Implemented all std::os::fd traits for mqueue::MqdT.

CHANGELOG.md

@@ -57,6 +57,8 @@ This project adheres to [Semantic Versioning](https://semver.org/).
   | RawFd         | BorrowedFd or OwnedFd |
 
   (#[1906](https://github.com/nix-rust/nix/pull/1906))
+- Implemented AsFd, AsRawFd, FromRawFd, and IntoRawFd for `mqueue::MqdT`.
+   See ([#2097](https://github.com/nix-rust/nix/pull/2097))
 
 ### Fixed
 - Fix: send `ETH_P_ALL` in htons format 

src/mqueue.rs

@@ -37,6 +37,13 @@ use crate::sys::stat::Mode;
 use libc::{self, c_char, mqd_t, size_t};
 use std::ffi::CStr;
 use std::mem;
+#[cfg(all(
+    unix,
+    any(target_os = "linux", target_os = "netbsd", target_os = "dragonfly")
+))]
+use std::os::unix::io::{
+    AsFd, AsRawFd, BorrowedFd, FromRawFd, IntoRawFd, RawFd,
+};
 
 libc_bitflags! {
     /// Used with [`mq_open`].
@@ -300,3 +307,55 @@ pub fn mq_remove_nonblock(mqd: &MqdT) -> Result<MqAttr> {
     );
     mq_setattr(mqd, &newattr)
 }
+
+#[cfg(all(
+    unix,
+    any(target_os = "linux", target_os = "netbsd", target_os = "dragonfly")
+))]
+impl AsFd for MqdT {
+    /// Borrow the underlying message queue descriptor.
+    fn as_fd(&self) -> BorrowedFd {
+        // SAFETY: [MqdT] will only contain a valid fd by construction.
+        unsafe { BorrowedFd::borrow_raw(self.0) }
+    }
+}
+
+#[cfg(all(
+    unix,
+    any(target_os = "linux", target_os = "netbsd", target_os = "dragonfly")
+))]
+impl AsRawFd for MqdT {
+    /// Return the underlying message queue descriptor.
+    ///
+    /// Returned descriptor is a "shallow copy" of the descriptor, so it refers
+    ///  to the same underlying kernel object as `self`.
+    fn as_raw_fd(&self) -> RawFd {
+        self.0
+    }
+}
+
+#[cfg(all(
+    unix,
+    any(target_os = "linux", target_os = "netbsd", target_os = "dragonfly")
+))]
+impl FromRawFd for MqdT {
+    /// Construct an [MqdT] from [RawFd].
+    ///
+    /// # Safety
+    /// The `fd` given must be a valid and open file descriptor for a message
+    ///  queue.
+    unsafe fn from_raw_fd(fd: RawFd) -> MqdT {
+        MqdT(fd)
+    }
+}
+
+#[cfg(all(
+    unix,
+    any(target_os = "linux", target_os = "netbsd", target_os = "dragonfly")
+))]
+impl IntoRawFd for MqdT {
+    /// Consume this [MqdT] and return a [RawFd].
+    fn into_raw_fd(self) -> RawFd {
+        self.0
+    }
+}