Cargo Features

tokio-sync has no features set by default.

tokio-sync = { version = "0.2.0-alpha.6", features = ["async-traits"] }
async-traits = futures-sink-preview

Features from optional dependencies

In crates that don't use the dep: syntax, optional dependencies automatically become Cargo features. These features may have been created by mistake, and this functionality may be removed in the future.

futures-sink-preview async-traits?