Compiler error in release mode when transmuting `Option<u32>`

[0xAtticus]

Code

fn main() {
    let variant: Option<u32> = None;
    let transmuted: u64 = unsafe {
        std::mem::transmute(variant)
    };
    println!("{transmuted}");
}

Meta

rustc --version --verbose:

rustc 1.79.0 (129f3b996 2024-06-10)
binary: rustc
commit-hash: 129f3b9964af4d4a709d1383930ade12dfe7c081
commit-date: 2024-06-10
host: x86_64-unknown-linux-gnu
release: 1.79.0
LLVM version: 18.1.7

Error output

   Compiling niche v0.1.0 (/home/atticus/code/niche)
thread 'rustc' panicked at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/compiler/rustc_codegen_ssa/src/mir/operand.rs:137:9:
assertion failed: alloc_align >= layout.align.abi
stack backtrace:
   0:     0x7f3794b89035 - std::backtrace_rs::backtrace::libunwind::trace::h1a07e5dba0da0cd2
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/../../backtrace/src/backtrace/libunwind.rs:105:5
   1:     0x7f3794b89035 - std::backtrace_rs::backtrace::trace_unsynchronized::h61b9b8394328c0bc
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f3794b89035 - std::sys_common::backtrace::_print_fmt::h1c5e18b460934cff
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/sys_common/backtrace.rs:68:5
   3:     0x7f3794b89035 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h1e1a1972118942ad
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x7f3794bd829b - core::fmt::rt::Argument::fmt::h07af2b4071d536cd
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/core/src/fmt/rt.rs:165:63
   5:     0x7f3794bd829b - core::fmt::write::hc090a2ffd6b28c4a
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/core/src/fmt/mod.rs:1157:21
   6:     0x7f3794b7dbdf - std::io::Write::write_fmt::h8898bac6ff039a23
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/io/mod.rs:1832:15
   7:     0x7f3794b88e0e - std::sys_common::backtrace::_print::h4e80c5803d4ee35b
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/sys_common/backtrace.rs:47:5
   8:     0x7f3794b88e0e - std::sys_common::backtrace::print::ha96650907276675e
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/sys_common/backtrace.rs:34:9
   9:     0x7f3794b8b779 - std::panicking::default_hook::{{closure}}::h215c2a0a8346e0e0
  10:     0x7f3794b8b4bd - std::panicking::default_hook::h207342be97478370
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/panicking.rs:298:9
  11:     0x7f37916891b7 - std[3c8ba8ebcf555201]::panicking::update_hook::<alloc[bfbae7e348dce413]::boxed::Box<rustc_driver_impl[c88438ade88661f4]::install_ice_hook::{closure#0}>>::{closure#0}
  12:     0x7f3794b8be76 - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::ha9c3bc81d312fd83
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/alloc/src/boxed.rs:2036:9
  13:     0x7f3794b8be76 - std::panicking::rust_panic_with_hook::hac8bdceee1e4fe2c
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/panicking.rs:799:13
  14:     0x7f3794b8bbeb - std::panicking::begin_panic_handler::{{closure}}::h00d785e82757ce3c
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/panicking.rs:656:13
  15:     0x7f3794b894f9 - std::sys_common::backtrace::__rust_end_short_backtrace::h1628d957bcd06996
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/sys_common/backtrace.rs:171:18
  16:     0x7f3794b8b957 - rust_begin_unwind
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/panicking.rs:652:5
  17:     0x7f3794bd4763 - core::panicking::panic_fmt::hdc63834ffaaefae5
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/core/src/panicking.rs:72:14
  18:     0x7f3794bd480c - core::panicking::panic::h75b3c9209f97d725
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/core/src/panicking.rs:146:5
  19:     0x7f379303163c - <rustc_codegen_ssa[8b1fc88ede96b23c]::mir::FunctionCx<rustc_codegen_llvm[6172e0583731cd24]::builder::Builder>>::codegen_operand
  20:     0x7f37935aab58 - rustc_codegen_ssa[8b1fc88ede96b23c]::mir::codegen_mir::<rustc_codegen_llvm[6172e0583731cd24]::builder::Builder>
  21:     0x7f37935977e4 - rustc_codegen_llvm[6172e0583731cd24]::base::compile_codegen_unit::module_codegen
  22:     0x7f3793593d83 - <rustc_codegen_llvm[6172e0583731cd24]::LlvmCodegenBackend as rustc_codegen_ssa[8b1fc88ede96b23c]::traits::backend::ExtraBackendMethods>::compile_codegen_unit
  23:     0x7f379386219e - <rustc_codegen_llvm[6172e0583731cd24]::LlvmCodegenBackend as rustc_codegen_ssa[8b1fc88ede96b23c]::traits::backend::CodegenBackend>::codegen_crate
  24:     0x7f3793831345 - rustc_interface[640972162e3c086f]::passes::start_codegen
  25:     0x7f3793830ad8 - <rustc_interface[640972162e3c086f]::queries::Queries>::codegen_and_build_linker
  26:     0x7f37935831be - rustc_interface[640972162e3c086f]::interface::run_compiler::<core[868bc93c3f2beb33]::result::Result<(), rustc_span[8c7415e9d33ddd75]::ErrorGuaranteed>, rustc_driver_impl[c88438ade88661f4]::run_compiler::{closure#0}>::{closure#1}
  27:     0x7f3793540f8b - std[3c8ba8ebcf555201]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[640972162e3c086f]::util::run_in_thread_with_globals<rustc_interface[640972162e3c086f]::interface::run_compiler<core[868bc93c3f2beb33]::result::Result<(), rustc_span[8c7415e9d33ddd75]::ErrorGuaranteed>, rustc_driver_impl[c88438ade88661f4]::run_compiler::{closure#0}>::{closure#1}, core[868bc93c3f2beb33]::result::Result<(), rustc_span[8c7415e9d33ddd75]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[868bc93c3f2beb33]::result::Result<(), rustc_span[8c7415e9d33ddd75]::ErrorGuaranteed>>
  28:     0x7f3793540d80 - <<std[3c8ba8ebcf555201]::thread::Builder>::spawn_unchecked_<rustc_interface[640972162e3c086f]::util::run_in_thread_with_globals<rustc_interface[640972162e3c086f]::interface::run_compiler<core[868bc93c3f2beb33]::result::Result<(), rustc_span[8c7415e9d33ddd75]::ErrorGuaranteed>, rustc_driver_impl[c88438ade88661f4]::run_compiler::{closure#0}>::{closure#1}, core[868bc93c3f2beb33]::result::Result<(), rustc_span[8c7415e9d33ddd75]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[868bc93c3f2beb33]::result::Result<(), rustc_span[8c7415e9d33ddd75]::ErrorGuaranteed>>::{closure#2} as core[868bc93c3f2beb33]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  29:     0x7f3794b95cab - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h09e5a4c541afa800
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/alloc/src/boxed.rs:2022:9
  30:     0x7f3794b95cab - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h9c8b03c22f4e7026
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/alloc/src/boxed.rs:2022:9
  31:     0x7f3794b95cab - std::sys::pal::unix::thread::Thread::new::thread_start::h522bc89a54da820a
                               at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/sys/pal/unix/thread.rs:108:17
  32:     0x7f37949521b7 - start_thread
  33:     0x7f37949d43cc - __GI___clone3
  34:                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 (129f3b996 2024-06-10) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type bin -C opt-level=3 -C embed-bitcode=no -C strip=debuginfo

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack
error: could not compile `niche` (bin "niche")

Backtrace

   Compiling niche v0.1.0 (/home/atticus/code/niche)
thread 'rustc' panicked at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/compiler/rustc_codegen_ssa/src/mir/operand.rs:137:9:
assertion failed: alloc_align >= layout.align.abi
stack backtrace:
   0: rust_begin_unwind
             at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/panicking.rs:652:5
   1: core::panicking::panic_fmt
             at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/core/src/panicking.rs:72:14
   2: core::panicking::panic
             at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/core/src/panicking.rs:146:5
   3: <rustc_codegen_ssa::mir::FunctionCx<rustc_codegen_llvm::builder::Builder>>::codegen_operand
   4: rustc_codegen_ssa::mir::codegen_mir::<rustc_codegen_llvm::builder::Builder>
   5: rustc_codegen_llvm::base::compile_codegen_unit::module_codegen
   6: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::ExtraBackendMethods>::compile_codegen_unit
   7: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
   8: rustc_interface::passes::start_codegen
   9: <rustc_interface::queries::Queries>::codegen_and_build_linker
  10: 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: rustc 1.79.0 (129f3b996 2024-06-10) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type bin -C opt-level=3 -C embed-bitcode=no -C strip=debuginfo

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack
error: could not compile `niche` (bin "niche"

[matthiaskrgr]

cc @cjgillot I suspect this might be related to #116012

[0xAtticus]

I did not know where to add this is the template but this may be useful:
Build works in debug mode, but not in release mode.

[GrigorenkoPV]

Regression in nightly-2023-12-31 (3cdd004...2a3e635)