Switch to ManuallyDropUnion

[NotGyro]

Out of the available methods of wrapping C unions in Bindgen, __BindgenUnionField generates code with questionable correctness. This was spotted by @workingjubilee in #1465. This pull request adds a .default_non_copy_union_style(NonCopyUnionStyle::ManuallyDrop) line to pgrx-pg-sys's build.rs. Since the semantics around these types are different, I have also changed some methods to work with the new type signature (particularly set_varsize_4b()).

[workingjubilee]

Hm, okay... so we cast the varlena pointer, then call as_mut(), then unwrap_unchecked it, then deref .va_4byte... since we're just derefing anyways, can't this just be

&mut (*varlena.cast::<pg_sys::varattrib_4b>()).va_4byte;

or something like that?

...also why is it a pointer? If that is important for soundness, this should actually be written with core::ptr::addr_of_mut! without &mut or as_mut. I think when I first reviewed this code, I didn't have my feet under me on the exact semantics of place expressions the way I do now: we want to create a place expression without expressing the strong semantics of &mut, and get a pointer to that place, which is what the macro I mentioned allows.

[workingjubilee]

This should be good after a little bit of cleanup.

Before:

#[repr(C)]
pub struct varattrib_4b {
    pub va_4byte: __BindgenUnionField<varattrib_4b__bindgen_ty_1>,
    pub va_compressed: __BindgenUnionField<varattrib_4b__bindgen_ty_2>,
    pub bindgen_union_field: [u32; 2usize],
}

After:

#[repr(C)]
pub union varattrib_4b {
    pub va_4byte: ::std::mem::ManuallyDrop<varattrib_4b__bindgen_ty_1>,
    pub va_compressed: ::std::mem::ManuallyDrop<varattrib_4b__bindgen_ty_2>,
}

[workingjubilee]

Great!