Implement AsFd and bidirectional conversion to/from OwnedFd

In particular, this makes it possible to get a `BorrowedFd` from an
`Inotify`, which allows integration with polling mechanisms that don't
use `RawFd`. This will be needed to work with the next version of
`async-io`.

.github/workflows/rust.yml

@@ -12,7 +12,7 @@ jobs:
     - name: Install rust
       uses: actions-rs/toolchain@v1
       with:
-        toolchain: 1.49.0
+        toolchain: 1.63.0
         override: true
         components: clippy
     - name: Check rust and cargo version

Cargo.toml

@@ -9,6 +9,7 @@ authors = [
   "Frank Denis <github@pureftpd.org>"
 ]
 edition = "2018"
+rust-version = "1.63"
 
 description   = "Idiomatic wrapper for inotify"
 documentation = "https://docs.rs/inotify"

src/fd_guard.rs

@@ -1,7 +1,9 @@
 use std::{
     ops::Deref,
     os::unix::io::{
+        AsFd,
         AsRawFd,
+        BorrowedFd,
         FromRawFd,
         IntoRawFd,
         RawFd,
@@ -77,6 +79,13 @@ impl AsRawFd for FdGuard {
     }
 }
 
+impl AsFd for FdGuard {
+    #[inline]
+    fn as_fd(&self) -> BorrowedFd<'_> {
+        unsafe { BorrowedFd::borrow_raw(self.fd) }
+    }
+}
+
 impl PartialEq for FdGuard {
     fn eq(&self, other: &FdGuard) -> bool {
         self.fd == other.fd

src/inotify.rs

@@ -1,9 +1,12 @@
 use std::{
     io,
     os::unix::io::{
+        AsFd,
         AsRawFd,
+        BorrowedFd,
         FromRawFd,
         IntoRawFd,
+        OwnedFd,
         RawFd,
     },
     path::Path,
@@ -370,3 +373,22 @@ impl IntoRawFd for Inotify {
         self.fd.fd
     }
 }
+
+impl AsFd for Inotify {
+    #[inline]
+    fn as_fd(&self) -> BorrowedFd<'_> {
+        self.fd.as_fd()
+    }
+}
+
+impl From<Inotify> for OwnedFd {
+    fn from(fd: Inotify) -> OwnedFd {
+        unsafe { OwnedFd::from_raw_fd(fd.into_raw_fd()) }
+    }
+}
+
+impl From<OwnedFd> for Inotify {
+    fn from(fd: OwnedFd) -> Inotify {
+        unsafe { Inotify::from_raw_fd(fd.into_raw_fd()) }
+    }
+}