ICE: `expected wide pointer extra data (e.g. slice length or trait object vtable)`

[Naserume]

Code

#[derive(Copy, Clone)]
struct Wide<'a>(&'a Foo, &'static VTable);

trait Cap<'a> {}

struct VTable {
    bar: dyn Cap,
}

trait Bar {
    fn bar(&self) -> u32;
}

struct Foo {
    foo: u32,
    bar: bool,
}

impl Bar for Foo {
    fn bar(&self) -> u32 {}
}

#[repr(C)]
union Transmute<T: Copy, U: Copy> {
    t: T,
    u: U,
}

const FOO: &dyn Bar = &Foo {
    foo: 128,
    bar: false,
};

const G: Wide = Transmute { t: FOO }.u;

fn main() {}

Meta

rustc --version --verbose:

rustc 1.81.0-nightly (fcaa6fdfb 2024-07-13)
binary: rustc
commit-hash: fcaa6fdfbee1316184e7ad98c53241d52cd30a5f
commit-date: 2024-07-13
host: x86_64-apple-darwin
release: 1.81.0-nightly
LLVM version: 18.1.7

Error output

error[E0106]: missing lifetime specifier
 --> ./A6D34.rs:7:14
  |
7 |     bar: dyn Cap,
  |              ^^^ expected named lifetime parameter
  |
  = note: for more information on higher-ranked polymorphism, visit https://doc.rust-lang.org/nomicon/hrtb.html
help: consider making the bound lifetime-generic with a new `'a` lifetime
  |
7 |     bar: dyn for<'a> Cap<'a>,
  |              +++++++    ++++
help: consider introducing a named lifetime parameter
  |
6 ~ struct VTable<'a> {
7 ~     bar: dyn Cap<'a>,
  |

Backtrace

error: internal compiler error: compiler/rustc_const_eval/src/interpret/place.rs:39:17: expected wide pointer extra data (e.g. slice length or trait object vtable)

thread 'rustc' panicked at compiler/rustc_const_eval/src/interpret/place.rs:39:17:
Box<dyn Any>
stack backtrace:
   0:        0x101ff6373 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h605d51fc858e32d3
   1:        0x1020412fb - core::fmt::write::he70b1f5b242a09e4
   2:        0x101fec0be - std::io::Write::write_fmt::hbfde528b9dfff91a
   3:        0x101ff8c1a - std::panicking::default_hook::{{closure}}::hfdc91bfcaaf40c09
   4:        0x101ff88ca - std::panicking::default_hook::h8673ceadddb8316e
   5:        0x10b37d8dc - std[8bfe66706d3bd117]::panicking::update_hook::<alloc[ea04c33bd6f71985]::boxed::Box<rustc_driver_impl[d8a49a9463f03e6]::install_ice_hook::{closure#0}>>::{closure#0}
   6:        0x101ff9a09 - std::panicking::rust_panic_with_hook::h206b639ed4704bb1
   7:        0x10b3f0bb7 - std[8bfe66706d3bd117]::panicking::begin_panic::<rustc_errors[7f155011d809a3bf]::ExplicitBug>::{closure#0}
   8:        0x10b3dc9f9 - std[8bfe66706d3bd117]::sys::backtrace::__rust_end_short_backtrace::<std[8bfe66706d3bd117]::panicking::begin_panic<rustc_errors[7f155011d809a3bf]::ExplicitBug>::{closure#0}, !>
   9:        0x10fe57469 - std[8bfe66706d3bd117]::panicking::begin_panic::<rustc_errors[7f155011d809a3bf]::ExplicitBug>
  10:        0x10b402c06 - <rustc_errors[7f155011d809a3bf]::diagnostic::BugAbort as rustc_errors[7f155011d809a3bf]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  11:        0x10c0c8b45 - rustc_middle[66eaac359ba8a608]::util::bug::opt_span_bug_fmt::<rustc_span[9df13b32b3ae4eb2]::span_encoding::Span>::{closure#0}
  12:        0x10c07f0b7 - rustc_middle[66eaac359ba8a608]::ty::context::tls::with_opt::<rustc_middle[66eaac359ba8a608]::util::bug::opt_span_bug_fmt<rustc_span[9df13b32b3ae4eb2]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  13:        0x10c07ebf5 - rustc_middle[66eaac359ba8a608]::ty::context::tls::with_context_opt::<rustc_middle[66eaac359ba8a608]::ty::context::tls::with_opt<rustc_middle[66eaac359ba8a608]::util::bug::opt_span_bug_fmt<rustc_span[9df13b32b3ae4eb2]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  14:        0x10ff1ff9b - rustc_middle[66eaac359ba8a608]::util::bug::bug_fmt
  15:        0x10b2f2c32 - <rustc_const_eval[b2dc86267216f489]::interpret::validity::ValidityVisitor<rustc_const_eval[b2dc86267216f489]::const_eval::machine::CompileTimeMachine>>::check_wide_ptr_meta
  16:        0x10b2f3141 - <rustc_const_eval[b2dc86267216f489]::interpret::validity::ValidityVisitor<rustc_const_eval[b2dc86267216f489]::const_eval::machine::CompileTimeMachine>>::check_safe_pointer
  17:        0x10b2f57d3 - <rustc_const_eval[b2dc86267216f489]::interpret::validity::ValidityVisitor<rustc_const_eval[b2dc86267216f489]::const_eval::machine::CompileTimeMachine> as rustc_const_eval[b2dc86267216f489]::interpret::visitor::ValueVisitor<rustc_const_eval[b2dc86267216f489]::const_eval::machine::CompileTimeMachine>>::visit_value
  18:        0x10b2f629e - <rustc_const_eval[b2dc86267216f489]::interpret::validity::ValidityVisitor<rustc_const_eval[b2dc86267216f489]::const_eval::machine::CompileTimeMachine> as rustc_const_eval[b2dc86267216f489]::interpret::visitor::ValueVisitor<rustc_const_eval[b2dc86267216f489]::const_eval::machine::CompileTimeMachine>>::visit_value
  19:        0x10b2f7e85 - <rustc_const_eval[b2dc86267216f489]::interpret::eval_context::InterpCx<rustc_const_eval[b2dc86267216f489]::const_eval::machine::CompileTimeMachine>>::validate_operand_internal
  20:        0x10b303b5b - rustc_const_eval[b2dc86267216f489]::const_eval::eval_queries::eval_to_allocation_raw_provider
  21:        0x10ca63d1c - rustc_query_impl[ab99e8e391ad42b2]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[ab99e8e391ad42b2]::query_impl::eval_to_allocation_raw::dynamic_query::{closure#2}::{closure#0}, rustc_middle[66eaac359ba8a608]::query::erase::Erased<[u8; 24usize]>>
  22:        0x10ca173de - <rustc_query_impl[ab99e8e391ad42b2]::query_impl::eval_to_allocation_raw::dynamic_query::{closure#2} as core[84f3396b7379d157]::ops::function::FnOnce<(rustc_middle[66eaac359ba8a608]::ty::context::TyCtxt, rustc_middle[66eaac359ba8a608]::ty::ParamEnvAnd<rustc_middle[66eaac359ba8a608]::mir::interpret::GlobalId>)>>::call_once
  23:        0x10c84e42d - rustc_query_system[55587a8be6cb95b6]::query::plumbing::try_execute_query::<rustc_query_impl[ab99e8e391ad42b2]::DynamicConfig<rustc_query_system[55587a8be6cb95b6]::query::caches::DefaultCache<rustc_middle[66eaac359ba8a608]::ty::ParamEnvAnd<rustc_middle[66eaac359ba8a608]::mir::interpret::GlobalId>, rustc_middle[66eaac359ba8a608]::query::erase::Erased<[u8; 24usize]>>, false, false, false>, rustc_query_impl[ab99e8e391ad42b2]::plumbing::QueryCtxt, false>
  24:        0x10ca8ed1c - rustc_query_impl[ab99e8e391ad42b2]::query_impl::eval_to_allocation_raw::get_query_non_incr::__rust_end_short_backtrace
  25:        0x10b22d5a2 - rustc_middle[66eaac359ba8a608]::query::plumbing::query_get_at::<rustc_query_system[55587a8be6cb95b6]::query::caches::DefaultCache<rustc_middle[66eaac359ba8a608]::ty::ParamEnvAnd<rustc_middle[66eaac359ba8a608]::mir::interpret::GlobalId>, rustc_middle[66eaac359ba8a608]::query::erase::Erased<[u8; 24usize]>>>
  26:        0x10b2ffe40 - rustc_const_eval[b2dc86267216f489]::const_eval::eval_queries::eval_to_const_value_raw_provider
  27:        0x10ca646bc - rustc_query_impl[ab99e8e391ad42b2]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[ab99e8e391ad42b2]::query_impl::eval_to_const_value_raw::dynamic_query::{closure#2}::{closure#0}, rustc_middle[66eaac359ba8a608]::query::erase::Erased<[u8; 24usize]>>
  28:        0x10ca1cbae - <rustc_query_impl[ab99e8e391ad42b2]::query_impl::eval_to_const_value_raw::dynamic_query::{closure#2} as core[84f3396b7379d157]::ops::function::FnOnce<(rustc_middle[66eaac359ba8a608]::ty::context::TyCtxt, rustc_middle[66eaac359ba8a608]::ty::ParamEnvAnd<rustc_middle[66eaac359ba8a608]::mir::interpret::GlobalId>)>>::call_once
  29:        0x10c84e42d - rustc_query_system[55587a8be6cb95b6]::query::plumbing::try_execute_query::<rustc_query_impl[ab99e8e391ad42b2]::DynamicConfig<rustc_query_system[55587a8be6cb95b6]::query::caches::DefaultCache<rustc_middle[66eaac359ba8a608]::ty::ParamEnvAnd<rustc_middle[66eaac359ba8a608]::mir::interpret::GlobalId>, rustc_middle[66eaac359ba8a608]::query::erase::Erased<[u8; 24usize]>>, false, false, false>, rustc_query_impl[ab99e8e391ad42b2]::plumbing::QueryCtxt, false>
  30:        0x10ca8f5cc - rustc_query_impl[ab99e8e391ad42b2]::query_impl::eval_to_const_value_raw::get_query_non_incr::__rust_end_short_backtrace
  31:        0x10b600c61 - <rustc_middle[66eaac359ba8a608]::hir::map::Map>::par_body_owners::<rustc_hir_analysis[c06ca58d4f3a4935]::check_crate::{closure#3}>::{closure#0}
  32:        0x10b74f4dc - rustc_hir_analysis[c06ca58d4f3a4935]::check_crate
  33:        0x10bd0ce37 - rustc_interface[f3798eed8f7e14ff]::passes::run_required_analyses
  34:        0x10bd0f2d3 - rustc_interface[f3798eed8f7e14ff]::passes::analysis
  35:        0x10ca6671c - rustc_query_impl[ab99e8e391ad42b2]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[ab99e8e391ad42b2]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[66eaac359ba8a608]::query::erase::Erased<[u8; 1usize]>>
  36:        0x10c83ebfe - rustc_query_system[55587a8be6cb95b6]::query::plumbing::try_execute_query::<rustc_query_impl[ab99e8e391ad42b2]::DynamicConfig<rustc_query_system[55587a8be6cb95b6]::query::caches::SingleCache<rustc_middle[66eaac359ba8a608]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[ab99e8e391ad42b2]::plumbing::QueryCtxt, false>
  37:        0x10ca70c77 - rustc_query_impl[ab99e8e391ad42b2]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  38:        0x10b323537 - <rustc_interface[f3798eed8f7e14ff]::queries::QueryResult<&rustc_middle[66eaac359ba8a608]::ty::context::GlobalCtxt>>::enter::<core[84f3396b7379d157]::result::Result<(), rustc_span[9df13b32b3ae4eb2]::ErrorGuaranteed>, rustc_driver_impl[d8a49a9463f03e6]::run_compiler::{closure#0}::{closure#1}::{closure#5}>
  39:        0x10b384c1b - rustc_interface[f3798eed8f7e14ff]::interface::run_compiler::<core[84f3396b7379d157]::result::Result<(), rustc_span[9df13b32b3ae4eb2]::ErrorGuaranteed>, rustc_driver_impl[d8a49a9463f03e6]::run_compiler::{closure#0}>::{closure#1}
  40:        0x10b36ffc1 - std[8bfe66706d3bd117]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[f3798eed8f7e14ff]::util::run_in_thread_with_globals<rustc_interface[f3798eed8f7e14ff]::util::run_in_thread_pool_with_globals<rustc_interface[f3798eed8f7e14ff]::interface::run_compiler<core[84f3396b7379d157]::result::Result<(), rustc_span[9df13b32b3ae4eb2]::ErrorGuaranteed>, rustc_driver_impl[d8a49a9463f03e6]::run_compiler::{closure#0}>::{closure#1}, core[84f3396b7379d157]::result::Result<(), rustc_span[9df13b32b3ae4eb2]::ErrorGuaranteed>>::{closure#0}, core[84f3396b7379d157]::result::Result<(), rustc_span[9df13b32b3ae4eb2]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[84f3396b7379d157]::result::Result<(), rustc_span[9df13b32b3ae4eb2]::ErrorGuaranteed>>
  41:        0x10b38b5e6 - <<std[8bfe66706d3bd117]::thread::Builder>::spawn_unchecked_<rustc_interface[f3798eed8f7e14ff]::util::run_in_thread_with_globals<rustc_interface[f3798eed8f7e14ff]::util::run_in_thread_pool_with_globals<rustc_interface[f3798eed8f7e14ff]::interface::run_compiler<core[84f3396b7379d157]::result::Result<(), rustc_span[9df13b32b3ae4eb2]::ErrorGuaranteed>, rustc_driver_impl[d8a49a9463f03e6]::run_compiler::{closure#0}>::{closure#1}, core[84f3396b7379d157]::result::Result<(), rustc_span[9df13b32b3ae4eb2]::ErrorGuaranteed>>::{closure#0}, core[84f3396b7379d157]::result::Result<(), rustc_span[9df13b32b3ae4eb2]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[84f3396b7379d157]::result::Result<(), rustc_span[9df13b32b3ae4eb2]::ErrorGuaranteed>>::{closure#2} as core[84f3396b7379d157]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  42:        0x1020029bb - std::sys::pal::unix::thread::Thread::new::thread_start::hff8a09cc296b9c2d
  43:     0x7ff801f5318b - __pthread_start

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 `/Users/sal/Documents/240714(240712-nightly)/rustc-ice-2024-07-15T00_43_16-58375.txt` to your bug report

query stack during panic:
#0 [eval_to_allocation_raw] const-evaluating + checking `G`
#1 [eval_to_const_value_raw] simplifying constant for the type system `G`
end of query stack
error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0106`.

Note

ICE location

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

Lines 33 to 42 in fcaa6fd

	impl<Prov: Provenance> MemPlaceMeta<Prov> {
	#[cfg_attr(debug_assertions, track_caller)] // only in debug builds due to perf (see #98980)
	pub fn unwrap_meta(self) -> Scalar<Prov> {
	match self {
	Self::Meta(s) => s,
	Self::None => {
	bug!("expected wide pointer extra data (e.g. slice length or trait object vtable)")
	}
	}
	}

[theemathas]

Minimized:

struct Vtable(dyn Cap);  // missing lifetime

trait Cap<'a> {}

union Transmute {
    t: u64,  // ICEs with u64, u128, or usize. Correctly errors with u32.
    u: &'static Vtable,
}

const G: &'static Vtable = unsafe { Transmute { t: 1 }.u };

[GrigorenkoPV]

searched nightlies: from nightly-2023-11-11 to nightly-2024-07-15
regressed nightly: nightly-2024-03-22
searched commit range: 1388d7a...0ad927c
regressed commit: 6a6cd65
The PR introducing the regression in this rollup is #122749: make type_flags(ReError) & HAS_ERROR

(both the original and the minimized version)

[adwinwhite]

@rustbot claim

[adwinwhite]

The error originates from layout miscalculation of ill-formed type.
It should return a ScalarPair for trait object reference but returned a Scalar here.

rust/compiler/rustc_ty_utils/src/layout.rs

Lines 204 to 205 in 176e545

	if pointee.is_sized(tcx, param_env) {
	return Ok(tcx.mk_layout(LayoutS::scalar(cx, data_ptr)));

[adwinwhite]

A more detailed analysis:
The ICE happened when trying to validate resulting const value which had been interpreted.
The result const is a trait object reference.
As we can see from the backtrace, the ICE is due to the trait object reference having empty meta data, which should contain a vtable pointer under normal circumstance.
Tracing down meta variable's origin, I found that it's directly split from the resulting const value.

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

Lines 407 to 414 in e57f309

	pub fn ref_to_mplace(
	&self,
	val: &ImmTy<'tcx, M::Provenance>,
	) -> InterpResult<'tcx, MPlaceTy<'tcx, M::Provenance>> {
	let pointee_type =
	val.layout.ty.builtin_deref(true).expect("`ref_to_mplace` called on non-ptr type");
	let layout = self.layout_of(pointee_type)?;
	let (ptr, meta) = val.to_scalar_and_meta();

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

Lines 107 to 110 in e57f309

	pub fn to_scalar_and_meta(self) -> (Scalar<Prov>, MemPlaceMeta<Prov>) {
	match self {
	Immediate::ScalarPair(val1, val2) => (val1, MemPlaceMeta::Meta(val2)),
	Immediate::Scalar(val) => (val, MemPlaceMeta::None),

Our resulting const value is a Scalar, allocated according to its layout. So its layout calculation must go wrong.
As I mentioned in previous comment, it mistook the pointee(trait object wrapper) as sized.
The cause is that our trait object type contains error, more specifically its region contains error.

rust/compiler/rustc_ty_utils/src/ty.rs

Lines 97 to 99 in 176e545

	let constraint_ty = sized_constraint_for_ty(tcx, tail_ty)?;
	if let Err(guar) = constraint_ty.error_reported() {
	return Some(ty::EarlyBinder::bind(Ty::new_error(tcx, guar)));

And is_trivially_sized thinks ty::Error is sized.

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

Lines 1805 to 1806 in 176e545

	| ty::Error(_)
	| ty::Dynamic(_, _, ty::DynStar) => true,

[adwinwhite]

The analysis is based on the minimized code above and the trait object I'm talking about is dyn Cap.