Test support module doesn't compile on recent nightlies

[hawkw]

See this CI build against rustc 1.27.0-nightly (eeea94c11 2018-04-06).

Build log

error[E0277]: the trait bound `(): std::convert::From<!>` is not satisfied
  --> tests/support/mod.rs:25:34
   |
25 |             Err(e) => return Err(From::from(e)),
   |                                  ^^^^^^^^^^ the trait `std::convert::From<!>` is not implemented for `()`
   | 
  ::: tests/support/util.rs:34:17
   |
34 |         let _ = try_ready!(self.stream().poll_capacity().map_err(|_| panic!()));
   |                 --------------------------------------------------------------- in this macro invocation
   |
   = help: the following implementations were found:
             <() as std::convert::From<support::mock_io::tokio::tokio_timer::TimerError>>
             <() as std::convert::From<support::mock_io::tokio::tokio_timer::TimeoutError<T>>>
   = note: required by `std::convert::From::from`
error: aborting due to previous error
For more information about this error, try `rustc --explain E0277`.
error: Could not compile `h2`.
warning: build failed, waiting for other jobs to finish...
error[E0277]: the trait bound `(): std::convert::From<!>` is not satisfied
  --> tests/support/mod.rs:25:34
   |
25 |             Err(e) => return Err(From::from(e)),
   |                                  ^^^^^^^^^^ the trait `std::convert::From<!>` is not implemented for `()`
   | 
  ::: tests/support/util.rs:34:17
   |
34 |         let _ = try_ready!(self.stream().poll_capacity().map_err(|_| panic!()));
   |                 --------------------------------------------------------------- in this macro invocation
   |
   = help: the following implementations were found:
             <() as std::convert::From<support::mock_io::tokio::tokio_timer::TimerError>>
             <() as std::convert::From<support::mock_io::tokio::tokio_timer::TimeoutError<T>>>
   = note: required by `std::convert::From::from`
error: aborting due to previous error
For more information about this error, try `rustc --explain E0277`.
error: Could not compile `h2`.
To learn more, run the command again with --verbose.
The command "cargo build --tests --features unstable" exited with 101.

[carllerche]

I would probably recommend filing a bug on the rust repo.

[hawkw]

@carllerche looks like there's already an issue (rust-lang/rust#49932) tracking this.

[carllerche]

This has been fixed in Rust.