warning: `extern` block uses type `Box<[u8; 42]>`, which is not FFI-safe
10 | fn uses_boxed_array(arr: *mut BoxedArray);
| ^^^^^^^^^^^^^^^ not FFI-safe
= note: `#[warn(improper_ctypes)]` on by default
= help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
= note: this struct has unspecified layout
I'm wondering if this is simply a false positive of improper_ctypes warning or if the code is indeed incorrect.
Note: I could store a *mut f64 in the WrappedBox by allocating a Box<[f64;42]> and calling into_raw and then use from_raw in the Drop implementation, but this seems less elegant to me than the above solution of just using Box<[f64;42]>.
on the one hand, it's kind of code-smelly to have a type with drop-glue be involved in FFI boundaries;
on the other hand, your use case seems legitimate (assuming you are the one allocating the stuff, and not the FFI!! In that case, btw, the FFI API looks a bit weird, but maybe you've minimized the code a little bit too much).
So I wouldn't blame the lint, but I'd glady #[allow(…)]-silence it in your case
Another approach would be to say that uses_boxed_array takes a &mut &mut [f64; 42], and then, when calling it, just do a &mut &mut *my_boxed_array.arr.
on the C side and the function is more of a int compute(RetValue& ret, /* other params */) so my function needs a pointer to the struct (which contains the allocated buffer) and not to the buffer directly.
So I'll just #[allow(..)] the lint. Thanks for your help!