Cargo Features

Signal-hook-tokio has no features set by default.

[dependencies]
signal-hook-tokio = { version = "0.3.1", features = ["futures-v0_3"] }
futures-v0_3 = futures-core-0_3

Features from optional dependencies

In crates that don't use the dep: syntax, optional dependencies automatically become Cargo features.

futures-core-0_3 futures-v0_3?

Enables default (std) of futures-core