That makes it impossible to figure the entire hierarchy at compile-time, which I believe is one of the main requirement for this feature (to work with SwiftUI).
I addressed that in the last paragraph. My point is that I see function builders as a performance optimization to a general builder functionality: If you need the performance, you can trade in access to some Swift functionality and increased implementation complexity. But I think the default should be the simple version, with unrestricted access to Swift features.