An additive feature is a Cargo feature that adds functionality without disabling anything already available. Cargo depends on this behavior when resolving crate dependencies. section 10.3
Feature-gated macro functionality
A macro crate can define independent features such as `struct` and `functional`, with `functional` enabled by default. The `struct` feature adds a configuration-struct macro and its implementation; it does not remove existing functionality.
Apply the rule when designing crate features:
- Do not use a feature to exclude an implementation that the default configuration already provides. section 10.3
