ICE: transmute: `assertion failed: min <= max'`

[matthiaskrgr]

auto-reduced (treereduce-rust):

mod assert {
    use std::mem::{Assume, BikeshedIntrinsicFrom};
    pub struct Context;

    pub fn is_maybe_transmutable<Src, Dst>()
    where
        Dst: BikeshedIntrinsicFrom<Src, Context>,
    {
    }
}

fn should_pad_explicitly_aligned_field() {
    #[repr(packed, align(0x100))]
    #[repr(u8)]
    enum V0u8 {
        V = 0,
    }

    #[repr(C)]
    pub union Uninit {
        a: (),
        b: V0u8,
    }

    #[repr(C)]
    #[repr(C)]
    struct ExplicitlyPadded(V0u8, Uninit, V0u8);

    assert::is_maybe_transmutable::<(), ExplicitlyPadded>();
}

original code

original:

// check-pass
//! The presence of an `align(X)` annotation must be accounted for.

#![crate_type = "lib"]
#![feature(transmutability)]
#![allow(dead_code, r#true, non_camel_case_types)]

mod assert {
    use std::mem::{Assume, BikeshedIntrinsicFrom};
    pub struct Context;

    pub fn is_maybe_transmutable<Src, Dst>()
    where
        Dst: BikeshedIntrinsicFrom<Src, Context, {
            Assume {
                alignment: true,
                lifetimes: true,
                safety: true,
                validity: true,
            }
        }>
    {}
}

fn should_pad_explicitly_aligned_field() {
    #[repr(packed, align(0x100))] #[repr(u8)] enum V0u8 { V = 0 }

    #[repr(C)]
    pub union Uninit {
        a: (),
        b: V0u8,
    }

    #[repr(C, align(2))] struct align_2(V0u8);

    #[repr(C)] struct ImplicitlyPadded(align_2, V0u8);
    #[repr(C)] struct ExplicitlyPadded(V0u8, Uninit, V0u8);

    // An implementation that (incorrectly) does not place a padding byte after
    // `align_2` will, incorrectly, reject the following transmutations.
    assert::is_maybe_transmutable::<ImplicitlyPadded, ExplicitlyPadded>();
    assert::is_maybe_transmutable::<Box<Alias>>();
}

Version information

rustc 1.75.0-nightly (1db4b1249 2023-11-11)
binary: rustc
commit-hash: 1db4b12494f698754b925f55061eb9e6b3241423
commit-date: 2023-11-11
host: x86_64-unknown-linux-gnu
release: 1.75.0-nightly
LLVM version: 17.0.4

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

Program output

warning: unused import: `Assume`
 --> /tmp/icemaker_global_tempdir.vhOXnwiqD1fn/rustc_testrunner_tmpdir_reporting.dwaAls4BhtVg/mvce.rs:2:20
  |
2 |     use std::mem::{Assume, BikeshedIntrinsicFrom};
  |                    ^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

error[E0601]: `main` function not found in crate `mvce`
  --> /tmp/icemaker_global_tempdir.vhOXnwiqD1fn/rustc_testrunner_tmpdir_reporting.dwaAls4BhtVg/mvce.rs:30:2
   |
30 | }
   |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.vhOXnwiqD1fn/rustc_testrunner_tmpdir_reporting.dwaAls4BhtVg/mvce.rs`

error[E0517]: attribute should be applied to a struct or union
  --> /tmp/icemaker_global_tempdir.vhOXnwiqD1fn/rustc_testrunner_tmpdir_reporting.dwaAls4BhtVg/mvce.rs:13:12
   |
13 |       #[repr(packed, align(0x100))]
   |              ^^^^^^
14 |       #[repr(u8)]
15 | /     enum V0u8 {
16 | |         V = 0,
17 | |     }
   | |_____- not a struct or union

error[E0658]: use of unstable library feature 'transmutability'
 --> /tmp/icemaker_global_tempdir.vhOXnwiqD1fn/rustc_testrunner_tmpdir_reporting.dwaAls4BhtVg/mvce.rs:2:20
  |
2 |     use std::mem::{Assume, BikeshedIntrinsicFrom};
  |                    ^^^^^^
  |
  = note: see issue #99571 <https://github.com/rust-lang/rust/issues/99571> for more information
  = help: add `#![feature(transmutability)]` to the crate attributes to enable

error[E0658]: use of unstable library feature 'transmutability'
 --> /tmp/icemaker_global_tempdir.vhOXnwiqD1fn/rustc_testrunner_tmpdir_reporting.dwaAls4BhtVg/mvce.rs:2:28
  |
2 |     use std::mem::{Assume, BikeshedIntrinsicFrom};
  |                            ^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #99571 <https://github.com/rust-lang/rust/issues/99571> for more information
  = help: add `#![feature(transmutability)]` to the crate attributes to enable

error[E0658]: use of unstable library feature 'transmutability'
 --> /tmp/icemaker_global_tempdir.vhOXnwiqD1fn/rustc_testrunner_tmpdir_reporting.dwaAls4BhtVg/mvce.rs:7:14
  |
7 |         Dst: BikeshedIntrinsicFrom<Src, Context>,
  |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #99571 <https://github.com/rust-lang/rust/issues/99571> for more information
  = help: add `#![feature(transmutability)]` to the crate attributes to enable

error[E0740]: field must implement `Copy` or be wrapped in `ManuallyDrop<...>` to be used in a union
  --> /tmp/icemaker_global_tempdir.vhOXnwiqD1fn/rustc_testrunner_tmpdir_reporting.dwaAls4BhtVg/mvce.rs:22:9
   |
22 |         b: V0u8,
   |         ^^^^^^^
   |
   = 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<...>`
   |
22 |         b: std::mem::ManuallyDrop<V0u8>,
   |            +++++++++++++++++++++++    +

thread 'rustc' panicked at /rustc/1db4b12494f698754b925f55061eb9e6b3241423/library/core/src/cmp.rs:876:9:
assertion failed: min <= max
stack backtrace:
   0:     0x7f838782e62c - std::backtrace_rs::backtrace::libunwind::trace::h60c98351b446b6fc
                               at /rustc/1db4b12494f698754b925f55061eb9e6b3241423/library/std/src/../../backtrace/src/backtrace/libunwind.rs:104:5
   1:     0x7f838782e62c - std::backtrace_rs::backtrace::trace_unsynchronized::h5f7f811450b9c6bf
                               at /rustc/1db4b12494f698754b925f55061eb9e6b3241423/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f838782e62c - std::sys_common::backtrace::_print_fmt::h0d5170c59727098e
                               at /rustc/1db4b12494f698754b925f55061eb9e6b3241423/library/std/src/sys_common/backtrace.rs:67:5
   3:     0x7f838782e62c - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h75fbf13e2e3eab42
                               at /rustc/1db4b12494f698754b925f55061eb9e6b3241423/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x7f8387881630 - core::fmt::rt::Argument::fmt::hf5a01755b8c97b0f
                               at /rustc/1db4b12494f698754b925f55061eb9e6b3241423/library/core/src/fmt/rt.rs:142:9
   5:     0x7f8387881630 - core::fmt::write::h21ffd88623dd8e8a
                               at /rustc/1db4b12494f698754b925f55061eb9e6b3241423/library/core/src/fmt/mod.rs:1120:17
   6:     0x7f83878224bf - std::io::Write::write_fmt::h192daa860eac75cb
                               at /rustc/1db4b12494f698754b925f55061eb9e6b3241423/library/std/src/io/mod.rs:1762:15
   7:     0x7f838782e414 - std::sys_common::backtrace::_print::h38432716ce8bcbbd
                               at /rustc/1db4b12494f698754b925f55061eb9e6b3241423/library/std/src/sys_common/backtrace.rs:47:5
   8:     0x7f838782e414 - std::sys_common::backtrace::print::h1f49e3e8b68963a0
                               at /rustc/1db4b12494f698754b925f55061eb9e6b3241423/library/std/src/sys_common/backtrace.rs:34:9
   9:     0x7f83878310a7 - std::panicking::default_hook::{{closure}}::h1ab1b9b9fcc59e11
  10:     0x7f8387830e0f - std::panicking::default_hook::h052ad26defc2b2d9
                               at /rustc/1db4b12494f698754b925f55061eb9e6b3241423/library/std/src/panicking.rs:292:9
  11:     0x7f83844ef410 - std[d0bee0584bbc47af]::panicking::update_hook::<alloc[dbc0aa70caf49578]::boxed::Box<rustc_driver_impl[cdb930f4dd583f9b]::install_ice_hook::{closure#0}>>::{closure#0}
  12:     0x7f83878317e8 - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::h80a3a3c1c2830f76
                               at /rustc/1db4b12494f698754b925f55061eb9e6b3241423/library/alloc/src/boxed.rs:2021:9
  13:     0x7f83878317e8 - std::panicking::rust_panic_with_hook::hd1cd0e2bf5e3508a
                               at /rustc/1db4b12494f698754b925f55061eb9e6b3241423/library/std/src/panicking.rs:783:13
  14:     0x7f8387831509 - std::panicking::begin_panic_handler::{{closure}}::h97ceddfe3dd8d703
                               at /rustc/1db4b12494f698754b925f55061eb9e6b3241423/library/std/src/panicking.rs:649:13
  15:     0x7f838782eaf6 - std::sys_common::backtrace::__rust_end_short_backtrace::ha30381b9e82dea56
                               at /rustc/1db4b12494f698754b925f55061eb9e6b3241423/library/std/src/sys_common/backtrace.rs:170:18
  16:     0x7f83878312a2 - rust_begin_unwind
                               at /rustc/1db4b12494f698754b925f55061eb9e6b3241423/library/std/src/panicking.rs:645:5
  17:     0x7f838787dd15 - core::panicking::panic_fmt::hfb14b7c9192d3596
                               at /rustc/1db4b12494f698754b925f55061eb9e6b3241423/library/core/src/panicking.rs:72:14
  18:     0x7f838787ddb3 - core::panicking::panic::h55b0e3fc25d6ee3f
                               at /rustc/1db4b12494f698754b925f55061eb9e6b3241423/library/core/src/panicking.rs:127:5
  19:     0x7f838506875e - <rustc_transmute[8d90f26179819fee]::layout::tree::Tree<rustc_transmute[8d90f26179819fee]::layout::rustc::Def, rustc_transmute[8d90f26179819fee]::layout::rustc::Ref>>::from_repr_c_variant
  20:     0x7f8385067bb1 - <rustc_transmute[8d90f26179819fee]::layout::tree::Tree<rustc_transmute[8d90f26179819fee]::layout::rustc::Def, rustc_transmute[8d90f26179819fee]::layout::rustc::Ref>>::from_ty
  21:     0x7f8385068473 - <rustc_transmute[8d90f26179819fee]::layout::tree::Tree<rustc_transmute[8d90f26179819fee]::layout::rustc::Def, rustc_transmute[8d90f26179819fee]::layout::rustc::Ref>>::from_repr_c_variant
  22:     0x7f8385067dc2 - <rustc_transmute[8d90f26179819fee]::layout::tree::Tree<rustc_transmute[8d90f26179819fee]::layout::rustc::Def, rustc_transmute[8d90f26179819fee]::layout::rustc::Ref>>::from_ty
  23:     0x7f838506b077 - <rustc_transmute[8d90f26179819fee]::rustc::TransmuteTypeEnv>::is_transmutable
  24:     0x7f83861685b4 - <rustc_trait_selection[6c7867fa258f0fc9]::traits::select::SelectionContext>::confirm_candidate
  25:     0x7f838623ad91 - <rustc_trait_selection[6c7867fa258f0fc9]::traits::select::SelectionContext>::evaluate_candidate
  26:     0x7f8385fd4b7f - <rustc_trait_selection[6c7867fa258f0fc9]::traits::select::SelectionContext>::evaluate_trait_predicate_recursively
  27:     0x7f8385fcdaa2 - <rustc_trait_selection[6c7867fa258f0fc9]::traits::select::SelectionContext>::evaluation_probe::<<rustc_trait_selection[6c7867fa258f0fc9]::traits::select::SelectionContext>::evaluate_root_obligation::{closure#0}>
  28:     0x7f8385a0899d - rustc_traits[1b3ad6effbd60470]::evaluate_obligation::evaluate_obligation
  29:     0x7f8385a08360 - rustc_query_impl[26ab2eb5a4b73d01]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[26ab2eb5a4b73d01]::query_impl::evaluate_obligation::dynamic_query::{closure#2}::{closure#0}, rustc_middle[8d21d7bf35089fff]::query::erase::Erased<[u8; 2usize]>>
  30:     0x7f8385a078d0 - rustc_query_system[b68b7660e1b83fb7]::query::plumbing::try_execute_query::<rustc_query_impl[26ab2eb5a4b73d01]::DynamicConfig<rustc_query_system[b68b7660e1b83fb7]::query::caches::DefaultCache<rustc_type_ir[eee2672c2f33704e]::canonical::Canonical<rustc_middle[8d21d7bf35089fff]::ty::context::TyCtxt, rustc_middle[8d21d7bf35089fff]::ty::ParamEnvAnd<rustc_middle[8d21d7bf35089fff]::ty::Predicate>>, rustc_middle[8d21d7bf35089fff]::query::erase::Erased<[u8; 2usize]>>, false, false, false>, rustc_query_impl[26ab2eb5a4b73d01]::plumbing::QueryCtxt, false>
  31:     0x7f8385a07555 - rustc_query_impl[26ab2eb5a4b73d01]::query_impl::evaluate_obligation::get_query_non_incr::__rust_end_short_backtrace
  32:     0x7f83828e2015 - <rustc_trait_selection[6c7867fa258f0fc9]::traits::fulfill::FulfillProcessor as rustc_data_structures[1a3c63c55c78a367]::obligation_forest::ObligationProcessor>::process_obligation
  33:     0x7f838580f7d2 - <rustc_data_structures[1a3c63c55c78a367]::obligation_forest::ObligationForest<rustc_trait_selection[6c7867fa258f0fc9]::traits::fulfill::PendingPredicateObligation>>::process_obligations::<rustc_trait_selection[6c7867fa258f0fc9]::traits::fulfill::FulfillProcessor>
  34:     0x7f83863895bd - <rustc_hir_typeck[48a7925dd0d8cef4]::fn_ctxt::FnCtxt>::check_argument_types
  35:     0x7f83860e7707 - <rustc_hir_typeck[48a7925dd0d8cef4]::fn_ctxt::FnCtxt>::check_call
  36:     0x7f83862f57e5 - <rustc_hir_typeck[48a7925dd0d8cef4]::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
  37:     0x7f8385dcc1ed - <rustc_hir_typeck[48a7925dd0d8cef4]::fn_ctxt::FnCtxt>::check_block_with_expected
  38:     0x7f83862f5ef4 - <rustc_hir_typeck[48a7925dd0d8cef4]::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
  39:     0x7f8385ff5cef - rustc_hir_typeck[48a7925dd0d8cef4]::check::check_fn
  40:     0x7f8385b197a9 - rustc_hir_typeck[48a7925dd0d8cef4]::typeck
  41:     0x7f8385b18b93 - rustc_query_impl[26ab2eb5a4b73d01]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[26ab2eb5a4b73d01]::query_impl::typeck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[8d21d7bf35089fff]::query::erase::Erased<[u8; 8usize]>>
  42:     0x7f83858eb4c3 - rustc_query_system[b68b7660e1b83fb7]::query::plumbing::try_execute_query::<rustc_query_impl[26ab2eb5a4b73d01]::DynamicConfig<rustc_query_system[b68b7660e1b83fb7]::query::caches::VecCache<rustc_span[c2bda87e20783fc8]::def_id::LocalDefId, rustc_middle[8d21d7bf35089fff]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[26ab2eb5a4b73d01]::plumbing::QueryCtxt, false>
  43:     0x7f83858eb150 - rustc_query_impl[26ab2eb5a4b73d01]::query_impl::typeck::get_query_non_incr::__rust_end_short_backtrace
  44:     0x7f8385ddf3c0 - rustc_hir_analysis[5a62dd6b63ca128a]::check_crate
  45:     0x7f8385b58517 - rustc_interface[97b84efa53a847bb]::passes::analysis
  46:     0x7f8385b58177 - rustc_query_impl[26ab2eb5a4b73d01]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[26ab2eb5a4b73d01]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[8d21d7bf35089fff]::query::erase::Erased<[u8; 1usize]>>
  47:     0x7f83867986e4 - rustc_query_system[b68b7660e1b83fb7]::query::plumbing::try_execute_query::<rustc_query_impl[26ab2eb5a4b73d01]::DynamicConfig<rustc_query_system[b68b7660e1b83fb7]::query::caches::SingleCache<rustc_middle[8d21d7bf35089fff]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[26ab2eb5a4b73d01]::plumbing::QueryCtxt, false>
  48:     0x7f83867984d5 - rustc_query_impl[26ab2eb5a4b73d01]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  49:     0x7f83867a10ff - rustc_interface[97b84efa53a847bb]::interface::run_compiler::<core[e724bad1857b6bd7]::result::Result<(), rustc_span[c2bda87e20783fc8]::ErrorGuaranteed>, rustc_driver_impl[cdb930f4dd583f9b]::run_compiler::{closure#1}>::{closure#0}
  50:     0x7f83867a841a - std[d0bee0584bbc47af]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[97b84efa53a847bb]::util::run_in_thread_with_globals<rustc_interface[97b84efa53a847bb]::util::run_in_thread_pool_with_globals<rustc_interface[97b84efa53a847bb]::interface::run_compiler<core[e724bad1857b6bd7]::result::Result<(), rustc_span[c2bda87e20783fc8]::ErrorGuaranteed>, rustc_driver_impl[cdb930f4dd583f9b]::run_compiler::{closure#1}>::{closure#0}, core[e724bad1857b6bd7]::result::Result<(), rustc_span[c2bda87e20783fc8]::ErrorGuaranteed>>::{closure#0}, core[e724bad1857b6bd7]::result::Result<(), rustc_span[c2bda87e20783fc8]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[e724bad1857b6bd7]::result::Result<(), rustc_span[c2bda87e20783fc8]::ErrorGuaranteed>>
  51:     0x7f83867a8243 - <<std[d0bee0584bbc47af]::thread::Builder>::spawn_unchecked_<rustc_interface[97b84efa53a847bb]::util::run_in_thread_with_globals<rustc_interface[97b84efa53a847bb]::util::run_in_thread_pool_with_globals<rustc_interface[97b84efa53a847bb]::interface::run_compiler<core[e724bad1857b6bd7]::result::Result<(), rustc_span[c2bda87e20783fc8]::ErrorGuaranteed>, rustc_driver_impl[cdb930f4dd583f9b]::run_compiler::{closure#1}>::{closure#0}, core[e724bad1857b6bd7]::result::Result<(), rustc_span[c2bda87e20783fc8]::ErrorGuaranteed>>::{closure#0}, core[e724bad1857b6bd7]::result::Result<(), rustc_span[c2bda87e20783fc8]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[e724bad1857b6bd7]::result::Result<(), rustc_span[c2bda87e20783fc8]::ErrorGuaranteed>>::{closure#1} as core[e724bad1857b6bd7]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  52:     0x7f838783b6a5 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hf4cdd73175754799
                               at /rustc/1db4b12494f698754b925f55061eb9e6b3241423/library/alloc/src/boxed.rs:2007:9
  53:     0x7f838783b6a5 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h6955d1f30cee8d29
                               at /rustc/1db4b12494f698754b925f55061eb9e6b3241423/library/alloc/src/boxed.rs:2007:9
  54:     0x7f838783b6a5 - std::sys::unix::thread::Thread::new::thread_start::h58dae6c91eb60bce
                               at /rustc/1db4b12494f698754b925f55061eb9e6b3241423/library/std/src/sys/unix/thread.rs:108:17
  55:     0x7f83816aa9eb - <unknown>
  56:     0x7f838172e7cc - <unknown>
  57:                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 1.75.0-nightly (1db4b1249 2023-11-11) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [evaluate_obligation] evaluating trait selection obligation `should_pad_explicitly_aligned_field::ExplicitlyPadded: core::mem::transmutability::BikeshedIntrinsicFrom<(), assert::Context, core::mem::transmutability::Assume { alignment: false, lifetimes: false, safety: false, validity: false }>`
#1 [typeck] type-checking `should_pad_explicitly_aligned_field`
#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: E0517, E0601, E0658, E0740.
For more information about an error, try `rustc --explain E0517`.

[matthiaskrgr]

smaller:

#![feature(transmutability)]

use std::mem::BikeshedIntrinsicFrom;
pub struct Context;

pub fn is_maybe_transmutable<Src, Dst>()
where
    Dst: BikeshedIntrinsicFrom<Src, Context>,
{
}

fn should_pad_explicitly_aligned_field() {
    #[repr(packed, align(0x100))]
    #[repr(u8)]
    enum V0u8 {
        V = 0,
    }

    #[repr(C)]
    struct ExplicitlyPadded(V0u8);

    is_maybe_transmutable::<(), ExplicitlyPadded>();
}

[matthiaskrgr]

Regression in nightly-2022-11-25

found 7 bors merge commits in the specified range
commit[0] 2022-11-23: Auto merge of #104410 - WaffleLapkin:unregress, r=estebank
commit[1] 2022-11-24: Auto merge of #104507 - WaffleLapkin:asderefsyou, r=wesleywiser
commit[2] 2022-11-24: Auto merge of #104610 - ouz-a:revert-overflow, r=compiler-errors
commit[3] 2022-11-24: Auto merge of #103808 - cjgillot:vec-cache, r=TaKO8Ki
commit[4] 2022-11-24: Auto merge of #104809 - matthiaskrgr:rollup-8abjdwh, r=matthiaskrgr
commit[5] 2022-11-24: Auto merge of #104321 - Swatinem:async-gen, r=oli-obk
commit[6] 2022-11-24: Auto merge of #103693 - HKalbasi:master, r=oli-obk
ERROR: no CI builds available between 70f8737 and b3bc6bf within last 167 days

[sunmy2019]

Panic on invalid code. Bisected to #103693

Nightly version: 1.75.0-nightly
(2023-11-10 edf0b1d)

Shorter

#![feature(transmutability)]

pub fn is_maybe_transmutable<Dst>()
where
    Dst: std::mem::BikeshedIntrinsicFrom<(), ()>,
{
}

fn should_pad_explicitly_aligned_field() {
    #[repr(packed, align(0x100))]
    #[repr(u8)]
    enum V0u8 {
        V,
    }

    is_maybe_transmutable::<V0u8>();
}

fn main() {}

The key difference is that V0u8 now has a valid TyAndLayout when querying layout_of. Before this PR, it does not.
This change caused an assertion failure in rustc_transmute.

[matthiaskrgr]

#117812 (comment) still ICEs 🥲

[workingjubilee]

This no longer ICEs because we no longer accept this combination of attributes on an enum, which is covered by this testcase:

rust/tests/ui/repr/attr-usage-repr.rs

Lines 24 to 25 in 3678036

	#[repr(packed)] //~ ERROR: attribute should be applied to a struct
	enum EPacked { A, B }