Conditionally compile peer credentials feature for supported platforms

library/std/src/sys/unix/ext/net.rs

@@ -443,6 +443,15 @@ pub fn peer_addr(&self) -> io::Result<SocketAddr> {
     /// }
     /// ```
     #[unstable(feature = "peer_credentials_unix_socket", issue = "42839", reason = "unstable")]
+    #[cfg(any(
+        target_os = "android",
+        target_os = "linux",
+        target_os = "dragonfly",
+        target_os = "freebsd",
+        target_os = "ios",
+        target_os = "macos",
+        target_os = "openbsd"
+    ))]
     pub fn peer_cred(&self) -> io::Result<UCred> {
         ucred::peer_cred(self)
     }

library/std/src/sys/unix/ext/ucred/tests.rs

@@ -2,6 +2,15 @@
 use libc::{getegid, geteuid};
 
 #[test]
+#[cfg(any(
+    target_os = "android",
+    target_os = "linux",
+    target_os = "dragonfly",
+    target_os = "freebsd",
+    target_os = "ios",
+    target_os = "macos",
+    target_os = "openbsd"
+))]
 fn test_socket_pair() {
     // Create two connected sockets and get their peer credentials. They should be equal.
     let (sock_a, sock_b) = UnixStream::pair().unwrap();