Undefined behavior in implement_buffer_content!

[avl]

The macro implement_buffer_content! contains the following code:

#[inline]
fn is_size_suitable(size: usize) -> bool {
    use std::mem;

    let fake_ptr: &$struct_name = unsafe { mem::transmute((0usize, 0usize)) };
    let min_size = mem::size_of_val(fake_ptr);

    let fake_ptr: &$struct_name = unsafe { mem::transmute((0usize, 1usize)) };
    let step = mem::size_of_val(fake_ptr) - min_size;

    size > min_size && (size - min_size) % step == 0
}

This is, as I understand, undefined behavior. It sort of works by accident in some/many cases. But with recent (>2021-03-04) rustc it does not work in the app I'm developing.

I've tried to solicit help from reddit: https://www.reddit.com/r/rust/comments/om6xs6/help_needed_fixing_ub_in_glium_graphics_library/

I've also made a github repo with a minimal(ish) test case: https://github.com/avl/rust-llvm12-regression

I'm trying to come up with a PR to fix this. I'm very interested in if others have the same problem.

The problem is of course the cast from (0usize,0usize) to a reference to a DST.

[avl]

Here is a PR: #1950
With this PR the code works, but the implement_buffer_content-macro is still UB. Just not as much UB :-) .

[est31]

Alternative PR: #1951