Cargo Features

futures-executor-preview = { version = "0.3.0-alpha.19", default-features = false, features = ["std"] }
default = std

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

std default = num_cpus

Enables std of futures-core-preview and futures-util-preview

Features from optional dependencies

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

num_cpus std