C variable length arrays (VLAs) get imported incorrectly

[tothambrus11]

The following C code declares a prototype of a function taking a variable length array argument:

int vla(int w, int xs[][3][w][5]);

Currently, it is imported as:

unsafe extern "C" {
    pub fn vla(
        w: ::std::os::raw::c_int,
        xs: *mut *mut *mut [::std::os::raw::c_int; 5usize],
    ) -> ::std::os::raw::c_int;
}

So the parameter type is transformed into a pointer to a pointer to a pointer to a sized array. This is not correct, because the pointer we need to pass into the function should point to the first element of the whole multidimensional array instead of pointing to some other pointers.

I guess this is very to represent in Rust's type system. It's not possible to compose code like this:
*mut [VLA<[c_int; 5usize]>; 3usize]
because the outer sized array needs to know the size of its element at compile time.

Swift's approach is to just import VLAs as opaque pointers, and gives the responsibility of filling it with data in the right size and layout to the programmer.

Do you have any other ideas for asking for VLAs in Rust that's any better than an opaque pointer?