Use WeakKeyDict for the subexpression dictionary

[odow]

A variant of #4204 that closes #4203

The downside to this approach is that we need to decide how frequently to filter the subexpression dictionary. If an element has been GC'd, it cannot be referenced in the future, so it is safe to remove.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.96%. Comparing base (8791c47) to head (3c997b7).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4207      +/-   ##
==========================================
+ Coverage   99.95%   99.96%   +0.01%     
==========================================
  Files          43       43              
  Lines        6308     6315       +7     
==========================================
+ Hits         6305     6313       +8     
+ Misses          3        2       -1     

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[blegat]

Where is this used ?

[odow]

It was used in the tests to simplify things. I guess I can remove.

[odow]

I think I'm just re-implementing Base.WeakKeyDict. Except that instead of having a finaliser on the key, I have the clean-up pass.

[blegat]

Surprising that this decreases when we change struct to mutable struct

[blegat]

Related JuliaLang/julia#62831

[odow]

I guess we should make a decision on this.

[blegat]

I feel that we can merge as is. We can add a function later that cleans the dict if needed. Not sure that we should do this cleaning automatically. If it's documented, then it should be fine. Another option would be to clean it before optimize!, maybe with an option to disable it. But then it won't work as well if you call optimize! from the same scope these objects are created. And we also need to assume GC has been run. So it's better to have a custom function, where we document that the user is recommended to call GC.gc() before calling that function