Coroutine benchmark fails to build

The benchmark at

https://github.com/atemerev/skynet/tree/master/rust-coroutine

fails to build as

error[E0308]: mismatched types
  --> /home/nordlow/.cargo/registry/src/github.com-1ecc6299db9ec823/fs2-0.2.2/src/unix.rs:63:67
   |
63 |     let ret = unsafe { libc::posix_fallocate(file.as_raw_fd(), 0, len as off_t) };
   |                                                                   ^^^^^^^^^^^^ expected i64, found u64

error: aborting due to previous error

error: Could not compile `fs2`.
warning: build failed, waiting for other jobs to finish...
error: build failed

The error occurs in another unix.rs module.

I'm using Rust version 1.21.

What to do?

It looks like incompatibility between fs2 and libc. Let fs2 author know about this.