Replace cfg macro with attribute.

src/libstd/sys/windows/c.rs

@@ -216,7 +216,6 @@ pub const SOCK_STREAM: c_int = 1;
 pub const SOL_SOCKET: c_int = 0xffff;
 pub const SO_RCVTIMEO: c_int = 0x1006;
 pub const SO_SNDTIMEO: c_int = 0x1005;
-pub const SO_REUSEADDR: c_int = 0x0004;
 pub const IPPROTO_IP: c_int = 0;
 pub const IPPROTO_TCP: c_int = 6;
 pub const IPPROTO_IPV6: c_int = 41;

src/libstd/sys_common/net.rs

@@ -368,12 +368,15 @@ impl TcpListener {
 
         let sock = Socket::new(addr, c::SOCK_STREAM)?;
 
-        // On platforms with Berkeley-derived sockets, this allows
-        // to quickly rebind a socket, without needing to wait for
-        // the OS to clean up the previous one.
-        if !cfg!(windows) {
-            setsockopt(&sock, c::SOL_SOCKET, c::SO_REUSEADDR, 1 as c_int)?;
-        }
+        // On platforms with Berkeley-derived sockets, this allows to quickly
+        // rebind a socket, without needing to wait for the OS to clean up the
+        // previous one.
+        //
+        // On Windows, this allows rebinding sockets which are actively in use,
+        // which allows “socket hijacking”, so we explicitly don't set it here.
+        // https://docs.microsoft.com/en-us/windows/win32/winsock/using-so-reuseaddr-and-so-exclusiveaddruse
+        #[cfg(not(windows))]
+        setsockopt(&sock, c::SOL_SOCKET, c::SO_REUSEADDR, 1 as c_int)?;
 
         // Bind our new socket
         let (addrp, len) = addr.into_inner();