DRAFT: Implement reparenting and disjoint access in tree_arena_safe

[PoignardAzur]

Add ArenaMutList::item_mut_disjoint.
Add ArenaMutList::reparent.
Make ArenaMapMut an explicit part of the API.

Overall, the goal of this is to make reparenting possible while in the middle of iterating the widget tree.
This is a key part of the groundwork for the lifecycle refactor as described in #masonry > Plan for widget lifecycle.

[PoignardAzur]

This is, obviously, not remotely close to compiling, since it changes the arena APIs without updating the Masonry code that uses them.

@KGrewal1 I'm uploading this because I'm especially interested in your take on this API so far. How feasible do you think it would be to make a sound version of this API on the unsafe side?

[KGrewal1]

apologies for taking so long to look at this

• from looking the main benefit is the ability to get disjoint mutable access to the sub trees
• I do not think this is unfeasible though in practice though may require rethinking the current API a bit - not sure how hard this would - the invariants for this should be stored in parents_map which is also in the safe version of the arena so the check for being disjoint in that should mirror over to disjoint in unsafe too, making this change easier

[PoignardAzur]

apologies for taking so long to look at this

No worries.

That said, I'm on a pretty tight schedule (one week left). Any chance you'd have time for a video call? You can DM on zulip if so.