Support creating an Async with AsFd rather than AsRawFd

Add a `new_with_asfd` constructor that works just like `new` but uses
`AsFd` rather than `AsRawFd`. This makes use of the newly factored out
`set_nonblocking` helper, with which the body of the new constructor is
just a few lines.

Author: joshtriplett

src/lib.rs

@@ -647,6 +647,42 @@ fn set_nonblocking(fd: BorrowedFd<'_>) -> io::Result<()> {
     Ok(())
 }
 
+#[cfg(unix)]
+impl<T: AsFd> Async<T> {
+    /// Creates an async I/O handle.
+    ///
+    /// This method will put the handle in non-blocking mode and register it in
+    /// [epoll]/[kqueue]/[event ports]/[IOCP].
+    ///
+    /// On Unix systems, the handle must implement `AsFd`, while on Windows it must implement
+    /// `AsSocket`.
+    ///
+    /// [epoll]: https://en.wikipedia.org/wiki/Epoll
+    /// [kqueue]: https://en.wikipedia.org/wiki/Kqueue
+    /// [event ports]: https://illumos.org/man/port_create
+    /// [IOCP]: https://learn.microsoft.com/en-us/windows/win32/fileio/i-o-completion-ports
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use async_io::Async;
+    /// use std::net::{SocketAddr, TcpListener};
+    ///
+    /// # futures_lite::future::block_on(async {
+    /// let listener = TcpListener::bind(SocketAddr::from(([127, 0, 0, 1], 0)))?;
+    /// let listener = Async::new(listener)?;
+    /// # std::io::Result::Ok(()) });
+    /// ```
+    pub fn new_with_asfd(io: T) -> io::Result<Async<T>> {
+        let fd = io.as_fd();
+        set_nonblocking(fd)?;
+        Ok(Async {
+            source: Reactor::get().insert_io(fd.as_raw_fd())?,
+            io: Some(io),
+        })
+    }
+}
+
 #[cfg(unix)]
 impl<T: AsRawFd> Async<T> {
     /// Creates an async I/O handle.
@@ -719,6 +755,42 @@ impl<T: Into<OwnedFd>> TryFrom<Async<T>> for OwnedFd {
     }
 }
 
+#[cfg(windows)]
+impl<T: AsSocket> Async<T> {
+    /// Creates an async I/O handle.
+    ///
+    /// This method will put the handle in non-blocking mode and register it in
+    /// [epoll]/[kqueue]/[event ports]/[IOCP].
+    ///
+    /// On Unix systems, the handle must implement `AsFd`, while on Windows it must implement
+    /// `AsSocket`.
+    ///
+    /// [epoll]: https://en.wikipedia.org/wiki/Epoll
+    /// [kqueue]: https://en.wikipedia.org/wiki/Kqueue
+    /// [event ports]: https://illumos.org/man/port_create
+    /// [IOCP]: https://learn.microsoft.com/en-us/windows/win32/fileio/i-o-completion-ports
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use async_io::Async;
+    /// use std::net::{SocketAddr, TcpListener};
+    ///
+    /// # futures_lite::future::block_on(async {
+    /// let listener = TcpListener::bind(SocketAddr::from(([127, 0, 0, 1], 0)))?;
+    /// let listener = Async::new(listener)?;
+    /// # std::io::Result::Ok(()) });
+    /// ```
+    pub fn new_with_asfd(io: T) -> io::Result<Async<T>> {
+        let borrowed = io.as_socket();
+        set_nonblocking(borrowed)?;
+        Ok(Async {
+            source: Reactor::get().insert_io(borrowed.as_raw_socket())?,
+            io: Some(io),
+        })
+    }
+}
+
 #[cfg(windows)]
 impl<T: AsRawSocket> Async<T> {
     /// Creates an async I/O handle.