Rust-bindgen: bindgen generates union with u128 member for alignment which is UB for FFI

[benpye]

Input C/C++ Header

typedef union __declspec(align(16)) TestUnion
{
    unsigned long Longs[2];
    unsigned int  Ints[4];
} TestUnion;

Bindgen Invocation

bindgen .\test.h -o test.rs

Actual Results

#[repr(C)]
#[repr(align(16))]
#[derive(Copy, Clone)]
pub union TestUnion {
    pub Longs: [::std::os::raw::c_ulong; 2usize],
    pub Ints: [::std::os::raw::c_uint; 4usize],
    _bindgen_union_align: u128,
}

Expected Results

I believe the u128 member of the union is unnessecary and actually triggers a warning (improper_ctypes) as u128s are not stable across the FFI boundary. My understanding here is that #[repr(align(16))] is sufficient to ensure the correct alignment for this union.

[emilio]

You're right. This should be trivial to fix. Relevant code is:

https://github.com/rust-lang/rust-bindgen/blob/b7e5d8d4d511066d0363ef72dbee2c66615b8dd4/src/codegen/mod.rs#L1601

[agodnic]

@emilio Would this be the desired behavior?

• If the Rust version is => 1.25: Use #[repr(align(n))]. Do not add fields to the struct.
• If the Rust version is < 1.25: Align up the struct size by adding fields to it, as long as these are stable across FFI boundaries (therefore, do not use u128). Do not use #[repr(align(n))]

[emilio]

Yes, that's right, though it's even simpler, given support for repr(align) predates support for u128 / i128. So we should just not generate that alignment field if repr(align) is supported, and add a test.