localhost does not resolve

I tried to open an async connection to a server running locally at ws://localhost:4000 and received the following error:

IoError(Error { repr: Os { code: 61, message: "Connection refused" } })

However, the following work:

• opening an async websocket connection to ws://127.0.0.1:4000. So my code connects once DNS is resolved.
• opening an async websocket connection to ws://echo.websocket.org. So websocket DNS resolution does work in some cases.
• opening a synchronous websocket connection to ws://localhost:4000. So local DNS resolution works but only for synchronous connections.
• opening an async HTTP connection to http://localhost:4000 with Hyper 0.11. Does Hyper 0.11 use a different Tokio primitive to resolve DNS?

I'm not exactly sure what's going on. It's not a big deal - I'm manually resolving localhost to 127.0.0.1 - but I'm kind of curious.

Start by reproducing the async websocket connection to ws://localhost:4000 and compare it with ws://127.0.0.1:4000, the synchronous websocket path, and the Hyper 0.11 HTTP path. Trace the async DNS resolution entry point and determine why localhost differs; done means async websocket connections resolve localhost successfully without manual substitution.