pico: speculative: multiple levels of guards

[rbalicki2]

It is nice that MemoRef has a 'db lifetime, meaning that you can't call .garbage_collect / db.set/db.remove when there is a MemoRef, all of which could change its value or remove it.

I wonder if we can have something similar for SourceRef<'db>, where:

• If a SourceRef exists, no deletions are allowed, but you can create/update sources

So there's a heirarchy:

• MemoRef exists -> no changes to DB state
• SourceRef exists -> no deletions of sources
• Nothing exists -> anything allowed

Furthermore, I wonder if we can do this via ownership rules.

Right now, we can enforce that if a MemoRef exists, you can't mutate the DB because of ownership rules. Can we have two structs (e.g. a SourceRef only takes a reference to a sub-struct?) to pull this off with ownership rules?

Anyway, speculative and low-importance