net: add TcpSocket for configuring a socket (#2920)

This enables the caller to configure the socket and to explicitly bind
the socket before converting it to a `TcpStream` or `TcpListener`.

Closes: #2902

tokio/tests/tcp_socket.rs

@@ -0,0 +1,60 @@
+#![warn(rust_2018_idioms)]
+#![cfg(feature = "full")]
+
+use tokio::net::TcpSocket;
+use tokio_test::assert_ok;
+
+#[tokio::test]
+async fn basic_usage_v4() {
+    // Create server
+    let addr = assert_ok!("127.0.0.1:0".parse());
+    let srv = assert_ok!(TcpSocket::new_v4());
+    assert_ok!(srv.bind(addr));
+
+    let mut srv = assert_ok!(srv.listen(128));
+
+    // Create client & connect
+    let addr = srv.local_addr().unwrap();
+    let cli = assert_ok!(TcpSocket::new_v4());
+    let _cli = assert_ok!(cli.connect(addr).await);
+
+    // Accept
+    let _ = assert_ok!(srv.accept().await);
+}
+
+#[tokio::test]
+async fn basic_usage_v6() {
+    // Create server
+    let addr = assert_ok!("[::1]:0".parse());
+    let srv = assert_ok!(TcpSocket::new_v6());
+    assert_ok!(srv.bind(addr));
+
+    let mut srv = assert_ok!(srv.listen(128));
+
+    // Create client & connect
+    let addr = srv.local_addr().unwrap();
+    let cli = assert_ok!(TcpSocket::new_v6());
+    let _cli = assert_ok!(cli.connect(addr).await);
+
+    // Accept
+    let _ = assert_ok!(srv.accept().await);
+}
+
+#[tokio::test]
+async fn bind_before_connect() {
+    // Create server
+    let any_addr = assert_ok!("127.0.0.1:0".parse());
+    let srv = assert_ok!(TcpSocket::new_v4());
+    assert_ok!(srv.bind(any_addr));
+
+    let mut srv = assert_ok!(srv.listen(128));
+
+    // Create client & connect
+    let addr = srv.local_addr().unwrap();
+    let cli = assert_ok!(TcpSocket::new_v4());
+    assert_ok!(cli.bind(any_addr));
+    let _cli = assert_ok!(cli.connect(addr).await);
+
+    // Accept
+    let _ = assert_ok!(srv.accept().await);
+}