Rollup merge of rust-lang#58551 - ssomers:master, r=oli-obk

Explain a panic in test case net::tcp::tests::double_bind

Those who try to build libstd on the Windows Subsystem for Linux experience a single failing test, where the point of failure is an explicit but anonymous panic, as reported in rust-lang#49367
This commit somewhat explains why and allows diagnosing a little.

Author: kennytm

src/libstd/net/tcp.rs

@@ -1187,9 +1187,13 @@ mod tests {
     #[test]
     fn double_bind() {
         each_ip(&mut |addr| {
-            let _listener = t!(TcpListener::bind(&addr));
+            let listener1 = t!(TcpListener::bind(&addr));
             match TcpListener::bind(&addr) {
-                Ok(..) => panic!(),
+                Ok(listener2) => panic!(
+                    "This system (perhaps due to options set by TcpListener::bind) \
+                     permits double binding: {:?} and {:?}",
+                    listener1, listener2
+                ),
                 Err(e) => {
                     assert!(e.kind() == ErrorKind::ConnectionRefused ||
                             e.kind() == ErrorKind::Other ||