C99 Initialize an array through a pointer with braces

I wrote a function that calculates all for vertex points for a square given its position with and height. Since one cannot return array's in C I have to do it through a pointer. This is the code I ended up writing:

// Creates a rectangle for mapping a texture. Array must be 20 elements long.
void make_vertex_rect(float x, float y, float w, float h, float *vertex_positions) {
    /*  -1.0,+1.0             +1.0,+1.0
        +----------------------+
        |                      |
        |                      |
        |                      |
        +----------------------+
        -1.0,-1.0             +1.0,-1.0 */
    float new_positions[20] = {
        // We start at the top left and go in clockwise direction.
        //  x,     y,        z,    u,    v
            x,     y,     0.0f, 0.0f, 0.0f,
            x + w, y,     0.0f, 1.0f, 0.0f,
            x + w, y - h, 0.0f, 1.0f, 1.0f,
            x,     y - h, 0.0f, 0.0f, 1.0f
    };
    for (int i = 0; i < 20; ++i) { vertex_positions[i] = new_positions[i]; }
}

Now since C99 provides designated initializers I thought there might be a way to do this without writing a for loop but could not figure it out. Is there a way to do this directly, like:

// Creates a rectangle for mapping a texture. Array must be 20 elements long.
void make_vertex_rect(float x, float y, float w, float h, float *vertex_positions) {
    // Does not compile, but is there a way to get it to compile with a cast or something?
    *vertex_positions = { ... }; 
}

No, initializers can only be used to initialize objects as they are declared. You can't use them to overwrite an array that already exists.

Either write your for loop, or use memcpy, or just write out the assignments to the destination array elements.