There are a few things to know about developing Rayon:

• You can only run tests (and use the demo) using the current stable compiler (or newer beta or nightly).
• To run benchmarks, go to rayon-demo and do cargo bench using the nightly* compiler.
• In general, benchmarks go in rayon-demo.

* Nightly version of rust can be installed with rustup default nightly (use rustup default stable to install stable release)