Packed structures with 0-sized arrays are not deriving clone/copy

[gpace1]

I think bindgen does this for any packed struct that doesn't contain a 0-sized array.

Input C/C++ Header

// test.h
#ifdef __cplusplus
extern "C" {
#endif

typedef struct {

        unsigned char data[0];

       // rustc will not complain about the generating bindings if the struct is "0-sized"
        unsigned short more_data;

}__attribute__((packed)) test_struct;

#ifdef __cplusplus
}
#endif

Bindgen Invocation

let bindings = bindgen::builder()
    .header("test.h")
    .generate()
    .unwrap();

Actual Results

Generated bindings:

 1  /* automatically generated by rust-bindgen */
 2
 3  #[repr(C)]
 4  #[derive(Default)]
 5  pub struct __IncompleteArrayField<T>(::std::marker::PhantomData<T>);
 6  impl<T> __IncompleteArrayField<T> {
 7      #[inline]
 8      pub fn new() -> Self {
 9          __IncompleteArrayField(::std::marker::PhantomData)
10      }
11      #[inline]
12      pub unsafe fn as_ptr(&self) -> *const T {
13          ::std::mem::transmute(self)
14      }
15      #[inline]
16      pub unsafe fn as_mut_ptr(&mut self) -> *mut T {
17          ::std::mem::transmute(self)
18      }
19      #[inline]
20      pub unsafe fn as_slice(&self, len: usize) -> &[T] {
21          ::std::slice::from_raw_parts(self.as_ptr(), len)
22      }
23      #[inline]
24      pub unsafe fn as_mut_slice(&mut self, len: usize) -> &mut [T] {
25          ::std::slice::from_raw_parts_mut(self.as_mut_ptr(), len)
26      }
27  }
28  impl<T> ::std::fmt::Debug for __IncompleteArrayField<T> {
29      fn fmt(&self, fmt: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
30          fmt.write_str("__IncompleteArrayField")
31      }
32  }
33  impl<T> ::std::clone::Clone for __IncompleteArrayField<T> {
34      #[inline]
35      fn clone(&self) -> Self {
36          Self::new()
37      }
38  }
39  impl<T> ::std::marker::Copy for __IncompleteArrayField<T> {}
40  #[repr(C, packed)]
41  #[derive(Debug)]
42  pub struct test_struct {
43      pub data: __IncompleteArrayField<::std::os::raw::c_uchar>,
44      pub more_data: ::std::os::raw::c_ushort,
45  }
46 // removed test

Rust warning about this

  --> src/bindings.rs:41:10
   |
41 | #[derive(Debug)]
   |          ^^^^^
   |
   = note: #[warn(safe_packed_borrows)] on by default
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #46043 <https://github.com/rust-lang/rust/issues/46043>

Expected Results

test_struct should derive both clone & copy to get rid of the safe_packed_borrows rustc warning

[emilio]

I think that the reason for not deriving copy / clone for incomplete arrays is because they are usually used to allocate past yourself when the array is the last member.

[gpace1]

This is still a problem isn't it? I'm now not sure how to go about getting around the eventual error without explicitly defining the structure yourself (which may be the only solution to this).

[emilio]

The way to go here would be to teach #[derive(Debug)] about these I think. I asked in that rustc issue.