Fine-grained variants of open for spec functions

[parno]

@jaylorch asks:

Is there a way to make a spec function f be open within the current module (or within the current module's parent) but closed for external users of that function? ... what I'd really like is to be able to say pub open(super) spec f(...) meaning "It's open for descendants of this module's parent, but closed for any other modules".

Daniel Schoepe asks:

Is there something like open(crate) that exposes the definition in the current crate but not externally?