Improve tree_arena's safety documentation#1755
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| // This is sound because `self.arena` is a `&DataMap<T>`, and the only other code | ||
| // path that can access `node_cell` is `ArenaMutList::find_mut_inner`. | ||
| // `ArenaMutList::parent_arena` is a `&mut DataMap<T>`, which means that an | ||
| // `ArenaRefList` can't be constructed as long as an `ArenaMutList` is live. |
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Come to think of it, this is already inaccurate because of ArenaMutList::reborrow.
Uuuugh.
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See #masonry > Refactoring tree arena for disjoint borrows for rationale.
Note that this PR neither changes the behavior of tree_arena_unsafe nor the conditions under which it's sound.
The PR makes
find_mut_innerunsafe, but I'd argue it was already unsafe in the sense that calling it with the wrong arguments would lead to UB. This was acceptable because the method was private and the rest of the code never called it with wrong arguments, but I would argue that marking the method as unsafe is more accurate and helpful.Similarly, the PR points out the box invalidation problem, but that problem already exists in our current implementation.