title: how to use nix::mount::mount() replace with libc::mount() in Rust
body: <p>I want to use the 'nix' library to replace with 'libc' in Rust FFI .</p>
  <p>I want to use nix::mount::mount() to replace with libc::mount(). Now I have the followed code :</p>
  <pre class="lang-rust prettyprint-override"><code>libc::mount(ptr::null(), path.as_ptr(), ptr::null(), libc:: MS_SLAVE, ptr::null())
  </code></pre>
  <p>I just want to know how to replace the ptr::null() in nix library, I try to use 'None' to do it, but failed. Please help me, thanks.</p>
  <p>Show the error:</p>
  <pre><code>39 |     nix::mount::mount(
     |     ^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `P1` declared on the function `mount`
     | 
    ::: /root/.cargo/registry/src/mirrors.ustc.edu.cn-15f9db60536bad60/nix-0.19.0/src/mount.rs:57:27
     |
  57 | pub fn mount&lt;P1: ?Sized + NixPath, P2: ?Sized + NixPath, P3: ?Sized + NixPath, P4: ?Sized + NixPath&gt;(
     |                           ------- required by this bound in `nix::mount::mount`
     |
     = note: cannot satisfy `_: nix::NixPath`
  </code></pre>