pub fn enabled_on(&self, platform_spec: &PlatformSpec) -> EnabledTernary {
        let required = self.required.enabled_on(platform_spec);
        let optional = self.optional.enabled_on(platform_spec);

        required | optional
    }
Evaluates whether this dependency is enabled (required or optional) on the given platform
spec.

Returns `Unknown` if the result was unknown, which may happen if evaluating against an
individual platform and its target features are unknown.
guppy/src/graph/graph_impl.rs