Cargo Features

[dependencies]
nippy = { version = "2.0.0", default-features = false, features = ["async-std-runtime", "tokio-runtime"] }
default = async-std

The async-std feature is set by default whenever nippy is added without default-features = false somewhere in the dependency tree.

async-std-runtime = async-std
tokio-runtime = tokio

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.

tokio tokio-runtime?
async-std default async-std-runtime?