ICE building `glam`: Projecting into SIMD type std::arch::aarch64::float32x4_t is banned by MCP#838

[kpreid]

Code

[package]
name = "foo"
version = "0.1.0"
edition = "2024"

[dependencies]
glam = { version = "0.30.5" }

[profile.dev]
opt-level = 2
overflow-checks = true

Then cargo check or cargo build.

Meta

rustc --version --verbose:

rustc 1.90.0-nightly (498ae9fed 2025-07-28)
binary: rustc
commit-hash: 498ae9fed2e7d90821d70a048f3770f91af08957
commit-date: 2025-07-28
host: aarch64-apple-darwin
release: 1.90.0-nightly
LLVM version: 20.1.8

Error output

error: internal compiler error: compiler/rustc_mir_transform/src/validate.rs:80:25: broken MIR in Item(DefId(0:35261 ~ glam[4582]::euler::{impl#44}::to_euler_angles)) (after phase change to runtime-optimized) at bb0[5]:
                                Projecting into SIMD type std::arch::aarch64::float32x4_t is banned by MCP#838
   --> /Users/kpreid/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/glam-0.30.5/src/euler.rs:410:34
    |
410 |                 $mat3::from_mat4(self).to_euler_angles(order)
    |                                  ^^^^
...
435 | impl_mat4_to_euler!(f32, Mat4, Mat3);
    | ------------------------------------ in this macro invocation
    |
    = note: this error: internal compiler error originates in the macro `impl_mat4_to_euler` (in Nightly builds, run with -Z macro-backtrace for more info)


thread 'rustc' panicked at compiler/rustc_mir_transform/src/validate.rs:80:25:
Box<dyn Any>

Backtrace

stack backtrace:
   0:        0x10e89cb4c - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h2c8dadb96be3f8dc
   1:        0x10bd78670 - core::fmt::write::haea1839b4421f097
   2:        0x10e891184 - std::io::Write::write_fmt::h829d20f6c5614c8f
   3:        0x10e89ca0c - std::sys::backtrace::BacktraceLock::print::h9d16fad313b69287
   4:        0x10e8a15c0 - std::panicking::default_hook::{{closure}}::hd647fc9fea98ee68
   5:        0x10e8a1298 - std::panicking::default_hook::h438fdb7239b92b6b
   6:        0x10c96f1ac - rustc_driver_impl[c764e17e85da8204]::install_ice_hook::{closure#1}
   7:        0x10e8a1f4c - std::panicking::rust_panic_with_hook::h14f7a21fdd463b8b
   8:        0x10c99f534 - std[b05d56cfd8fca740]::panicking::begin_panic::<rustc_errors[1c7534ae315a5042]::ExplicitBug>::{closure#0}
   9:        0x10c98df40 - std[b05d56cfd8fca740]::sys::backtrace::__rust_end_short_backtrace::<std[b05d56cfd8fca740]::panicking::begin_panic<rustc_errors[1c7534ae315a5042]::ExplicitBug>::{closure#0}, !>
  10:        0x11153f890 - std[b05d56cfd8fca740]::panicking::begin_panic::<rustc_errors[1c7534ae315a5042]::ExplicitBug>
  11:        0x11153fdd0 - <rustc_errors[1c7534ae315a5042]::diagnostic::BugAbort as rustc_errors[1c7534ae315a5042]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  12:        0x1115ae330 - <rustc_errors[1c7534ae315a5042]::DiagCtxtHandle>::span_bug::<rustc_span[383248f629c63a7f]::span_encoding::Span, alloc[e2705810b410854d]::string::String>
  13:        0x1115b7208 - rustc_middle[ff00f450670e31a2]::util::bug::opt_span_bug_fmt::<rustc_span[383248f629c63a7f]::span_encoding::Span>::{closure#0}
  14:        0x10d61951c - rustc_middle[ff00f450670e31a2]::ty::context::tls::with_opt::<rustc_middle[ff00f450670e31a2]::util::bug::opt_span_bug_fmt<rustc_span[383248f629c63a7f]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  15:        0x10d618fe4 - rustc_middle[ff00f450670e31a2]::ty::context::tls::with_context_opt::<rustc_middle[ff00f450670e31a2]::ty::context::tls::with_opt<rustc_middle[ff00f450670e31a2]::util::bug::opt_span_bug_fmt<rustc_span[383248f629c63a7f]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  16:        0x1115b7178 - rustc_middle[ff00f450670e31a2]::util::bug::span_bug_fmt::<rustc_span[383248f629c63a7f]::span_encoding::Span>
  17:        0x10d9a6a08 - <rustc_mir_transform[c3f80c3e19acb41d]::validate::CfgChecker>::fail::<alloc[e2705810b410854d]::string::String>
  18:        0x10d9a5b34 - <rustc_mir_transform[c3f80c3e19acb41d]::validate::Validator as rustc_mir_transform[c3f80c3e19acb41d]::pass_manager::MirPass>::run_pass
  19:        0x10d8f7198 - rustc_mir_transform[c3f80c3e19acb41d]::pass_manager::run_passes_inner
  20:        0x10d91bf5c - rustc_mir_transform[c3f80c3e19acb41d]::run_optimization_passes
  21:        0x10d91c3c0 - rustc_mir_transform[c3f80c3e19acb41d]::optimized_mir
  22:        0x10e00eb50 - rustc_query_impl[710479fe5bc1800b]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[710479fe5bc1800b]::query_impl::optimized_mir::dynamic_query::{closure#2}::{closure#0}, rustc_middle[ff00f450670e31a2]::query::erase::Erased<[u8; 8usize]>>
  23:        0x10dde78e0 - rustc_query_system[8d269a2fdd66b5b0]::query::plumbing::try_execute_query::<rustc_query_impl[710479fe5bc1800b]::DynamicConfig<rustc_query_system[8d269a2fdd66b5b0]::query::caches::DefIdCache<rustc_middle[ff00f450670e31a2]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[710479fe5bc1800b]::plumbing::QueryCtxt, false>
  24:        0x10e02b5f8 - rustc_query_impl[710479fe5bc1800b]::query_impl::optimized_mir::get_query_non_incr::__rust_end_short_backtrace
  25:        0x10d8f8f5c - rustc_mir_transform[c3f80c3e19acb41d]::cross_crate_inline::cross_crate_inlinable
  26:        0x10e011cd4 - rustc_query_impl[710479fe5bc1800b]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[710479fe5bc1800b]::query_impl::cross_crate_inlinable::dynamic_query::{closure#2}::{closure#0}, rustc_middle[ff00f450670e31a2]::query::erase::Erased<[u8; 1usize]>>
  27:        0x10dddc258 - rustc_query_system[8d269a2fdd66b5b0]::query::plumbing::try_execute_query::<rustc_query_impl[710479fe5bc1800b]::DynamicConfig<rustc_query_system[8d269a2fdd66b5b0]::query::caches::DefIdCache<rustc_middle[ff00f450670e31a2]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[710479fe5bc1800b]::plumbing::QueryCtxt, false>
  28:        0x10e0a4a0c - rustc_query_impl[710479fe5bc1800b]::query_impl::cross_crate_inlinable::get_query_non_incr::__rust_end_short_backtrace
  29:        0x10dc61150 - rustc_passes[dfb39456de595ce0]::reachable::recursively_reachable
  30:        0x10dc63b3c - rustc_passes[dfb39456de595ce0]::reachable::reachable_set
  31:        0x10e00ebcc - rustc_query_impl[710479fe5bc1800b]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[710479fe5bc1800b]::query_impl::reachable_set::dynamic_query::{closure#2}::{closure#0}, rustc_middle[ff00f450670e31a2]::query::erase::Erased<[u8; 8usize]>>
  32:        0x10ddf4c54 - rustc_query_system[8d269a2fdd66b5b0]::query::plumbing::try_execute_query::<rustc_query_impl[710479fe5bc1800b]::DynamicConfig<rustc_query_system[8d269a2fdd66b5b0]::query::caches::SingleCache<rustc_middle[ff00f450670e31a2]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[710479fe5bc1800b]::plumbing::QueryCtxt, false>
  33:        0x10e04e888 - rustc_query_impl[710479fe5bc1800b]::query_impl::reachable_set::get_query_non_incr::__rust_end_short_backtrace
  34:        0x10d49eff8 - <rustc_metadata[7a8e59b443ce0166]::rmeta::encoder::EncodeContext>::encode_crate_root
  35:        0x111598df4 - rustc_metadata[7a8e59b443ce0166]::rmeta::encoder::encode_metadata::{closure#3}::{closure#0}
  36:        0x10d47bf10 - <rustc_metadata[7a8e59b443ce0166]::rmeta::encoder::encode_metadata::{closure#3} as core[9fdc13437c3067b0]::ops::function::FnOnce<(rustc_middle[ff00f450670e31a2]::ty::context::TyCtxt, &std[b05d56cfd8fca740]::path::Path)>>::call_once
  37:        0x10d5039e0 - rustc_metadata[7a8e59b443ce0166]::rmeta::encoder::encode_metadata
  38:        0x10d4ca17c - rustc_metadata[7a8e59b443ce0166]::fs::encode_and_write_metadata
  39:        0x10d259450 - <rustc_interface[57fce32a49ee6398]::queries::Linker>::codegen_and_build_linker
  40:        0x10c8d8060 - rustc_interface[57fce32a49ee6398]::passes::create_and_enter_global_ctxt::<core[9fdc13437c3067b0]::option::Option<rustc_interface[57fce32a49ee6398]::queries::Linker>, rustc_driver_impl[c764e17e85da8204]::run_compiler::{closure#0}::{closure#2}>
  41:        0x10c91fd88 - rustc_interface[57fce32a49ee6398]::interface::run_compiler::<(), rustc_driver_impl[c764e17e85da8204]::run_compiler::{closure#0}>::{closure#1}
  42:        0x10c9151f4 - std[b05d56cfd8fca740]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[57fce32a49ee6398]::util::run_in_thread_with_globals<rustc_interface[57fce32a49ee6398]::util::run_in_thread_pool_with_globals<rustc_interface[57fce32a49ee6398]::interface::run_compiler<(), rustc_driver_impl[c764e17e85da8204]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  43:        0x10c9220c4 - <<std[b05d56cfd8fca740]::thread::Builder>::spawn_unchecked_<rustc_interface[57fce32a49ee6398]::util::run_in_thread_with_globals<rustc_interface[57fce32a49ee6398]::util::run_in_thread_pool_with_globals<rustc_interface[57fce32a49ee6398]::interface::run_compiler<(), rustc_driver_impl[c764e17e85da8204]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[9fdc13437c3067b0]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  44:        0x10e8a52bc - std::sys::pal::unix::thread::Thread::new::thread_start::h538430a11caebc44
  45:        0x18b1bec0c - __pthread_cond_wait

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 `/Users/kpreid/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/glam-0.30.5/rustc-ice-2025-07-29T03_57_24-72327.txt` to your bug report

note: compiler flags: --crate-type lib -C opt-level=2 -C embed-bitcode=no -C debuginfo=2 -C split-debuginfo=unpacked -C debug-assertions=on

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

query stack during panic:
#0 [optimized_mir] optimizing MIR for `euler::<impl at /Users/kpreid/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/glam-0.30.5/src/euler.rs:404:9: 404:31>::to_euler_angles`
#1 [cross_crate_inlinable] whether the item should be made inlinable across crates
... and 1 other queries... use `env RUST_BACKTRACE=1` to see the full query stack
error: could not compile `glam` (lib)

Caused by:
  process didn't exit successfully: `/Users/kpreid/.rustup/toolchains/nightly-aarch64-apple-darwin/bin/rustc --crate-name glam --edition=2021 /Users/kpreid/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/glam-0.30.5/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=211 --crate-type lib --emit=dep-info,metadata,link -C opt-level=2 -C embed-bitcode=no -C debuginfo=2 -C split-debuginfo=unpacked --warn=unexpected_cfgs --check-cfg 'cfg(target_arch, values("spirv"))' -C debug-assertions=on --cfg 'feature="default"' --cfg 'feature="std"' --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values("approx", "bytecheck", "bytemuck", "core-simd", "cuda", "debug-glam-assert", "default", "fast-math", "glam-assert", "libm", "mint", "nostd-libm", "rand", "rkyv", "scalar-math", "serde", "speedy", "std"))' -C metadata=53063e6929dcaebc -C extra-filename=-28fe2bab7d0849f1 --out-dir /Users/kpreid/Projects/rust/scratch/foo/target/debug/deps -L dependency=/Users/kpreid/Projects/rust/scratch/foo/target/debug/deps --cap-lints allow` (exit status: 101)

@rustbot label regression-from-stable-to-nightly

[jieyouxu]

cc @scottmcm

[scottmcm]

Well no need to bisect at least, it's clearly #143833 (cc rust-lang/compiler-team#838) Actually, based on timing, it's not, at least not directly.

But darn, I thought I'd gotten all these in rust-lang/stdarch#1740

Crate code: https://docs.rs/glam/0.30.5/src/glam/euler.rs.html#402-414

I'll try to take a look, though doing so from x64 will probably be annoying. If anyone can isolate to where it should be an easy fix, though.

EDIT: Oh, fun, it's still using non-array SIMD on some targets (https://docs.rs/glam/0.30.5/src/glam/f32/vec3.rs.html#23) which hasn't worked on nightly in almost a year (#129403).

[stefanboca]

I also ran into this, here's another repro if it helps at all. For me this only errors in release builds; debug still builds fine.

#![feature(portable_simd)]

use std::simd::Simd;

pub(crate) fn foo(simds: &[Simd<u8, 16>], _unused: Simd<u8, 16>) {
    let a = simds[0];
}

The above compiles after removing the _unused argument.

Meta

rustc --version --verbose:

rustc 1.90.0-nightly (498ae9fed 2025-07-28)
binary: rustc
commit-hash: 498ae9fed2e7d90821d70a048f3770f91af08957
commit-date: 2025-07-28
host: x86_64-unknown-linux-gnu
release: 1.90.0-nightly
LLVM version: 20.1.8

Error output

error: internal compiler error: compiler/rustc_mir_transform/src/validate.rs:80:25: broken MIR in Item(DefId(0:4 ~ playground[d875]::foo)) (after phase change to runtime-optimized) at bb1[0]:
                                Projecting into SIMD type std::simd::Simd is banned by MCP#838
 --> src/lib.rs:6:13
  |
6 |     let a = simds[0];
  |             ^^^^^^^^

Backtrace

thread 'rustc' panicked at compiler/rustc_mir_transform/src/validate.rs:80:25:
Box<dyn Any>
stack backtrace:
   0:     0x7fea457befb5 - std::backtrace::Backtrace::create::hf91db3f640b5c750
   1:     0x7fea457bef05 - std::backtrace::Backtrace::force_capture::h21cb8315e241075f
   2:     0x7fea44858545 - std[a40869ec86c0be37]::panicking::update_hook::<alloc[6bff5a853f1091a5]::boxed::Box<rustc_driver_impl[1e14af3d6ff318b8]::install_ice_hook::{closure#1}>>::{closure#0}
   3:     0x7fea457d8fce - std::panicking::rust_panic_with_hook::h940eac454294b603
   4:     0x7fea44891841 - std[a40869ec86c0be37]::panicking::begin_panic::<rustc_errors[63d76ee921538c85]::ExplicitBug>::{closure#0}
   5:     0x7fea448867d6 - std[a40869ec86c0be37]::sys::backtrace::__rust_end_short_backtrace::<std[a40869ec86c0be37]::panicking::begin_panic<rustc_errors[63d76ee921538c85]::ExplicitBug>::{closure#0}, !>
   6:     0x7fea44886665 - std[a40869ec86c0be37]::panicking::begin_panic::<rustc_errors[63d76ee921538c85]::ExplicitBug>
   7:     0x7fea4489b2b1 - <rustc_errors[63d76ee921538c85]::diagnostic::BugAbort as rustc_errors[63d76ee921538c85]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
   8:     0x7fea44e0bcdc - <rustc_errors[63d76ee921538c85]::DiagCtxtHandle>::span_bug::<rustc_span[37dba8674e9310c0]::span_encoding::Span, alloc[6bff5a853f1091a5]::string::String>
   9:     0x7fea44ea0b67 - rustc_middle[8189fbb6e8ddda52]::util::bug::opt_span_bug_fmt::<rustc_span[37dba8674e9310c0]::span_encoding::Span>::{closure#0}
  10:     0x7fea44e8005a - rustc_middle[8189fbb6e8ddda52]::ty::context::tls::with_opt::<rustc_middle[8189fbb6e8ddda52]::util::bug::opt_span_bug_fmt<rustc_span[37dba8674e9310c0]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  11:     0x7fea44e7fecb - rustc_middle[8189fbb6e8ddda52]::ty::context::tls::with_context_opt::<rustc_middle[8189fbb6e8ddda52]::ty::context::tls::with_opt<rustc_middle[8189fbb6e8ddda52]::util::bug::opt_span_bug_fmt<rustc_span[37dba8674e9310c0]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  12:     0x7fea42f57597 - rustc_middle[8189fbb6e8ddda52]::util::bug::span_bug_fmt::<rustc_span[37dba8674e9310c0]::span_encoding::Span>
  13:     0x7fea435992d8 - <rustc_mir_transform[b10d8462625e20cf]::validate::CfgChecker>::fail::<alloc[6bff5a853f1091a5]::string::String>
  14:     0x7fea4730bbd0 - <rustc_mir_transform[b10d8462625e20cf]::validate::Validator as rustc_mir_transform[b10d8462625e20cf]::pass_manager::MirPass>::run_pass
  15:     0x7fea46011836 - rustc_mir_transform[b10d8462625e20cf]::run_optimization_passes
  16:     0x7fea46d28af8 - rustc_mir_transform[b10d8462625e20cf]::optimized_mir
  17:     0x7fea46d282dd - rustc_query_impl[371f4acb5ba9ab34]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[371f4acb5ba9ab34]::query_impl::optimized_mir::dynamic_query::{closure#2}::{closure#0}, rustc_middle[8189fbb6e8ddda52]::query::erase::Erased<[u8; 8usize]>>
  18:     0x7fea46018f25 - rustc_query_system[195bfac6b6ef1586]::query::plumbing::try_execute_query::<rustc_query_impl[371f4acb5ba9ab34]::DynamicConfig<rustc_query_system[195bfac6b6ef1586]::query::caches::DefIdCache<rustc_middle[8189fbb6e8ddda52]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[371f4acb5ba9ab34]::plumbing::QueryCtxt, false>
  19:     0x7fea4601841b - rustc_query_impl[371f4acb5ba9ab34]::query_impl::optimized_mir::get_query_non_incr::__rust_end_short_backtrace
  20:     0x7fea43757bfd - rustc_mir_transform[b10d8462625e20cf]::deduce_param_attrs::deduced_param_attrs
  21:     0x7fea4619dc2a - rustc_query_impl[371f4acb5ba9ab34]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[371f4acb5ba9ab34]::query_impl::deduced_param_attrs::dynamic_query::{closure#2}::{closure#0}, rustc_middle[8189fbb6e8ddda52]::query::erase::Erased<[u8; 16usize]>>
  22:     0x7fea4619dc0f - <rustc_query_impl[371f4acb5ba9ab34]::query_impl::deduced_param_attrs::dynamic_query::{closure#2} as core[729d18482630865e]::ops::function::FnOnce<(rustc_middle[8189fbb6e8ddda52]::ty::context::TyCtxt, rustc_span[37dba8674e9310c0]::def_id::DefId)>>::call_once
  23:     0x7fea4619ef7b - rustc_query_system[195bfac6b6ef1586]::query::plumbing::try_execute_query::<rustc_query_impl[371f4acb5ba9ab34]::DynamicConfig<rustc_query_system[195bfac6b6ef1586]::query::caches::DefIdCache<rustc_middle[8189fbb6e8ddda52]::query::erase::Erased<[u8; 16usize]>>, false, false, false>, rustc_query_impl[371f4acb5ba9ab34]::plumbing::QueryCtxt, false>
  24:     0x7fea4619e868 - rustc_query_impl[371f4acb5ba9ab34]::query_impl::deduced_param_attrs::get_query_non_incr::__rust_end_short_backtrace
  25:     0x7fea46b7331a - <rustc_metadata[7bd0cc19634daba3]::rmeta::encoder::EncodeContext>::encode_crate_root
  26:     0x7fea4748fc70 - <rustc_metadata[7bd0cc19634daba3]::rmeta::encoder::encode_metadata::{closure#3} as core[729d18482630865e]::ops::function::FnOnce<(rustc_middle[8189fbb6e8ddda52]::ty::context::TyCtxt, &std[a40869ec86c0be37]::path::Path)>>::call_once
  27:     0x7fea4748ceb0 - rustc_metadata[7bd0cc19634daba3]::rmeta::encoder::encode_metadata
  28:     0x7fea46f1a417 - rustc_metadata[7bd0cc19634daba3]::fs::encode_and_write_metadata
  29:     0x7fea47135c43 - <rustc_interface[f6a0648cee61be4a]::queries::Linker>::codegen_and_build_linker
  30:     0x7fea4713383c - rustc_interface[f6a0648cee61be4a]::passes::create_and_enter_global_ctxt::<core[729d18482630865e]::option::Option<rustc_interface[f6a0648cee61be4a]::queries::Linker>, rustc_driver_impl[1e14af3d6ff318b8]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  31:     0x7fea472cacde - rustc_interface[f6a0648cee61be4a]::interface::run_compiler::<(), rustc_driver_impl[1e14af3d6ff318b8]::run_compiler::{closure#0}>::{closure#1}
  32:     0x7fea47198141 - std[a40869ec86c0be37]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[f6a0648cee61be4a]::util::run_in_thread_with_globals<rustc_interface[f6a0648cee61be4a]::util::run_in_thread_pool_with_globals<rustc_interface[f6a0648cee61be4a]::interface::run_compiler<(), rustc_driver_impl[1e14af3d6ff318b8]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  33:     0x7fea47197e22 - <<std[a40869ec86c0be37]::thread::Builder>::spawn_unchecked_<rustc_interface[f6a0648cee61be4a]::util::run_in_thread_with_globals<rustc_interface[f6a0648cee61be4a]::util::run_in_thread_pool_with_globals<rustc_interface[f6a0648cee61be4a]::interface::run_compiler<(), rustc_driver_impl[1e14af3d6ff318b8]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[729d18482630865e]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  34:     0x7fea4719df7d - std::sys::pal::unix::thread::Thread::new::thread_start::hccd3bb61f89c035f
  35:     0x7fea4089af08 - start_thread
  36:     0x7fea409230ac - __clone3
  37:                0x0 - <unknown>


rustc version: 1.90.0-nightly (498ae9fed 2025-07-28)
platform: x86_64-unknown-linux-gnu

query stack during panic:
#0 [optimized_mir] optimizing MIR for `foo`
#1 [deduced_param_attrs] deducing parameter attributes for foo
end of query stack

[scottmcm]

@stefanboca Thank you! Isolating to not be neon-only and not need an extra crate helps a ton :)

Repro link: https://play.rust-lang.org/?version=nightly&mode=release&edition=2024&gist=0846e5f86dd52777f25b6bcd13942f7e

Needs release -- in debug it doesn't trigger.

Hmm, not triggering in 7-27 https://rust.godbolt.org/z/4W6evss9M, so maybe it's actually #144543 instead. That'd be good -- must easier to fix :)

Ah, yup. With -Z validate-mir=yes it confirms it's SRoA:

error: internal compiler error: compiler\rustc_mir_transform\src\validate.rs:80:25: broken MIR in Item(DefId(0:4 ~ uhoh[0d16]::foo)) (after pass ScalarReplacementOfAggregates) at bb1[0]:
                                Projecting into SIMD type std::simd::Simd is banned by MCP#838
 --> uhoh.rs:6:13
  |
6 |     let a = simds[0];
  |             ^^^^^^^^

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