Macro feature 

macro_rules! feature {
    ($name:literal, $default:expr $(,)?) => { ... };
    ($name:literal $(,)?) => { ... };
}

Define a feature flag at compile-time.

The macro takes two arguments: the name of the feature, and an optional default value. The default value argument is evaluated each time the feature is using its default value.

If the feature-registry feature is enabled, the feature will be registered globally and can be accessed using the known_features function.