Rust 1.39 now is in beta channel and async/await keywords are stabilized.
future-preview released version 0.3.0-alpha.19 for this beta version.
Tokio version tokio v0.2.0-alpha.5 cannot complie because it depends on futures-core-preview 0.3.0-alpha.18 which cannot be compiled .
Please release a new alpha or beta version to update dependencies.
I've managed to compile tokio on beta by just removing #![feature(async_await)] from every tokio (and its dependencies) file
This has been done 0.2.0-alpha.6 is now out!