Failed to build `core` crate with `-C opt-level=0`

In one environment, I use RUSTFLAGS="-C opt-level=0 -C debuginfo=0". Building rustc with this when using rustc_codegen_gcc, I get a failure to build core:

thread 'rustc' (75759) panicked at /rustc-dev/67c208e9eec6b5add33b94f593fe18b23bf9fa3c/compiler/rustc_codegen_ssa/src/mir/rvalue.rs:1034:5:
assertion `left == right` failed
  left: __int32_t  __attribute__((aligned(4))) *  __attribute__((aligned(8)))
 right: void *
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt.localalias
   2: core::panicking::assert_failed_inner
   3: core::panicking::assert_failed::<gccjit::types::Type, gccjit::types::Type>
   4: rustc_codegen_ssa::mir::rvalue::transmute_scalar::<rustc_codegen_gcc::builder::Builder>
   5: <rustc_codegen_ssa::mir::FunctionCx<rustc_codegen_gcc::builder::Builder>>::codegen_transmute_operand
   6: <rustc_codegen_ssa::mir::FunctionCx<rustc_codegen_gcc::builder::Builder>>::codegen_rvalue_operand
   7: <rustc_codegen_ssa::mir::FunctionCx<rustc_codegen_gcc::builder::Builder>>::codegen_statement
   8: <rustc_codegen_ssa::mir::FunctionCx<rustc_codegen_gcc::builder::Builder>>::codegen_block.localalias
   9: rustc_codegen_ssa::mir::codegen_mir::<rustc_codegen_gcc::builder::Builder>
  10: rustc_codegen_ssa::base::codegen_instance::<rustc_codegen_gcc::builder::Builder>
  11: <rustc_middle::mir::mono::MonoItem as rustc_codegen_ssa::mono_item::MonoItemExt>::define::<rustc_codegen_gcc::builder::Builder>
  12: rustc_codegen_gcc::base::compile_codegen_unit::module_codegen
  13: <rustc_query_system::dep_graph::graph::DepGraph<rustc_middle::dep_graph::DepsType>>::with_task::<rustc_middle::ty::context::TyCtxt, (rustc_span::symbol::Symbol, rustc_codegen_gcc::LockedTargetInfo), rustc_codegen_ssa::ModuleCodegen<rustc_codegen_gcc::GccContext>>
  14: rustc_codegen_gcc::base::compile_codegen_unit
  15: <rustc_codegen_gcc::GccCodegenBackend as rustc_codegen_ssa::traits::backend::ExtraBackendMethods>::compile_codegen_unit
  16: rustc_codegen_ssa::base::codegen_crate::<rustc_codegen_gcc::GccCodegenBackend>
  17: <rustc_codegen_gcc::GccCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
  18: rustc_interface::passes::start_codegen::{closure#0}
  19: <rustc_session::session::Session>::time::<alloc::boxed::Box<dyn core::any::Any>, rustc_interface::passes::start_codegen::{closure#0}>
  20: rustc_interface::passes::start_codegen
  21: <rustc_interface::queries::Linker>::codegen_and_build_linker
  22: rustc_driver_impl::run_compiler::{closure#0}::{closure#2}
  23: rustc_interface::passes::create_and_enter_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  24: <rustc_middle::ty::context::GlobalCtxt>::enter::<rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}, core::option::Option<rustc_interface::queries::Linker>>::{closure#1}
  25: rustc_middle::ty::context::tls::enter_context::<<rustc_middle::ty::context::GlobalCtxt>::enter<rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}, core::option::Option<rustc_interface::queries::Linker>>::{closure#1}, core::option::Option<rustc_interface::queries::Linker>>::{closure#0}
  26: <std::thread::local::LocalKey<core::cell::Cell<*const ()>>>::try_with::<rustc_middle::ty::context::tls::enter_context<<rustc_middle::ty::context::GlobalCtxt>::enter<rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}, core::option::Option<rustc_interface::queries::Linker>>::{closure#1}, core::option::Option<rustc_interface::queries::Linker>>::{closure#0}, core::option::Option<rustc_interface::queries::Linker>>.localalias
  27: <std::thread::local::LocalKey<core::cell::Cell<*const ()>>>::with::<rustc_middle::ty::context::tls::enter_context<<rustc_middle::ty::context::GlobalCtxt>::enter<rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}, core::option::Option<rustc_interface::queries::Linker>>::{closure#1}, core::option::Option<rustc_interface::queries::Linker>>::{closure#0}, core::option::Option<rustc_interface::queries::Linker>>
  28: rustc_middle::ty::context::tls::enter_context::<<rustc_middle::ty::context::GlobalCtxt>::enter<rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}, core::option::Option<rustc_interface::queries::Linker>>::{closure#1}, core::option::Option<rustc_interface::queries::Linker>>
  29: <rustc_middle::ty::context::GlobalCtxt>::enter::<rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}, core::option::Option<rustc_interface::queries::Linker>>.localalias
  30: <rustc_middle::ty::context::TyCtxt>::create_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}>
  31: rustc_interface::passes::create_and_enter_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}
  32: <rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core::ops::function::FnOnce<(&rustc_session::session::Session, rustc_middle::ty::context::CurrentGcx, alloc::sync::Arc<rustc_data_structures::jobserver::Proxy>, &std::sync::once_lock::OnceLock<rustc_middle::ty::context::GlobalCtxt>, &rustc_data_structures::sync::worker_local::WorkerLocal<rustc_middle::arena::Arena>, &rustc_data_structures::sync::worker_local::WorkerLocal<rustc_hir::Arena>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
  33: <alloc::boxed::Box<dyn for<'a> core::ops::function::FnOnce<(&'a rustc_session::session::Session, rustc_middle::ty::context::CurrentGcx, alloc::sync::Arc<rustc_data_structures::jobserver::Proxy>, &'a std::sync::once_lock::OnceLock<rustc_middle::ty::context::GlobalCtxt<'a>>, &'a rustc_data_structures::sync::worker_local::WorkerLocal<rustc_middle::arena::Arena<'a>>, &'a rustc_data_structures::sync::worker_local::WorkerLocal<rustc_hir::Arena<'a>>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}), Output = core::option::Option<rustc_interface::queries::Linker>>> as core::ops::function::FnOnce<(&rustc_session::session::Session, rustc_middle::ty::context::CurrentGcx, alloc::sync::Arc<rustc_data_structures::jobserver::Proxy>, &std::sync::once_lock::OnceLock<rustc_middle::ty::context::GlobalCtxt>, &rustc_data_structures::sync::worker_local::WorkerLocal<rustc_middle::arena::Arena>, &rustc_data_structures::sync::worker_local::WorkerLocal<rustc_hir::Arena>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2})>>::call_once
  34: rustc_interface::passes::create_and_enter_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>
  35: rustc_driver_impl::run_compiler::{closure#0}
  36: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}::{closure#0}.localalias
  37: <core::panic::unwind_safe::AssertUnwindSafe<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}::{closure#0}> as core::ops::function::FnOnce<()>>::call_once
  38: std::panicking::catch_unwind::do_call::<core::panic::unwind_safe::AssertUnwindSafe<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}::{closure#0}>, ()>
  39: __rust_try
  40: std::panic::catch_unwind::<core::panic::unwind_safe::AssertUnwindSafe<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}::{closure#0}>, ()>
  41: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
  42: rustc_interface::util::run_in_thread_pool_with_globals::<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}.localalias
  43: rustc_interface::util::run_in_thread_with_globals::<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}::{closure#0}
  44: <scoped_tls::ScopedKey<rustc_span::SessionGlobals>>::set::<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}::{closure#0}, ()>
  45: rustc_span::create_session_globals_then::<(), rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}::{closure#0}>
  46: rustc_interface::util::run_in_thread_with_globals::<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}
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: please make sure that you have updated to the latest nightly

note: please attach the file at `/var/tmp/portage/dev-lang/rust-1.91.0_beta20251009/work/rustc-beta-src/rustc-ice-2025-10-27T12_31_28-75726.txt` to your bug report

note: compiler flags: --crate-type lib -C opt-level=3 -C embed-bitcode=no -C codegen-units=1 -C linker=x86_64-pc-linux-gnu-gcc -C opt-level=0 -C debuginfo=0 -C symbol-mangling-version=legacy -Z unstable-options -Z macro-backtrace -C split-debuginfo=off -C prefer-dynamic -Z inline-mir -Z inline-mir-preserve-debug -Z mir_strip_debuginfo=locals-in-tiny-functions -C link-args=-Wl,-z,origin -C link-args=-Wl,-rpath,$ORIGIN/../lib -C embed-bitcode=yes -C lto=off -Z unstable-options -C force-frame-pointers=non-leaf -Z crate-attr=doc(html_root_url="https://doc.rust-lang.org/nightly/") -Z binary-dep-depinfo -Z force-unstable-if-unmarked
note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack

Did not run successfully: exit status: 101
LD_LIBRARY_PATH="/var/tmp/portage/dev-lang/rust-1.91.0_beta20251009/work/rustc-beta-src/build/x86_64-unknown-linux-gnu/stage1/lib:/var/tmp/portage/dev-lang/rust-1.91.0_beta20251009/work/rustc-beta-src/build/x86_64-unknown-linux-gnu/stage1-std/release/deps" "/var/tmp/portage/dev-lang/rust-1.91.0_beta20251009/work/rustc-beta-src/build/x86_64-unknown-linux-gnu/stage1/bin/rustc" "--crate-name" "core" "--edition=2024" "library/core/src/lib.rs" "--error-format=json" "--json=diagnostic-rendered-ansi,artifacts,future-incompat" "--crate-type" "lib" "--emit=dep-info,metadata,link" "-C" "opt-level=3" "-C" "embed-bitcode=no" "-C" "codegen-units=1" "--warn=unexpected_cfgs" "--check-cfg" "cfg(no_fp_fmt_parse)" "--check-cfg" "cfg(feature, values(any()))" "--check-cfg" "cfg(target_has_reliable_f16)" "--check-cfg" "cfg(target_has_reliable_f16_math)" "--check-cfg" "cfg(target_has_reliable_f128)" "--check-cfg" "cfg(target_has_reliable_f128_math)" "--check-cfg" "cfg(docsrs,test)" "--check-cfg" "cfg(feature, values(\"debug_refcell\", \"optimize_for_size\", \"panic_immediate_abort\"))" "-C" "metadata=e4c2771f4ac279b7" "-C" "extra-filename=-847cdeb8a8d320b3" "--out-dir" "/var/tmp/portage/dev-lang/rust-1.91.0_beta20251009/work/rustc-beta-src/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/release/deps" "--target" "x86_64-unknown-linux-gnu" "-C" "linker=x86_64-pc-linux-gnu-gcc" "-L" "dependency=/var/tmp/portage/dev-lang/rust-1.91.0_beta20251009/work/rustc-beta-src/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/release/deps" "-L" "dependency=/var/tmp/portage/dev-lang/rust-1.91.0_beta20251009/work/rustc-beta-src/build/x86_64-unknown-linux-gnu/stage1-std/release/deps" "-C" "opt-level=0" "-C" "debuginfo=0" "-Lnative=/usr/lib/llvm/21/lib64" "-Csymbol-mangling-version=legacy" "--check-cfg=cfg(feature,values(any()))" "-Zunstable-options" "-Zmacro-backtrace" "-Csplit-debuginfo=off" "-Cprefer-dynamic" "-Zinline-mir" "-Zinline-mir-preserve-debug" "-Zmir_strip_debuginfo=locals-in-tiny-functions" "-Clink-args=-Wl,-z,origin" "-Clink-args=-Wl,-rpath,$ORIGIN/../lib" "-Alinker-messages" "-Cembed-bitcode=yes" "-Clto=off" "-Zunstable-options" "-Cforce-frame-pointers=non-leaf" "-Zcrate-attr=doc(html_root_url=\"https://doc.rust-lang.org/nightly/\")" "-Z" "binary-dep-depinfo" "-Wrust_2018_idioms" "-Wunused_lifetimes" "-Dwarnings" "--sysroot" "/var/tmp/portage/dev-lang/rust-1.91.0_beta20251009/work/rustc-beta-src/build/x86_64-unknown-linux-gnu/stage1" "--remap-path-prefix" "/var/tmp/portage/dev-lang/rust-1.91.0_beta20251009/work/rustc-beta-src=/rustc/67c208e9eec6b5add33b94f593fe18b23bf9fa3c" "--remap-path-prefix" "/var/tmp/portage/dev-lang/rust-1.91.0_beta20251009/work/rustc-beta-src/vendor=/rust/deps" "-Z" "force-unstable-if-unmarked"
-------------
error: could not compile `core` (lib)

Caused by:
  process didn't exit successfully: `CARGO=/usr/lib/rust/1.91.0/bin/cargo CARGO_CRATE_NAME=core CARGO_MANIFEST_DIR=/var/tmp/portage/dev-lang/rust-1.91.0_beta20251009/work/rustc-beta-src/library/core CARGO_MANIFEST_PATH=/var/tmp/portage/dev-lang/rust-1.91.0_beta20251009/work/rustc-beta-src/library/core/Cargo.toml CARGO_PKG_AUTHORS='' CARGO_PKG_DESCRIPTION='The Rust Core Library' CARGO_PKG_HOMEPAGE='' CARGO_PKG_LICENSE='MIT OR Apache-2.0' CARGO_PKG_LICENSE_FILE='' CARGO_PKG_NAME=core CARGO_PKG_README='' CARGO_PKG_REPOSITORY='https://github.com/rust-lang/rust.git' CARGO_PKG_RUST_VERSION='' CARGO_PKG_VERSION=0.0.0 CARGO_PKG_VERSION_MAJOR=0 CARGO_PKG_VERSION_MINOR=0 CARGO_PKG_VERSION_PATCH=0 CARGO_PKG_VERSION_PRE='' LD_LIBRARY_PATH=/var/tmp/portage/dev-lang/rust-1.91.0_beta20251009/work/rustc-beta-src/build/x86_64-unknown-linux-gnu/stage1-std/release/deps /var/tmp/portage/dev-lang/rust-1.91.0_beta20251009/work/rustc-beta-src/build/bootstrap/debug/rustc /var/tmp/portage/dev-lang/rust-1.91.0_beta20251009/work/rustc-beta-src/build/bootstrap/debug/rustc --crate-name core --edition=2024 library/core/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type lib --emit=dep-info,metadata,link -C opt-level=3 -C embed-bitcode=no -C codegen-units=1 --warn=unexpected_cfgs --check-cfg 'cfg(no_fp_fmt_parse)' --check-cfg 'cfg(feature, values(any()))' --check-cfg 'cfg(target_has_reliable_f16)' --check-cfg 'cfg(target_has_reliable_f16_math)' --check-cfg 'cfg(target_has_reliable_f128)' --check-cfg 'cfg(target_has_reliable_f128_math)' --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values("debug_refcell", "optimize_for_size", "panic_immediate_abort"))' -C metadata=e4c2771f4ac279b7 -C extra-filename=-847cdeb8a8d320b3 --out-dir /var/tmp/portage/dev-lang/rust-1.91.0_beta20251009/work/rustc-beta-src/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/release/deps --target x86_64-unknown-linux-gnu -C linker=x86_64-pc-linux-gnu-gcc -L dependency=/var/tmp/portage/dev-lang/rust-1.91.0_beta20251009/work/rustc-beta-src/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/release/deps -L dependency=/var/tmp/portage/dev-lang/rust-1.91.0_beta20251009/work/rustc-beta-src/build/x86_64-unknown-linux-gnu/stage1-std/release/deps -C opt-level=0 -C debuginfo=0 -Lnative=/usr/lib/llvm/21/lib64 -Csymbol-mangling-version=legacy '--check-cfg=cfg(feature,values(any()))' -Zunstable-options -Zmacro-backtrace -Csplit-debuginfo=off -Cprefer-dynamic -Zinline-mir -Zinline-mir-preserve-debug -Zmir_strip_debuginfo=locals-in-tiny-functions -Clink-args=-Wl,-z,origin '-Clink-args=-Wl,-rpath,$ORIGIN/../lib' -Alinker-messages -Cembed-bitcode=yes -Clto=off -Zunstable-options -Cforce-frame-pointers=non-leaf '-Zcrate-attr=doc(html_root_url="https://doc.rust-lang.org/nightly/")' -Z binary-dep-depinfo` (exit status: 101)

It works OK with rustc_codegen_llvm.

$ rustc -vV
rustc 1.91.0-nightly (067965d6c 2025-10-27) (gentoo)
binary: rustc
commit-hash: 067965d6cc76bfd49637f46d05ae2111bbd2f121
commit-date: 2025-10-27
host: x86_64-unknown-linux-gnu
release: 1.91.0-nightly

This is with the rust 1.91.0 beta from 20251009 and github.com/rust-lang/rust from b93730ce97c738bdb9209aa973d3fc6fd4ce1d81.

Reproduce the failure while building library/core/src/lib.rs with rustc_codegen_gcc and the reported RUSTFLAGS. Start at rustc_codegen_ssa/src/mir/rvalue.rs:1034, especially transmute_scalar, and trace the type mismatch shown in the panic. Done means the core crate builds without this compiler panic under -C opt-level=0.