impl ToSocketAddrs for String

`ToSocketAddrs` is implemented for a number of different types,
including `(IpAddr, u16)`, `&str`, and various others, for the
convenience of being able to run things like
`TcpListener::bind("10.11.12.13:1415")`.  However, because this is a
generic parameter with a trait bound, if you have a `String` you cannot
pass it in, either directly as `TcpListener::bind(string)`, or the
`TcpListener::bind(&string)` as you might expect due to deref coercion;
you have to use `TcpListener::bind(&*string)`, which is noisy and hard
to discover (though #39029 suggests better error messages to make it
more discoverable).

Rather than making people stumble over this, just implement
`ToSocketAddrs` for `String`.

src/libstd/net/addr.rs

@@ -760,6 +760,14 @@ impl<'a, T: ToSocketAddrs + ?Sized> ToSocketAddrs for &'a T {
     }
 }
 
+#[stable(feature = "string_to_socket_addrs", since = "1.16.0")]
+impl ToSocketAddrs for String {
+    type Iter = vec::IntoIter<SocketAddr>;
+    fn to_socket_addrs(&self) -> io::Result<vec::IntoIter<SocketAddr>> {
+        (&**self).to_socket_addrs()
+    }
+}
+
 #[cfg(all(test, not(target_os = "emscripten")))]
 mod tests {
     use net::*;
@@ -797,6 +805,18 @@ mod tests {
         assert!(tsa("localhost:23924").unwrap().contains(&a));
     }
 
+    #[test]
+    fn to_socket_addr_string() {
+        let a = sa4(Ipv4Addr::new(77, 88, 21, 11), 24352);
+        assert_eq!(Ok(vec![a]), tsa(&*format!("{}:{}", "77.88.21.11", "24352")));
+        assert_eq!(Ok(vec![a]), tsa(&format!("{}:{}", "77.88.21.11", "24352")));
+        assert_eq!(Ok(vec![a]), tsa(format!("{}:{}", "77.88.21.11", "24352")));
+
+        let s = format!("{}:{}", "77.88.21.11", "24352");
+        assert_eq!(Ok(vec![a]), tsa(s));
+        // s has been moved into the tsa call
+    }
+
     // FIXME: figure out why this fails on openbsd and bitrig and fix it
     #[test]
     #[cfg(not(any(windows, target_os = "openbsd", target_os = "bitrig")))]