Make `Base._unsetindex!` (or a function like it) Public

[tecosaur]

Currently, in order to implement an array-backed dictionary that actually frees elements when they're delete!d, there is (as far as I am aware) no alternative to Base._unsetindex!.

As a consequence, you can see this function used in basically every dict-like construction outside of Base: it's in few key packages like DataStructures.jl, OrderedCollections.jl, AbstractAlgebra.jl.

I think unless we want to take the stance that "only Base is allowed to create array-backed dict-like structures that don't hold on to ghost references", there needs to be a public version of Base._unsetindex!.

[Tokazama]

Isn’t it an approach to setting values to a null ptr? It would be nice to have made public but we’d need to explicitly spell out what can be “unset” which requires a consistent memory layout for objects. This has historically been considered a matter of internal design subject to change.

[tecosaur]

Isn’t it an approach to setting values to a null ptr?

Essentially, the key thing is that it frees an item from GC, allowing it and associated resources to be cleaned up.

It would be nice to have made public but we’d need to explicitly spell out what can be “unset” which requires a consistent memory layout for objects. This has historically been considered a matter of internal design subject to change.

Right, this makes sense, the problem is that without any public API it is not possible for anyone outside Base to create a Dict-like structure with array-based storage without using Base internals.

I say "or a function like it" and "public version of" for good reason. I don't think we want to start docutmenting the particular internal design behaviour, but maybe we could have something like this:

    Base.unsetindex!(array, index::Integer)

Attempt to unset the entry at position `index` in `array`.

This is performed on a best-effort basis, and leaves the entry in an undefined state.

With const unsetindex! = _unsetindex! internally.

This way (1) the key functionality is publicly available in such a way that OrderedCollections etc. don't need to reach into Base internals, but (2) we avoid locking ourselves into a particular design/approach of "unsetting" an index.