net: document that *Fd traits on TcpSocket are unix-only (#6294)

tokio/src/macros/cfg.rs

@@ -25,6 +25,18 @@ macro_rules! cfg_windows {
     }
 }
 
+/// Enables Unix-specific code.
+/// Use this macro instead of `cfg(unix)` to generate docs properly.
+macro_rules! cfg_unix {
+    ($($item:item)*) => {
+        $(
+            #[cfg(any(all(doc, docsrs), unix))]
+            #[cfg_attr(docsrs, doc(cfg(unix)))]
+            $item
+        )*
+    }
+}
+
 /// Enables unstable Windows-specific code.
 /// Use this macro instead of `cfg(windows)` to generate docs properly.
 macro_rules! cfg_unstable_windows {

tokio/src/net/tcp/socket.rs

@@ -777,21 +777,19 @@ impl fmt::Debug for TcpSocket {
     }
 }
 
-#[cfg(unix)]
+cfg_unix! {
     impl AsRawFd for TcpSocket {
         fn as_raw_fd(&self) -> RawFd {
             self.inner.as_raw_fd()
         }
     }
 
-#[cfg(unix)]
     impl AsFd for TcpSocket {
         fn as_fd(&self) -> BorrowedFd<'_> {
             unsafe { BorrowedFd::borrow_raw(self.as_raw_fd()) }
         }
     }
 
-#[cfg(unix)]
     impl FromRawFd for TcpSocket {
         /// Converts a `RawFd` to a `TcpSocket`.
         ///
@@ -805,12 +803,12 @@ impl FromRawFd for TcpSocket {
         }
     }
 
-#[cfg(unix)]
     impl IntoRawFd for TcpSocket {
         fn into_raw_fd(self) -> RawFd {
             self.inner.into_raw_fd()
         }
     }
+}
 
 cfg_windows! {
     impl IntoRawSocket for TcpSocket {