How should libraries expose unstable features for those using nightly?

[gnzlbg]

Should they use a feature flag, e.g., unstable ? Should they use a --cfg flag, e.g., --cfg unstable ? Should the flag be unique for each library, e.g., --cfg rayon-unstable --cfg tokio-unstable ?

[L-as]

Isn't it common to use a nightly feature?