ICE: should be sized but is not?

[matthiaskrgr]

auto-reduced (treereduce-rust):

pub union BagOfBits<T: Copy> {
    uninit: (),
    write: Copy,
}

pub fn check_bag<T: Copy>(bag: &BagOfBits<T>) {
    let val = unsafe { (bag as *const _ as *const u8).read() };
}

original:

// run-pass

#![feature(const_ptr_write)]
#![feature(const_mut_refs)]

// Or, equivalently: `MaybeUninit`.
pub union BagOfBits<T: Copy> {
    uninit: (),
    write: Copy,
}

pub const fn make_1u8_bag<T: Copy>() -> BagOfBits<T> {
    assert!(core::mem::size_of::<T>() >= 1);
    let mut bag = BagOfBits { uninit: () };
    unsafe { (&mut bag as *mut _ as *mut u8).write(1); };
    bag
}

pub fn check_bag<T: Copy>(bag: &BagOfBits<T>) {
    let val = unsafe { (bag as *const _ as *const u8).read() };
    assert_eq!(val, 1);
}

fn main() {
    check_bag(&make_1u8_bag::<[usize; 1]>(1)); // Fine
    check_bag(&make_1u8_bag::<usize>()); // Fine

    const CONST_ARRAY_BAG: BagOfBits<[usize; 1]> = make_1u8_bag();
    check_bag(&CONST_ARRAY_BAG); // Fine
    const CONST_USIZE_BAG: BagOfBits<usize> = make_1u8_bag();

    // Used to panic since CTFE would make the entire `BagOfBits<usize>` uninit
    check_bag(&CONST_USIZE_BAG);
}

Version information

rustc 1.78.0-nightly (c03ea3dfd 2024-03-16)
binary: rustc
commit-hash: c03ea3dfd907e7dc6305ebf20c94f3a1f1d9fed7
commit-date: 2024-03-16
host: x86_64-unknown-linux-gnu
release: 1.78.0-nightly
LLVM version: 18.1.0

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc

Program output

error[E0601]: `main` function not found in crate `mvce`
 --> /tmp/icemaker_global_tempdir.rFVXcYalVTX7/rustc_testrunner_tmpdir_reporting.LvGGT1V3WKvx/mvce.rs:8:2
  |
8 | }
  |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.rFVXcYalVTX7/rustc_testrunner_tmpdir_reporting.LvGGT1V3WKvx/mvce.rs`

warning: trait objects without an explicit `dyn` are deprecated
 --> /tmp/icemaker_global_tempdir.rFVXcYalVTX7/rustc_testrunner_tmpdir_reporting.LvGGT1V3WKvx/mvce.rs:3:12
  |
3 |     write: Copy,
  |            ^^^^
  |
  = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021!
  = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/warnings-promoted-to-error.html>
  = note: `#[warn(bare_trait_objects)]` on by default
help: if this is an object-safe trait, use `dyn`
  |
3 |     write: dyn Copy,
  |            +++

error[E0038]: the trait `Copy` cannot be made into an object
 --> /tmp/icemaker_global_tempdir.rFVXcYalVTX7/rustc_testrunner_tmpdir_reporting.LvGGT1V3WKvx/mvce.rs:3:12
  |
3 |     write: Copy,
  |            ^^^^ `Copy` cannot be made into an object
  |
  = note: the trait cannot be made into an object because it requires `Self: Sized`
  = note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>

error[E0277]: the size for values of type `(dyn Copy + 'static)` cannot be known at compilation time
 --> /tmp/icemaker_global_tempdir.rFVXcYalVTX7/rustc_testrunner_tmpdir_reporting.LvGGT1V3WKvx/mvce.rs:3:12
  |
3 |     write: Copy,
  |            ^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `(dyn Copy + 'static)`
  = note: no field of a union may have a dynamically sized type
  = help: change the field's type to have a statically known size
help: borrowed types always have a statically known size
  |
3 |     write: &Copy,
  |            +
help: the `Box` type always has a statically known size and allocates its contents in the heap
  |
3 |     write: Box<Copy>,
  |            ++++    +

error[E0392]: type parameter `T` is never used
 --> /tmp/icemaker_global_tempdir.rFVXcYalVTX7/rustc_testrunner_tmpdir_reporting.LvGGT1V3WKvx/mvce.rs:1:21
  |
1 | pub union BagOfBits<T: Copy> {
  |                     ^ unused type parameter
  |
  = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`

error[E0740]: field must implement `Copy` or be wrapped in `ManuallyDrop<...>` to be used in a union
 --> /tmp/icemaker_global_tempdir.rFVXcYalVTX7/rustc_testrunner_tmpdir_reporting.LvGGT1V3WKvx/mvce.rs:3:5
  |
3 |     write: Copy,
  |     ^^^^^^^^^^^
  |
  = note: union fields must not have drop side-effects, which is currently enforced via either `Copy` or `ManuallyDrop<...>`
help: wrap the field type in `ManuallyDrop<...>`
  |
3 |     write: std::mem::ManuallyDrop<Copy>,
  |            +++++++++++++++++++++++    +

error: internal compiler error: `BagOfBits<T/#0>` should be sized but is not?
 --> /tmp/icemaker_global_tempdir.rFVXcYalVTX7/rustc_testrunner_tmpdir_reporting.LvGGT1V3WKvx/mvce.rs:7:24
  |
7 |     let val = unsafe { (bag as *const _ as *const u8).read() };
  |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

thread 'rustc' panicked at compiler/rustc_hir_typeck/src/cast.rs:142:28:
Box<dyn Any>
stack backtrace:
   0:     0x7f9ddfb8c912 - std::backtrace_rs::backtrace::libunwind::trace::h720e7f0c15df28db
                               at /rustc/c03ea3dfd907e7dc6305ebf20c94f3a1f1d9fed7/library/std/src/../../backtrace/src/backtrace/libunwind.rs:105:5
   1:     0x7f9ddfb8c912 - std::backtrace_rs::backtrace::trace_unsynchronized::ha9779099a18695a3
                               at /rustc/c03ea3dfd907e7dc6305ebf20c94f3a1f1d9fed7/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f9ddfb8c912 - std::sys_common::backtrace::_print_fmt::h639c233731a9e3cb
                               at /rustc/c03ea3dfd907e7dc6305ebf20c94f3a1f1d9fed7/library/std/src/sys_common/backtrace.rs:68:5
   3:     0x7f9ddfb8c912 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h63ddc66545b2f581
                               at /rustc/c03ea3dfd907e7dc6305ebf20c94f3a1f1d9fed7/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x7f9ddfbddc8c - core::fmt::rt::Argument::fmt::h06c657b2e59a0031
                               at /rustc/c03ea3dfd907e7dc6305ebf20c94f3a1f1d9fed7/library/core/src/fmt/rt.rs:142:9
   5:     0x7f9ddfbddc8c - core::fmt::write::h231dab7207b03bd2
                               at /rustc/c03ea3dfd907e7dc6305ebf20c94f3a1f1d9fed7/library/core/src/fmt/mod.rs:1153:17
   6:     0x7f9ddfb8180f - std::io::Write::write_fmt::h2228d6605854434b
                               at /rustc/c03ea3dfd907e7dc6305ebf20c94f3a1f1d9fed7/library/std/src/io/mod.rs:1843:15
   7:     0x7f9ddfb8c6e4 - std::sys_common::backtrace::_print::hac0689b492be0682
                               at /rustc/c03ea3dfd907e7dc6305ebf20c94f3a1f1d9fed7/library/std/src/sys_common/backtrace.rs:47:5
   8:     0x7f9ddfb8c6e4 - std::sys_common::backtrace::print::he630bda5375f1365
                               at /rustc/c03ea3dfd907e7dc6305ebf20c94f3a1f1d9fed7/library/std/src/sys_common/backtrace.rs:34:9
   9:     0x7f9ddfb8f3eb - std::panicking::default_hook::{{closure}}::h9d04fb7bce2d5305
  10:     0x7f9ddfb8f143 - std::panicking::default_hook::h142cecc409c8d640
                               at /rustc/c03ea3dfd907e7dc6305ebf20c94f3a1f1d9fed7/library/std/src/panicking.rs:292:9
  11:     0x7f9de2bb4a9e - std[6ebe52f946dd541]::panicking::update_hook::<alloc[54004d19b3858571]::boxed::Box<rustc_driver_impl[f7a8b498200e65bb]::install_ice_hook::{closure#0}>>::{closure#0}
  12:     0x7f9ddfb8fb50 - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::h4d1c095d3d6e9872
                               at /rustc/c03ea3dfd907e7dc6305ebf20c94f3a1f1d9fed7/library/alloc/src/boxed.rs:2034:9
  13:     0x7f9ddfb8fb50 - std::panicking::rust_panic_with_hook::h69a135916b9adbd2
                               at /rustc/c03ea3dfd907e7dc6305ebf20c94f3a1f1d9fed7/library/std/src/panicking.rs:783:13
  14:     0x7f9de2be0f34 - std[6ebe52f946dd541]::panicking::begin_panic::<rustc_errors[5de0d917113e7fb6]::ExplicitBug>::{closure#0}
  15:     0x7f9de2bddb86 - std[6ebe52f946dd541]::sys_common::backtrace::__rust_end_short_backtrace::<std[6ebe52f946dd541]::panicking::begin_panic<rustc_errors[5de0d917113e7fb6]::ExplicitBug>::{closure#0}, !>
  16:     0x7f9de2bd9096 - std[6ebe52f946dd541]::panicking::begin_panic::<rustc_errors[5de0d917113e7fb6]::ExplicitBug>
  17:     0x7f9de2bea3b1 - <rustc_errors[5de0d917113e7fb6]::diagnostic::BugAbort as rustc_errors[5de0d917113e7fb6]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  18:     0x7f9de2d0f2c8 - <rustc_errors[5de0d917113e7fb6]::DiagCtxt>::span_bug::<rustc_span[6e2b61d9e5bcc941]::span_encoding::Span, alloc[54004d19b3858571]::string::String>
  19:     0x7f9de514996e - <rustc_hir_typeck[6fac15a89779e2b3]::fn_ctxt::FnCtxt>::pointer_kind.cold.0
  20:     0x7f9de1a7bebe - <rustc_hir_typeck[6fac15a89779e2b3]::cast::CastCheck>::do_check
  21:     0x7f9de48e7845 - <rustc_hir_typeck[6fac15a89779e2b3]::cast::CastCheck>::check
  22:     0x7f9de40b0c1e - rustc_hir_typeck[6fac15a89779e2b3]::typeck
  23:     0x7f9de40ae05f - rustc_query_impl[d63bb6c0f452187f]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[d63bb6c0f452187f]::query_impl::typeck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[ea608354392e650]::query::erase::Erased<[u8; 8usize]>>
  24:     0x7f9de40ac52e - rustc_query_system[2114d95bfb45a679]::query::plumbing::try_execute_query::<rustc_query_impl[d63bb6c0f452187f]::DynamicConfig<rustc_query_system[2114d95bfb45a679]::query::caches::VecCache<rustc_span[6e2b61d9e5bcc941]::def_id::LocalDefId, rustc_middle[ea608354392e650]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[d63bb6c0f452187f]::plumbing::QueryCtxt, false>
  25:     0x7f9de40abf8c - rustc_query_impl[d63bb6c0f452187f]::query_impl::typeck::get_query_non_incr::__rust_end_short_backtrace
  26:     0x7f9de43ca5d3 - <rustc_middle[ea608354392e650]::hir::map::Map>::par_body_owners::<rustc_hir_analysis[f2100830a8b528cb]::check_crate::{closure#4}>::{closure#0}
  27:     0x7f9de43c926c - rustc_hir_analysis[f2100830a8b528cb]::check_crate
  28:     0x7f9de43a452a - rustc_interface[6fcfc136e89dc42e]::passes::analysis
  29:     0x7f9de43a4165 - rustc_query_impl[d63bb6c0f452187f]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[d63bb6c0f452187f]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[ea608354392e650]::query::erase::Erased<[u8; 1usize]>>
  30:     0x7f9de4c088a5 - rustc_query_system[2114d95bfb45a679]::query::plumbing::try_execute_query::<rustc_query_impl[d63bb6c0f452187f]::DynamicConfig<rustc_query_system[2114d95bfb45a679]::query::caches::SingleCache<rustc_middle[ea608354392e650]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[d63bb6c0f452187f]::plumbing::QueryCtxt, false>
  31:     0x7f9de4c08609 - rustc_query_impl[d63bb6c0f452187f]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  32:     0x7f9de4963284 - rustc_interface[6fcfc136e89dc42e]::interface::run_compiler::<core[7931fcdfa73bbef6]::result::Result<(), rustc_span[6e2b61d9e5bcc941]::ErrorGuaranteed>, rustc_driver_impl[f7a8b498200e65bb]::run_compiler::{closure#0}>::{closure#0}
  33:     0x7f9de4e54385 - std[6ebe52f946dd541]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[6fcfc136e89dc42e]::util::run_in_thread_with_globals<rustc_interface[6fcfc136e89dc42e]::util::run_in_thread_pool_with_globals<rustc_interface[6fcfc136e89dc42e]::interface::run_compiler<core[7931fcdfa73bbef6]::result::Result<(), rustc_span[6e2b61d9e5bcc941]::ErrorGuaranteed>, rustc_driver_impl[f7a8b498200e65bb]::run_compiler::{closure#0}>::{closure#0}, core[7931fcdfa73bbef6]::result::Result<(), rustc_span[6e2b61d9e5bcc941]::ErrorGuaranteed>>::{closure#0}, core[7931fcdfa73bbef6]::result::Result<(), rustc_span[6e2b61d9e5bcc941]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[7931fcdfa73bbef6]::result::Result<(), rustc_span[6e2b61d9e5bcc941]::ErrorGuaranteed>>
  34:     0x7f9de4e541b2 - <<std[6ebe52f946dd541]::thread::Builder>::spawn_unchecked_<rustc_interface[6fcfc136e89dc42e]::util::run_in_thread_with_globals<rustc_interface[6fcfc136e89dc42e]::util::run_in_thread_pool_with_globals<rustc_interface[6fcfc136e89dc42e]::interface::run_compiler<core[7931fcdfa73bbef6]::result::Result<(), rustc_span[6e2b61d9e5bcc941]::ErrorGuaranteed>, rustc_driver_impl[f7a8b498200e65bb]::run_compiler::{closure#0}>::{closure#0}, core[7931fcdfa73bbef6]::result::Result<(), rustc_span[6e2b61d9e5bcc941]::ErrorGuaranteed>>::{closure#0}, core[7931fcdfa73bbef6]::result::Result<(), rustc_span[6e2b61d9e5bcc941]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[7931fcdfa73bbef6]::result::Result<(), rustc_span[6e2b61d9e5bcc941]::ErrorGuaranteed>>::{closure#1} as core[7931fcdfa73bbef6]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  35:     0x7f9ddfb991e5 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h2cd6598022142910
                               at /rustc/c03ea3dfd907e7dc6305ebf20c94f3a1f1d9fed7/library/alloc/src/boxed.rs:2020:9
  36:     0x7f9ddfb991e5 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hfc8920891f2f584d
                               at /rustc/c03ea3dfd907e7dc6305ebf20c94f3a1f1d9fed7/library/alloc/src/boxed.rs:2020:9
  37:     0x7f9ddfb991e5 - std::sys::pal::unix::thread::Thread::new::thread_start::h31aea03b1d783db7
                               at /rustc/c03ea3dfd907e7dc6305ebf20c94f3a1f1d9fed7/library/std/src/sys/pal/unix/thread.rs:108:17
  38:     0x7f9ddf98155a - <unknown>
  39:     0x7f9ddf9fea3c - <unknown>
  40:                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: rustc 1.78.0-nightly (c03ea3dfd 2024-03-16) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [typeck] type-checking `check_bag`
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 6 previous errors; 1 warning emitted

Some errors have detailed explanations: E0038, E0277, E0392, E0601, E0740.
For more information about an error, try `rustc --explain E0038`.

[matthiaskrgr]

#121208 cc @nnethercote

[lukas-code]

This is incidentally fixed by #122493 by making unions always sized.

[lukas-code]

smaller:

union Union {
    val: [u8],
}

fn cast(ptr: *const ()) -> *const Union {
    ptr as _
}

The union is currently in fact not sized, but this code assumes that non-struct ADTs must be sized:

rust/compiler/rustc_hir_typeck/src/cast.rs

Lines 101 to 144 in 7aa1de7

	Ok(match *t.kind() {
	ty::Slice(_) | ty::Str => Some(PointerKind::Length),
	ty::Dynamic(tty, _, ty::Dyn) => Some(PointerKind::VTable(tty.principal_def_id())),
	ty::Adt(def, args) if def.is_struct() => match def.non_enum_variant().tail_opt() {
	None => Some(PointerKind::Thin),
	Some(f) => {
	let field_ty = self.field_ty(span, f, args);
	self.pointer_kind(field_ty, span)?
	}
	},
	ty::Tuple(fields) => match fields.last() {
	None => Some(PointerKind::Thin),
	Some(&f) => self.pointer_kind(f, span)?,
	},
	// Pointers to foreign types are thin, despite being unsized
	ty::Foreign(..) => Some(PointerKind::Thin),
	// We should really try to normalize here.
	ty::Alias(_, pi) => Some(PointerKind::OfAlias(pi)),
	ty::Param(p) => Some(PointerKind::OfParam(p)),
	// Insufficient type information.
	ty::Placeholder(..) | ty::Bound(..) | ty::Infer(_) => None,
	ty::Bool
	| ty::Char
	| ty::Int(..)
	| ty::Uint(..)
	| ty::Float(_)
	| ty::Array(..)
	| ty::CoroutineWitness(..)
	| ty::RawPtr(_)
	| ty::Ref(..)
	| ty::FnDef(..)
	| ty::FnPtr(..)
	| ty::Closure(..)
	| ty::CoroutineClosure(..)
	| ty::Coroutine(..)
	| ty::Adt(..)
	| ty::Never
	| ty::Dynamic(_, _, ty::DynStar)
	| ty::Error(_) => {
	self.dcx().span_bug(span, format!("`{t:?}` should be sized but is not?"));
	}
	})