ICE: `assertion left == right failed` in `rustc_const_eval/src/interpret/cast.rs unsize_into`

[cushionbadak]

Code

#![feature(coerce_unsized)]
use std::ops::CoerceUnsized;

struct Foo<T: ?Sized> {
    a: T,
}

impl<T, U> CoerceUnsized<U> for Foo<T> {}

union U {
    a: usize,
}

const C: U = Foo { a: 10 };

fn main() {}

Meta

rustc --version --verbose:

rustc 1.81.0-nightly (fda509e81 2024-06-25)
binary: rustc
commit-hash: fda509e817abeeecb5b76bc1de844f355675c81e
commit-date: 2024-06-25
host: x86_64-apple-darwin
release: 1.81.0-nightly
LLVM version: 18.1.7

Error output

error[E0376]: the trait `CoerceUnsized` may only be implemented for a coercion between structures
 --> cast442.rs:8:1
  |
8 | impl<T, U> CoerceUnsized<U> for Foo<T> {}
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Backtrace

thread 'rustc' panicked at compiler/rustc_const_eval/src/interpret/cast.rs:442:17:
assertion `left == right` failed
  left: Foo
 right: U
stack backtrace:
   0: _rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::panicking::assert_failed_inner
   3: core::panicking::assert_failed::<&rustc_middle::ty::adt::AdtDef, &rustc_middle::ty::adt::AdtDef>
   4: <rustc_const_eval::interpret::eval_context::InterpCx<rustc_const_eval::const_eval::machine::CompileTimeMachine>>::unsize_into
   5: <rustc_const_eval::interpret::eval_context::InterpCx<rustc_const_eval::const_eval::machine::CompileTimeMachine>>::statement
   6: rustc_const_eval::const_eval::eval_queries::eval_to_allocation_raw_provider
      [... omitted 2 frames ...]
   7: rustc_middle::query::plumbing::query_get_at::<rustc_query_system::query::caches::DefaultCache<rustc_middle::ty::ParamEnvAnd<rustc_middle::mir::interpret::GlobalId>, rustc_middle::query::erase::Erased<[u8; 24]>>>
   8: rustc_const_eval::const_eval::eval_queries::eval_to_const_value_raw_provider
      [... omitted 2 frames ...]
   9: <rustc_middle::hir::map::Map>::par_body_owners::<rustc_hir_analysis::check_crate::{closure#3}>::{closure#0}
  10: rustc_hir_analysis::check_crate
  11: rustc_interface::passes::run_required_analyses
  12: rustc_interface::passes::analysis
      [... omitted 1 frame ...]
  13: <rustc_interface::queries::QueryResult<&rustc_middle::ty::context::GlobalCtxt>>::enter::<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#0}::{closure#1}::{closure#5}>
  14: rustc_interface::interface::run_compiler::<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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: please attach the file at `/Volumes/T7/workspace/placeholder_rustexec/0625/rustc-ice-2024-06-26T06_34_30-28249.txt` to your bug report

query stack during panic:
#0 [eval_to_allocation_raw] const-evaluating + checking `C`
#1 [eval_to_const_value_raw] simplifying constant for the type system `C`
#2 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0376`.

Note

• ICE location: compiler/rustc_const_eval/src/interpret/cast.rs#L442
  

  rust/compiler/rustc_const_eval/src/interpret/cast.rs

  Lines 431 to 442 in fda509e

  	pub fn unsize_into(
  	&mut self,
  	src: &OpTy<'tcx, M::Provenance>,
  	cast_ty: TyAndLayout<'tcx>,
  	dest: &PlaceTy<'tcx, M::Provenance>,
  	) -> InterpResult<'tcx> {
  	trace!("Unsizing {:?} of type {} into {}", *src, src.layout.ty, cast_ty.ty);
  	match (&src.layout.ty.kind(), &cast_ty.ty.kind()) {
  	(&ty::Ref(_, s, _), &ty::Ref(_, c, _) | &ty::RawPtr(c, _))
  	| (&ty::RawPtr(s, _), &ty::RawPtr(c, _)) => self.unsize_into_ptr(src, dest, *s, *c),
  	(&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => {
  	assert_eq!(def_a, def_b); // implies same number of fields

• ICE: unsize_into: invalid conversion in rustc_const_eval/src/interpret/cast.rs #126269 might be related

@rustbot label +F-coerce_unsized

[lqd]

This bisects to #121087 so I'll cc @oli-obk, but that PR surfaced a bunch of pre-existing stable ICEs. It seems possible to be the case here as well, though the usual simple transforms to surface the stable ICEs didn't seem to be applicable here, so maybe not.

[oli-obk]

Yea, this needed that PR on top of my "don't hide errors from later phases" PRs. But even without #121087 it would then have been reproducible with

use std::ops::CoerceUnsized;

struct Foo<T: ?Sized> {
    a: T,
}

impl<T, U> CoerceUnsized<U> for Foo<T> {}

union U {
    a: usize,
}

const C: U = Foo { a: 10 };

const _: [(); {
    C;
    0
}] = [];

fn main() {}

[lqd]

That's what I expected but it didn't happen, the rewritten example doesn't ICE on 1.77 for example, but maybe it would only with the other PR that makes compilation go further (if I understood you correctly).