ICE in improper_ctypes on AIX

[Noratrieb]

Compiling core for AIX leads to an ICE in the improper_ctypes lint.

https://does-it-build.noratrieb.dev/build?nightly=2025-10-04&target=powerpc64-ibm-aix&mode=std

Likely some of the AIX layout special cases causing it.

Regressed in nightly-2025-09-08: https://does-it-build.noratrieb.dev/target?target=powerpc64-ibm-aix

[Noratrieb]

@daltenty @gilamn5tr

[folkertdev]

It looks like this function

rust/compiler/rustc_lint/src/types/improper_ctypes.rs

Lines 217 to 243 in e2c96cc

	fn check_struct_for_power_alignment<'tcx>(
	cx: &LateContext<'tcx>,
	item: &'tcx hir::Item<'tcx>,
	adt_def: AdtDef<'tcx>,
	) {
	let tcx = cx.tcx;
	// repr(C) structs also with packed or aligned representation
	// should be ignored.
	if adt_def.repr().c()
	&& !adt_def.repr().packed()
	&& adt_def.repr().align.is_none()
	&& tcx.sess.target.os == "aix"
	&& !adt_def.all_fields().next().is_none()
	{
	let struct_variant_data = item.expect_struct().2;
	for field_def in struct_variant_data.fields().iter().skip(1) {
	// Struct fields (after the first field) are checked for the
	// power alignment rule, as fields after the first are likely
	// to be the fields that are misaligned.
	let def_id = field_def.def_id;
	let ty = tcx.type_of(def_id).instantiate_identity();
	if check_arg_for_power_alignment(cx, ty) {
	cx.emit_span_lint(USES_POWER_ALIGNMENT, field_def.span, UsesPowerAlignment);
	}
	}
	}
	}

is called on a union? Or in any case, nothing guards against that being the case, and the error message does appear to contain unions.

[workingjubilee]

This is fixed by #142310 also.

[RalfJung]

So libcore did not build on AIX for a month and nobody noticed? I guess that platform really doesn't have many users.^^

[Noratrieb]

If you include std, it has been broken for 3 months now :). Maybe Big Blue has a patch that they use for their build but haven't upstreamed, or just hasn't updated to newer versions yet.
I'm sure the target maintainers will be able to say more :).