Cargo Features

tls-api = { version = "0.9.0", default-features = false, features = ["runtime-async-std", "runtime-tokio"] }
default = runtime-tokio

The runtime-tokio feature is set by default whenever tls-api is added without default-features = false somewhere in the dependency tree.

runtime-async-std = futures-util
runtime-tokio default = 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.

futures-util runtime-async-std?

Enables futures-util

Note technically there's no dependency on async-std

tokio runtime-tokio