Add Slice and Array support to LuaJIT FFI gen, clean up the type generation code.

[dgavedissian]

It will probably be easier to review this commit-by-commit.

[HaronK]

Overall looks good! Arrays/slices support will be a nice addition.

[dgavedissian]

@HaronK I just pushed another commit which adds support for [T; N], &[T; N] and &mut [T; N]. It works by creating a slice from the pointer passed from FFI by first converting it into a slice with a hardcoded size of N, then assumes the length is correct by using .try_into().unwrap(). That will obviously cause crashes if the length passed from Lua is wrong, but I think that's a sensible compromise to allow fixed size arrays.

If you would prefer, an alternative is to still pass the size to FFI, and simply let the FFI crash with a panic if the size doesn't match the expected one.

    pub fn move_primitive_array(&mut self, data: [f32; 3]) {
    }

    pub fn set_primitive_array(&mut self, data: &[f32; 3]) {
    }

    pub fn get_primitive_array(&self, out: &mut [f32; 3]) {
    }

    pub fn move_custom_array(&mut self, data: [Data; 3]) {
    }

    pub fn set_custom_array(&mut self, data: &[Data; 3]) {
    }

    pub fn get_custom_array(&self, out: &mut [Data; 3]) {
    }

            void TestStructArrays_MovePrimitiveArray (TestStructArrays*, float const* data);
            void TestStructArrays_SetPrimitiveArray  (TestStructArrays*, float const* data);
            void TestStructArrays_GetPrimitiveArray  (TestStructArrays const*, float* out);
            void TestStructArrays_MoveCustomArray    (TestStructArrays*, Data const* data);
            void TestStructArrays_SetCustomArray     (TestStructArrays*, Data const* data);
            void TestStructArrays_GetCustomArray     (TestStructArrays const*, Data* out);

[dgavedissian]

OK, I got it to a state where this happens:

thread 'test_primitive_array' panicked at engine/lib/luajit-ffi-gen/tests/test_impl_arrays.rs:21:1:
expected an array of 3 elements, got 2

When calling:

        TestStructArrays_MovePrimitiveArray(&mut ts, data_array.as_ptr(), 2);

This also does the right thing if the index is too large.

[HaronK]

• Also it would be better to do assert_eq check before calling from_raw_parts_mut to prevent potential undefined behavior.
• additional check for 0 size for slices (and Arrays?)
• check for null ptr

[dgavedissian]

Done all those things. Thanks for the feedback. I'll merge once CI passes.