Wrong code generated for union with bitfields

The below outputs are with bindgen 0.71.1 (no flags) on x86_64-pc-windows-msvc.

Consider the union below. There are two issues:

• The accessors for bitfld_b and bitfld_c use bit offsets of 4 and 8, respectively, while they should be 0.
• The generated type has the wrong size, resulting in a compile error. The C type has size 4 and alignment 1, but the generated type has size 2. (On x86_64-unknown-linux-gnu it happens to compile(!) because the alignment is 4 which makes the size also 4.) Therefore, padding needs to be added.

Don't ask me why someone put a bitfield in a union. (The case I found was an anonymous union in a struct, but that doesn't seem to make a difference.)

Header file

union Union {
    int bitfld_a : 4;
    int bitfld_b : 4;
    int bitfld_c : 4;
};

Generated code (excerpt)

#[repr(C)]
#[derive(Copy, Clone)]
pub union Union {
    pub _bitfield_align_1: [u8; 0],
    pub _bitfield_1: __BindgenBitfieldUnit<[u8; 2usize]>,
    //                                          ^^^^^^ should be 4usize
}
#[allow(clippy::unnecessary_operation, clippy::identity_op)]
const _: () = {
    ["Size of Union"][::std::mem::size_of::<Union>() - 4usize];
    ["Alignment of Union"][::std::mem::align_of::<Union>() - 1usize];
};
impl Union {
    // ...
    pub fn bitfld_b(&self) -> ::std::os::raw::c_int {
        unsafe { ::std::mem::transmute(self._bitfield_1.get(4usize, 4u8) as u32) }
        //                                                  ^^^^^^ should be 0usize
    }
    // ...
}

Compile error

error[E0080]: evaluation of constant value failed
   --> bindings.rs:148:23
    |
148 |     ["Size of Union"][::std::mem::size_of::<Union>() - 4usize];
    |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `2_usize - 4_usize`, which would overflow

Full output

Generated code
Debug log

Start by reproducing the issue from the C union in the report with bindgen 0.71.1, then inspect the generated bindings.rs output and the bitfield layout/accessor generation path. Add regression coverage for the union case and verify that the generated accessors use the correct offsets and that the type has size 4 and alignment 1.