Bad error message? "captured variable cannot escape `FnMut` closure body"

I was trying to build a simple server using Hyper when I wrote some code that had an error. I thought the Rust compiler's error felt less helpful than usual so I thought I'd ask about it here.

Here's the code: https://gist.github.com/polybuildr/e9966d4e06a92e6ab6db9c4ccdaa2575

The error is in the gist too, but it looks like this:

error: captured variable cannot escape `FnMut` closure body
  --> src/main.rs:18:13
   |
18 | /             match (req.method(), req.uri().path()) {
19 | |                 (&Method::POST, "/foo") => {
20 | |                     // let data_map = Arc::clone(&data_map);
21 | |                     let response_future =
...  |
36 | |                 )),
37 | |             }
   | |_____________^ returns a reference to a captured variable which escapes the closure body
   |
   = note: `FnMut` closures only have access to their captured variables while they are executing...
   = note: ...therefore, they cannot allow references to captured variables to escape

error: aborting due to previous error

error: could not compile `test`.

To learn more, run the command again with --verbose.

There's no helpful text pointing out which captured variable is accidentally escaping, there's not even the usual E00XX error that I can use in rustc --explain. Is there a reason the error message doesn't have this information?

Also, in this case I finally fixed it by doing what's in the comments in the code: cloning the Arc and then using move for the closure. Was that the right fix or did I misunderstand something?

That's the right fix. That error message does look bad

Thanks for confirming, @RustyYato!

I don't think I am familiar enough with closures and futures to do this, but is there a neat minimal example of this code I can use to file an issue in the rust repo?

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