules.

I am not sure what alternative fix you are suggesting though. We would have to entirely remove BackendRepr::Uninhabited, or at least stop using it for the ABI.

To me this seems like a clear oversight -- we intended to only allow fields that are irrelevant to the language (ignoring safety invariants, the user is responsible for that), but forgot about uninhabited types.