The `check_abi` function in typeck is not called for ABIs in trait declarations

[nagisa]

Consider the following code:

trait T {
    extern "amdgpu-kernel" fn mu();
}

type TAU = extern "amdgpu-kernel" fn();

The check_abi function…

rust/compiler/rustc_typeck/src/check/check.rs

Lines 37 to 59 in 0a8629b

	pub(super) fn check_abi(tcx: TyCtxt<'_>, span: Span, abi: Abi) {
	if !tcx.sess.target.is_abi_supported(abi) {
	struct_span_err!(
	tcx.sess,
	span,
	E0570,
	"The ABI `{}` is not supported for the current target",
	abi
	)
	.emit()
	}
	// This ABI is only allowed on function pointers
	if abi == Abi::CCmseNonSecureCall {
	struct_span_err!(
	tcx.sess,
	span,
	E0781,
	"the `\"C-cmse-nonsecure-call\"` ABI is only allowed on function pointers."
	)
	.emit()
	}
	}

… is not called for the following two ABI lines, meaning that the checks present therein do not activate, potentially allowing circumvention of the checks this function implements. In particular one thing that is allowed is taking the TAU function pointer as an argument and calling it on architectures where this ABI is unsupported.

This becomes especially relevant after #86231 lands.

[RalfJung]

#130260 seems to address this for function pointer types, including in type aliases.

However, trait declarations still seem to be missed:

#![feature(abi_gpu_kernel)]
trait T {
    extern "gpu-kernel" fn mu();
}

Note that attempting to implement such a trait already emits a hard error.

Cc @tdittr -- in case you have an appetite for another lint similar to #128784 :D