Function resolve_all_features 

pub fn resolve_all_features(
    resolve_with_overrides: &Resolve,
    resolved_features: &ResolvedFeatures,
    package_set: &PackageSet<'_>,
    package_id: PackageId,
    has_dev_units: HasDevUnits,
    requested_kinds: &[CompileKind],
    target_data: &RustcTargetData<'_>,
    force_all_targets: ForceAllTargets,
) -> HashSet<String>

Gets all of the features enabled for a package, plus its dependencies’ features.

Dependencies are added as dep_name/feat_name because required-features wants to support that syntax.