`aarch64_vector_pcs` function pointers become a zero-sized opaque; rustc drops the argument

Input C/C++ Header

typedef int (*fp)(int) __attribute__((aarch64_vector_pcs));
int apply(fp f, int x);

Bindgen Invocation

$ bindgen input.h -- -std=gnu11

Actual Results

bindgen 0.72.1 and 0.73.1 both turn the function-pointer type into a zero-sized opaque blob:

pub type fp = __BindgenOpaqueArray<[u32; 0]>;
pub fn apply(f: fp, x: c_int) -> c_int;

aarch64_vector_pcs is CXCallingConv_AArch64VectorCall, which get_abi maps to Abi::Vectorcall. On a stable rustc that ABI is UnsupportedAbi, so the function-pointer type becomes a 0-size opaque instead of *mut c_void / Option<unsafe extern "C" fn(i32) -> i32>. rustc then drops the ZST from the FFI signature:

; clang
define i32 @apply(ptr noundef %0, i32 noundef %1)

; rustc of the generated bindings (twice)
declare i32 @apply(i32)

A C caller apply(dummy, 7) passes the function pointer and 7. Generated Rust apply(Default::default(), 7) becomes apply(7) and the 7 lands in the pointer register.

This is not #727 / #2693: those abort or skip when the function itself has an unknown/unsupported CC. Here bindgen exits 0 and emits a callable apply with the wrong arity.

The same 0-size opaque happens for an aarch64_vector_pcs function-pointer field (struct S { int (*fp)(int,int) __attribute__((aarch64_vector_pcs)); int t; } — fp is [u32; 0] at offset 0 with t still at offset 8).

Expected Results

If vectorcall is not available, still emit a pointer-sized function-pointer type (or skip apply with a warning), not a ZST that rustc deletes from the signature.

Environment

bindgen: 0.72.1 and 0.73.1 (rust-lang/rust-bindgen 77cbc723)
clang/libclang: Homebrew clang 21.1.8
rustc: 1.97.1
target: aarch64-apple-darwin
OS: macOS

Start at get_abi, where CXCallingConv_AArch64VectorCall becomes Abi::Vectorcall, then trace how UnsupportedAbi is represented for function-pointer types and fields. Reproduce the provided header with bindgen and inspect the generated Rust and LLVM signature. Done means unsupported vectorcall pointers remain pointer-sized or the declaration is skipped with a warning instead of losing an argument.