Add support for Windows Registered I/O

Author: DBLouis

Cargo.toml

@@ -65,6 +65,8 @@ features = [
 [features]
 # Enable all API, even ones not available on all OSs.
 all = []
+# Enable Windows Registered I/O support.
+registered-io = []
 
 [package.metadata.cargo_check_external_types]
 allowed_external_types = [

src/sys/windows.rs

@@ -25,7 +25,7 @@ use windows_sys::Win32::Networking::WinSock::{
     self, tcp_keepalive, FIONBIO, IN6_ADDR, IN6_ADDR_0, INVALID_SOCKET, IN_ADDR, IN_ADDR_0,
     POLLERR, POLLHUP, POLLRDNORM, POLLWRNORM, SD_BOTH, SD_RECEIVE, SD_SEND, SIO_KEEPALIVE_VALS,
     SOCKET_ERROR, WSABUF, WSAEMSGSIZE, WSAESHUTDOWN, WSAPOLLFD, WSAPROTOCOL_INFOW,
-    WSA_FLAG_NO_HANDLE_INHERIT, WSA_FLAG_OVERLAPPED,
+    WSA_FLAG_NO_HANDLE_INHERIT, WSA_FLAG_OVERLAPPED, WSA_FLAG_REGISTERED_IO,
 };
 #[cfg(feature = "all")]
 use windows_sys::Win32::Networking::WinSock::{
@@ -136,6 +136,19 @@ impl Type {
     pub(crate) const fn _no_inherit(self) -> Type {
         Type(self.0 | Type::NO_INHERIT)
     }
+
+    /// Our custom flag to set `WSA_FLAG_REGISTERED_IO` on socket creation.
+    const REGISTERED_IO: c_int = 1 << ((size_of::<c_int>() * 8) - 2); // Second last bit.
+
+    /// Set `WSA_FLAG_REGISTERED_IO` on the socket.
+    #[cfg(feature = "registered-io")]
+    pub const fn registered_io(self) -> Type {
+        self._registered_io()
+    }
+
+    pub(crate) const fn _registered_io(self) -> Type {
+        Type(self.0 | Type::REGISTERED_IO)
+    }
 }
 
 impl_debug!(
@@ -253,13 +266,19 @@ pub(crate) fn socket_into_raw(socket: Socket) -> RawSocket {
 pub(crate) fn socket(family: c_int, mut ty: c_int, protocol: c_int) -> io::Result<RawSocket> {
     init();
 
-    // Check if we set our custom flag.
+    // Check if we set our custom flags.
     let flags = if ty & Type::NO_INHERIT != 0 {
         ty = ty & !Type::NO_INHERIT;
         WSA_FLAG_NO_HANDLE_INHERIT
     } else {
         0
     };
+    let flags = if ty & Type::REGISTERED_IO != 0 {
+        ty = ty & !Type::REGISTERED_IO;
+        flags | WSA_FLAG_REGISTERED_IO
+    } else {
+        flags
+    };
 
     syscall!(
         WSASocketW(