functions that get pointers to variables don't modify the variables

[abirvalarg]

unsafe {
  let mut vao = 0;
  gl::GenVertexArrays(1, &mut vao);
  assert!(vao != 0);
}

this code panics. But if i change the initial value of vao to anything else(for example 1), it doesn't panic. Similar thing with other functions that get pointers as arguments, they don't seem to modify anything so i can't create any objects or get error logs.