RFC: feature flags in kimchi

[mimoo]

Because I keep getting confused on how this is implemented, here's some doc that's relevant at the moment of this writing.

We use the following struct in kimchi:

pub struct FeatureFlags<F> {
    /// ChaCha gates
    pub chacha: bool,
    /// Range check gates
    pub range_check: bool,
    /// Foreign field addition gate
    pub foreign_field_add: bool,
    /// Foreign field multiplication gate
    pub foreign_field_mul: bool,
    /// XOR gate
    pub xor: bool,
    /// Lookup features
    pub lookup_features: LookupFeatures,
    /// Lookups
    pub lookup_configuration: Option<LookupConfiguration<F>>,
}

passing it to the setup is optional. If it is passed, kimchi will check each feature and:

• a feature that's set means that a constraint will be created (it's enabled)
• a feature that's not set means that a constraint will not be created (it's disabled)

fair enough? On the other hand, if we don't pass this FeatureFlags struct, it means that we want to decide things at runtime. And so constraints will be generated and wrapped by a special EnabledIf(FeatureFlag::TheFeature, _) (and an alpha element will be reserved for each of them)

Later at runtime, we can decide if we're enabling this or not.

I believe we don't use this feature to really do compile time vs runtime things, but rather in the generation of OCaml code and the toggling of things on the OCaml side. Perhaps though, this feature will make sense in Rust as well one day.