copying memory in phobos

[Mike]

I ran accross the following code in phobos std.array:
void trustedMemcopy(T[] dest, T[] src) @trusted
{
    assert(src.length == dest.length);
    if (!__ctfe)
        memcpy(dest.ptr, src.ptr, src.length * T.sizeof);
    else
    {
        dest[] = src[];
    }
}

Why two different implementations for copying memory (i.e. memcpy 
vs. dest[] = src[])? Why not use dest[] = src[] exclusively.

Thanks for the help,
Mike