ENTITY* x_arrow; // x direction arrow attached to entity
VECTOR to;

while(1)
{
var distance = c_trace(player.x, x_arrow.x, IGNORE_ME);
vec_set(to, mouse_dir3d);
vec_scale(to, distance);
vec_add(to, mouse_pos3d);

x_arrow.x = to.x;
wait(1);
}