Arena::alloc_with_id

[fitzgen]

+cc @alexcrichton

[alexcrichton]

Hm so one thing I ran into transitioning, I'm not sure how we'd implement insert_with_id around here.

[fitzgen]

Hm so one thing I ran into transitioning, I'm not sure how we'd implement insert_with_id around here.

Right because we couldn't construct the value to check whether it is already in the set without potentially adding duplicates.

I can make Arena::next_id public again, and then this wouldn't be a breaking change either... sound good?

[matklad]

How that API is actually used? It is a sort of self-fulfilling proficy: if you construct an entity with a fresh id, it wouldn’t be in the set. If this is expected behavior, perhaps that could could change it’s next_id to with_id as well?

[fitzgen]

How that API is actually used?

It is used by things whose equality/hash don't depend on the Id, for when we would like to de-duplicate items with a set whose storage is ultimately backed by an arena.

For example, function type declartations (which are separated and de-duped from the functions themselves): https://github.com/rustwasm/walrus/blob/ce985f826c89d2cc8e56e0d00b0a47c83bbd83af/src/module/types.rs#L56

[alexcrichton]

Leaving next_id is fine by me yeah!