#![feature(c_variadic)]
unsafe extern "C" fn write_through(
    ptr_to_val: *mut i32,
    hidden_mut_ref_to_val: ...
) {
    // UB: this write access invalidates a
    // protected mutable reference within
    // the list of variable arguments.
    unsafe { *ptr_to_val = 32; }
}

fn main() {
    let mut val: i32 = 0;
    let mut_ref_to_val = &mut val;
    let ptr_to_val = mut_ref_to_val as *mut _;
    unsafe {
        write_through(ptr_to_val, mut_ref_to_val);
    }
}