ICE: `!base.layout().is_sized()`

[matthiaskrgr]

auto-reduced (treereduce-rust):

use std::mem;

#[repr(C)]

const BAD_ENUM2_OPTION_PTR: Option<Enum2> = unsafe {};

enum UninhDiscriminant {
    A,
    B(!),
    C,
    D(Copy),
}

const BAD_UNINHABITED_VARIANT2: UninhDiscriminant = unsafe { mem::transmute(3u8) };

// # other

original code

original:

// Strip out raw byte dumps to make comparison platform-independent:
//@ normalize-stderr-test "(the raw bytes of the constant) \(size: [0-9]*, align: [0-9]*\)" -> "$1 (size: $$SIZE, align: $$ALIGN)"
//@ normalize-stderr-test "([0-9a-f][0-9a-f] |╾─*ALLOC[0-9]+(\+[a-z0-9]+)?(<imm>)?─*╼ )+ *│.*" -> "HEX_DUMP"
//@ normalize-stderr-test "0x0+" -> "0x0"
#![feature(never_type)]
#![allow(invalid_value)]

use std::mem;

#[repr(transparent)]
#[derive(Copy, Clone)]
struct Wrap<T>(T);

#[derive(Copy, Clone)]
enum Never {}

// # simple enum with discriminant 0

#[repr(usize)]
#[derive(Copy, Clone)]
enum Enum {
    A = 0,
}

const GOOD_ENUM: Enum = unsafe { mem::transmute(0usize) };

const BAD_ENUM: Enum = unsafe { mem::transmute(1usize) };
//~^ ERROR is undefined behavior

const BAD_ENUM_PTR: Enum = unsafe { mem::transmute(&1) };
//~^ ERROR evaluation of constant value failed

const BAD_ENUM_WRAPPED: Wrap<Enum> = unsafe { mem::transmute(&1) };
//~^ ERROR evaluation of constant value failed

// # simple enum with discriminant 2

// (Potentially) invalid enum discriminant
#[repr(usize)]
#[derive(Copy, Clone)]
enum Enum2 {
    A = 2,
}

const BAD_ENUM2: Enum2 = unsafe { mem::transmute(0usize) };
//~^ ERROR is undefined behavior
const BAD_ENUM2_PTR: Enum2 = unsafe { mem::transmute(&0) };
//~^ ERROR evaluation of constant value failed
// something wrapping the enum so that we test layout first, not enum
const BAD_ENUM2_WRAPPED: Wrap<Enum2> = unsafe { mem::transmute(&0) };
//~^ ERROR evaluation of constant value failed

// Undef enum discriminant.
#[repr(C)]
union MaybeUninit<T: Copy> {
    uninit: (),
    init: T,
}
const BAD_ENUM2_UNDEF : Enum2 = unsafe { MaybeUninit { uninit: () }.init };
//~^ ERROR evaluation of constant value failed
//~| uninitialized

// Pointer value in an enum with a niche that is not just 0.
const BAD_ENUM2_OPTION_PTR: Option<Enum2> = unsafe { mem::transmute(&0) };
//~^ ERROR evaluation of constant value failed

// # valid discriminant for uninhabited variant

// An enum with uninhabited variants but also at least 2 inhabited variants -- so the uninhabited
// variants *do* have a discriminant.
enum UninhDiscriminant {
    A,
    B(!),
    C,
    D(Copy),
}

const GOOD_INHABITED_VARIANT1: UninhDiscriminant = unsafe { mem::transmute(0u8) }; // variant A
const GOOD_INHABITED_VARIANT2: UninhDiscriminant = unsafe { mem::transmute(2u8) }; // variant C

const BAD_UNINHABITED_VARIANT1: UninhDiscriminant = unsafe { mem::transmute(1u8) };
//~^ ERROR is undefined behavior
const BAD_UNINHABITED_VARIANT2: UninhDiscriminant = unsafe { mem::transmute(3u8) };
//~^ ERROR is undefined behavior

// # other

// Invalid enum field content (mostly to test printing of paths for enum tuple
// variants and tuples).
// Need to create something which does not clash with enum layout optimizations.
const BAD_OPTION_CHAR: Option<(usize, char)> = Some(('x', unsafe { mem::transmute(!0u32) }));
//~^ ERROR is undefined behavior

// All variants are uninhabited but also have data.
// Use `0` as constant to make behavior endianness-independent.
const BAD_UNINHABITED_WITH_DATA1: Result<(i32, Never), (i32, !)> = unsafe { mem::transmute(0u64) };
//~^ ERROR evaluation of constant value failed
const BAD_UNINHABITED_WITH_DATA2: Result<(i32, !), (i32, Never)> = unsafe { mem::transmute(0u64) };
//~^ ERROR evaluation of constant value failed

const TEST_ICE_89765: () = {
    // This is a regression test for https://github.com/rust-lang/rust/issues/89765.
    unsafe { std::mem::discriminant(&*(&() as *const () as *const Never)); };
    //~^ inside `TEST_ICE_89765`
};

fn main() {
}

Version information

rustc 1.79.0-nightly (35936c483 2024-03-25)
binary: rustc
commit-hash: 35936c4839dd272646394779d151e060d0f6b3ac
commit-date: 2024-03-25
host: x86_64-unknown-linux-gnu
release: 1.79.0-nightly
LLVM version: 18.1.2

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

Program output

error[E0412]: cannot find type `Enum2` in this scope
 --> /tmp/icemaker_global_tempdir.vKOVyrYun5C7/rustc_testrunner_tmpdir_reporting.hFu1a4fPxKRm/mvce.rs:5:36
  |
5 | const BAD_ENUM2_OPTION_PTR: Option<Enum2> = unsafe {};
  |                                    ^^^^^ not found in this scope
  |
help: you might be missing a type parameter
  |
5 | const BAD_ENUM2_OPTION_PTR<Enum2>: Option<Enum2> = unsafe {};
  |                           +++++++

error[E0658]: the `!` type is experimental
 --> /tmp/icemaker_global_tempdir.vKOVyrYun5C7/rustc_testrunner_tmpdir_reporting.hFu1a4fPxKRm/mvce.rs:9:7
  |
9 |     B(!),
  |       ^
  |
  = note: see issue #35121 <https://github.com/rust-lang/rust/issues/35121> for more information
  = help: add `#![feature(never_type)]` to the crate attributes to enable
  = note: this compiler was built on 2024-03-25; consider upgrading it if it is out of date

error[E0601]: `main` function not found in crate `mvce`
  --> /tmp/icemaker_global_tempdir.vKOVyrYun5C7/rustc_testrunner_tmpdir_reporting.hFu1a4fPxKRm/mvce.rs:14:84
   |
14 | const BAD_UNINHABITED_VARIANT2: UninhDiscriminant = unsafe { mem::transmute(3u8) };
   |                                                                                    ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.vKOVyrYun5C7/rustc_testrunner_tmpdir_reporting.hFu1a4fPxKRm/mvce.rs`

error[E0517]: attribute should be applied to a struct, enum, or union
 --> /tmp/icemaker_global_tempdir.vKOVyrYun5C7/rustc_testrunner_tmpdir_reporting.hFu1a4fPxKRm/mvce.rs:3:8
  |
3 | #[repr(C)]
  |        ^
4 |
5 | const BAD_ENUM2_OPTION_PTR: Option<Enum2> = unsafe {};
  | ------------------------------------------------------ not a struct, enum, or union

warning: trait objects without an explicit `dyn` are deprecated
  --> /tmp/icemaker_global_tempdir.vKOVyrYun5C7/rustc_testrunner_tmpdir_reporting.hFu1a4fPxKRm/mvce.rs:11:7
   |
11 |     D(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`
   |
11 |     D(dyn Copy),
   |       +++

error[E0038]: the trait `Copy` cannot be made into an object
  --> /tmp/icemaker_global_tempdir.vKOVyrYun5C7/rustc_testrunner_tmpdir_reporting.hFu1a4fPxKRm/mvce.rs:11:7
   |
11 |     D(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.vKOVyrYun5C7/rustc_testrunner_tmpdir_reporting.hFu1a4fPxKRm/mvce.rs:11:7
   |
11 |     D(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 an enum variant 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
   |
11 |     D(&Copy),
   |       +
help: the `Box` type always has a statically known size and allocates its contents in the heap
   |
11 |     D(Box<Copy>),
   |       ++++    +

thread 'rustc' panicked at compiler/rustc_const_eval/src/interpret/projection.rs:158:13:
assertion failed: !base.layout().is_sized()
stack backtrace:
   0:     0x75b6e602efd5 - std::backtrace_rs::backtrace::libunwind::trace::h358d27b0beb91d52
                               at /rustc/35936c4839dd272646394779d151e060d0f6b3ac/library/std/src/../../backtrace/src/backtrace/libunwind.rs:105:5
   1:     0x75b6e602efd5 - std::backtrace_rs::backtrace::trace_unsynchronized::hb23626f2832be2ef
                               at /rustc/35936c4839dd272646394779d151e060d0f6b3ac/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x75b6e602efd5 - std::sys_common::backtrace::_print_fmt::h12a2241e36ed88c8
                               at /rustc/35936c4839dd272646394779d151e060d0f6b3ac/library/std/src/sys_common/backtrace.rs:68:5
   3:     0x75b6e602efd5 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h00f4d0202e49b73c
                               at /rustc/35936c4839dd272646394779d151e060d0f6b3ac/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x75b6e608013b - core::fmt::rt::Argument::fmt::h50b99c7ac9b6f780
                               at /rustc/35936c4839dd272646394779d151e060d0f6b3ac/library/core/src/fmt/rt.rs:142:9
   5:     0x75b6e608013b - core::fmt::write::h052bbc034f024029
                               at /rustc/35936c4839dd272646394779d151e060d0f6b3ac/library/core/src/fmt/mod.rs:1153:17
   6:     0x75b6e6023d6f - std::io::Write::write_fmt::h8a7055241a914793
                               at /rustc/35936c4839dd272646394779d151e060d0f6b3ac/library/std/src/io/mod.rs:1843:15
   7:     0x75b6e602edae - std::sys_common::backtrace::_print::hc7429e59d20aef3d
                               at /rustc/35936c4839dd272646394779d151e060d0f6b3ac/library/std/src/sys_common/backtrace.rs:47:5
   8:     0x75b6e602edae - std::sys_common::backtrace::print::h098f1297ce95d7fd
                               at /rustc/35936c4839dd272646394779d151e060d0f6b3ac/library/std/src/sys_common/backtrace.rs:34:9
   9:     0x75b6e6031a29 - std::panicking::default_hook::{{closure}}::hfacad2d95a15b54f
  10:     0x75b6e6031793 - std::panicking::default_hook::h335f6dbc0bdf0870
                               at /rustc/35936c4839dd272646394779d151e060d0f6b3ac/library/std/src/panicking.rs:292:9
  11:     0x75b6e29d127f - std[8986ecfb476bfa7d]::panicking::update_hook::<alloc[d5b01f14e800d591]::boxed::Box<rustc_driver_impl[4b2f8406c0f6a8ac]::install_ice_hook::{closure#0}>>::{closure#0}
  12:     0x75b6e603212c - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::h981ebc744fbdca0e
                               at /rustc/35936c4839dd272646394779d151e060d0f6b3ac/library/alloc/src/boxed.rs:2029:9
  13:     0x75b6e603212c - std::panicking::rust_panic_with_hook::h1985d360b67b5f37
                               at /rustc/35936c4839dd272646394779d151e060d0f6b3ac/library/std/src/panicking.rs:793:13
  14:     0x75b6e6031e9d - std::panicking::begin_panic_handler::{{closure}}::hf64883af21dee792
                               at /rustc/35936c4839dd272646394779d151e060d0f6b3ac/library/std/src/panicking.rs:650:13
  15:     0x75b6e602f499 - std::sys_common::backtrace::__rust_end_short_backtrace::h48fdcafa2a5427ef
                               at /rustc/35936c4839dd272646394779d151e060d0f6b3ac/library/std/src/sys_common/backtrace.rs:171:18
  16:     0x75b6e6031c07 - rust_begin_unwind
                               at /rustc/35936c4839dd272646394779d151e060d0f6b3ac/library/std/src/panicking.rs:646:5
  17:     0x75b6e607c5d6 - core::panicking::panic_fmt::h01c5db37aea323f1
                               at /rustc/35936c4839dd272646394779d151e060d0f6b3ac/library/core/src/panicking.rs:72:14
  18:     0x75b6e607c67f - core::panicking::panic::haef388cfe0bf5f86
                               at /rustc/35936c4839dd272646394779d151e060d0f6b3ac/library/core/src/panicking.rs:141:5
  19:     0x75b6e3e40d39 - <rustc_const_eval[62a5e1dbaf8a1d0a]::interpret::eval_context::InterpCx<rustc_const_eval[62a5e1dbaf8a1d0a]::const_eval::machine::CompileTimeInterpreter>>::project_field::<rustc_const_eval[62a5e1dbaf8a1d0a]::interpret::operand::OpTy>
  20:     0x75b6e3e3ca50 - <rustc_const_eval[62a5e1dbaf8a1d0a]::interpret::validity::ValidityVisitor<rustc_const_eval[62a5e1dbaf8a1d0a]::const_eval::machine::CompileTimeInterpreter> as rustc_const_eval[62a5e1dbaf8a1d0a]::interpret::visitor::ValueVisitor<rustc_const_eval[62a5e1dbaf8a1d0a]::const_eval::machine::CompileTimeInterpreter>>::visit_value
  21:     0x75b6e3e3d0b2 - <rustc_const_eval[62a5e1dbaf8a1d0a]::interpret::validity::ValidityVisitor<rustc_const_eval[62a5e1dbaf8a1d0a]::const_eval::machine::CompileTimeInterpreter> as rustc_const_eval[62a5e1dbaf8a1d0a]::interpret::visitor::ValueVisitor<rustc_const_eval[62a5e1dbaf8a1d0a]::const_eval::machine::CompileTimeInterpreter>>::visit_value
  22:     0x75b6e49a19b0 - rustc_const_eval[62a5e1dbaf8a1d0a]::const_eval::eval_queries::eval_to_allocation_raw_provider
  23:     0x75b6e49a0380 - rustc_query_impl[a0c31ac015223081]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[a0c31ac015223081]::query_impl::eval_to_allocation_raw::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b97de266c643709b]::query::erase::Erased<[u8; 24usize]>>
  24:     0x75b6e499d3d6 - rustc_query_system[97bff28321f2bac]::query::plumbing::try_execute_query::<rustc_query_impl[a0c31ac015223081]::DynamicConfig<rustc_query_system[97bff28321f2bac]::query::caches::DefaultCache<rustc_middle[b97de266c643709b]::ty::ParamEnvAnd<rustc_middle[b97de266c643709b]::mir::interpret::GlobalId>, rustc_middle[b97de266c643709b]::query::erase::Erased<[u8; 24usize]>>, false, false, false>, rustc_query_impl[a0c31ac015223081]::plumbing::QueryCtxt, false>
  25:     0x75b6e499cfac - rustc_query_impl[a0c31ac015223081]::query_impl::eval_to_allocation_raw::get_query_non_incr::__rust_end_short_backtrace
  26:     0x75b6e499cb0e - rustc_middle[b97de266c643709b]::query::plumbing::query_get_at::<rustc_query_system[97bff28321f2bac]::query::caches::DefaultCache<rustc_middle[b97de266c643709b]::ty::ParamEnvAnd<rustc_middle[b97de266c643709b]::mir::interpret::GlobalId>, rustc_middle[b97de266c643709b]::query::erase::Erased<[u8; 24usize]>>>
  27:     0x75b6e499b03f - rustc_const_eval[62a5e1dbaf8a1d0a]::const_eval::eval_queries::eval_to_const_value_raw_provider
  28:     0x75b6e499afb6 - rustc_query_impl[a0c31ac015223081]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[a0c31ac015223081]::query_impl::eval_to_const_value_raw::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b97de266c643709b]::query::erase::Erased<[u8; 24usize]>>
  29:     0x75b6e499d399 - rustc_query_system[97bff28321f2bac]::query::plumbing::try_execute_query::<rustc_query_impl[a0c31ac015223081]::DynamicConfig<rustc_query_system[97bff28321f2bac]::query::caches::DefaultCache<rustc_middle[b97de266c643709b]::ty::ParamEnvAnd<rustc_middle[b97de266c643709b]::mir::interpret::GlobalId>, rustc_middle[b97de266c643709b]::query::erase::Erased<[u8; 24usize]>>, false, false, false>, rustc_query_impl[a0c31ac015223081]::plumbing::QueryCtxt, false>
  30:     0x75b6e499ceac - rustc_query_impl[a0c31ac015223081]::query_impl::eval_to_const_value_raw::get_query_non_incr::__rust_end_short_backtrace
  31:     0x75b6e40ec2e9 - <rustc_middle[b97de266c643709b]::query::plumbing::TyCtxtEnsure>::const_eval_poly
  32:     0x75b6e3fcb775 - rustc_hir_analysis[56e3c688dc2c0c48]::check_crate
  33:     0x75b6e42a656a - rustc_interface[c38f70925b3e6389]::passes::analysis
  34:     0x75b6e42a60c7 - rustc_query_impl[a0c31ac015223081]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[a0c31ac015223081]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b97de266c643709b]::query::erase::Erased<[u8; 1usize]>>
  35:     0x75b6e4ba8f03 - rustc_query_system[97bff28321f2bac]::query::plumbing::try_execute_query::<rustc_query_impl[a0c31ac015223081]::DynamicConfig<rustc_query_system[97bff28321f2bac]::query::caches::SingleCache<rustc_middle[b97de266c643709b]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[a0c31ac015223081]::plumbing::QueryCtxt, false>
  36:     0x75b6e4ba8c7f - rustc_query_impl[a0c31ac015223081]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  37:     0x75b6e4a8c299 - rustc_interface[c38f70925b3e6389]::interface::run_compiler::<core[8cbc9b2d85f665a9]::result::Result<(), rustc_span[ebc71fb0dc5af8f5]::ErrorGuaranteed>, rustc_driver_impl[4b2f8406c0f6a8ac]::run_compiler::{closure#0}>::{closure#0}
  38:     0x75b6e4b62bd9 - std[8986ecfb476bfa7d]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[c38f70925b3e6389]::util::run_in_thread_with_globals<rustc_interface[c38f70925b3e6389]::util::run_in_thread_pool_with_globals<rustc_interface[c38f70925b3e6389]::interface::run_compiler<core[8cbc9b2d85f665a9]::result::Result<(), rustc_span[ebc71fb0dc5af8f5]::ErrorGuaranteed>, rustc_driver_impl[4b2f8406c0f6a8ac]::run_compiler::{closure#0}>::{closure#0}, core[8cbc9b2d85f665a9]::result::Result<(), rustc_span[ebc71fb0dc5af8f5]::ErrorGuaranteed>>::{closure#0}, core[8cbc9b2d85f665a9]::result::Result<(), rustc_span[ebc71fb0dc5af8f5]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[8cbc9b2d85f665a9]::result::Result<(), rustc_span[ebc71fb0dc5af8f5]::ErrorGuaranteed>>
  39:     0x75b6e4b62a06 - <<std[8986ecfb476bfa7d]::thread::Builder>::spawn_unchecked_<rustc_interface[c38f70925b3e6389]::util::run_in_thread_with_globals<rustc_interface[c38f70925b3e6389]::util::run_in_thread_pool_with_globals<rustc_interface[c38f70925b3e6389]::interface::run_compiler<core[8cbc9b2d85f665a9]::result::Result<(), rustc_span[ebc71fb0dc5af8f5]::ErrorGuaranteed>, rustc_driver_impl[4b2f8406c0f6a8ac]::run_compiler::{closure#0}>::{closure#0}, core[8cbc9b2d85f665a9]::result::Result<(), rustc_span[ebc71fb0dc5af8f5]::ErrorGuaranteed>>::{closure#0}, core[8cbc9b2d85f665a9]::result::Result<(), rustc_span[ebc71fb0dc5af8f5]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[8cbc9b2d85f665a9]::result::Result<(), rustc_span[ebc71fb0dc5af8f5]::ErrorGuaranteed>>::{closure#1} as core[8cbc9b2d85f665a9]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  40:     0x75b6e603ba49 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h6dc9de06dfc501ab
                               at /rustc/35936c4839dd272646394779d151e060d0f6b3ac/library/alloc/src/boxed.rs:2015:9
  41:     0x75b6e603ba49 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hc5bbd99ced815c6f
                               at /rustc/35936c4839dd272646394779d151e060d0f6b3ac/library/alloc/src/boxed.rs:2015:9
  42:     0x75b6e603ba49 - std::sys::pal::unix::thread::Thread::new::thread_start::h0243db2ae6ac2b16
                               at /rustc/35936c4839dd272646394779d151e060d0f6b3ac/library/std/src/sys/pal/unix/thread.rs:108:17
  43:     0x75b6df8a955a - <unknown>
  44:     0x75b6df926a3c - <unknown>
  45:                0x0 - <unknown>

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: rustc 1.79.0-nightly (35936c483 2024-03-25) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [eval_to_allocation_raw] const-evaluating + checking `BAD_UNINHABITED_VARIANT2`
#1 [eval_to_const_value_raw] simplifying constant for the type system `BAD_UNINHABITED_VARIANT2`
#2 [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, E0412, E0517, E0601, E0658.
For more information about an error, try `rustc --explain E0038`.

[matthiaskrgr]

regression in #122493 cc @lukas-code

[lukas-code]

#122493 makes this easier to trigger, but isn't the root cause. If you change it like this, it also ICEs on beta:

use std::mem;

#[repr(C)]

const BAD_ENUM2_OPTION_PTR: Option<Enum2> = unsafe {};

enum UninhDiscriminant {
    A,
    B(!),
    C,
-    D(Copy),
+    D(Copy, ()),
}

const BAD_UNINHABITED_VARIANT2: UninhDiscriminant = unsafe { mem::transmute(3u8) };

// # other

[lukas-code]

minimized:

struct S {
    a: [u8],
    b: ()
}

const C: S = unsafe { std::mem::transmute(()) };

[GrigorenkoPV]

minimized:

struct S {
    a: [u8],
    b: ()
}

const C: S = unsafe { std:mem::transmute(()) };

There is a typo in std:mem, but it does not really matter.

searched nightlies: from nightly-2024-01-01 to nightly-2024-03-15
regressed nightly: nightly-2024-02-21
searched commit range: 3246e79...bb59453
regressed commit: cce6a6e (#121087, cc @oli-obk)

bisected with cargo-bisect-rustc v0.6.8

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc -vv --preserve --access=github --start=2024-01-01 --end=2024-03-15 --regress=ice 

[oli-obk]

Not a regression from #121087, just exposed more easily 😆 .

struct S {
    a: [u8],
    b: ()
}

const C: S = unsafe { std::mem::transmute(()) };
const _: [(); {
    C;
    0
}] = [];

has been ICEing much longer

[GrigorenkoPV]

has been ICEing much longer

Damn, we need to go deeper. Judging by Godbolt, the regression is all the way back between 1.25 & 1.26.

It bisects to nightly-2018-03-15, which is 2789b06...521d91c. There are 34 merges in range, and god knows how many rollups.

[GrigorenkoPV]

Did some git bisecting, it bisects to #46882 (cc @oli-obk once again).

Out of 110 commits in that PR, most seem to not build, so I cannot pinpoint the breakage point more precisely.

[oli-obk]

lol, that's amazing. thanks!