nit: suppress async_await nightly warning.
//#![deny(warnings, missing_docs, trivial_casts, unused_qualifications)]
#![forbid(unsafe_code)]
+// async_await is considered stable on nightly releases, but the corresponding
+// release (1.39.0) is still in beta. Suppress the warning for now.
+#![allow(stable_features)]
#![feature(async_await)]
#[macro_use]
