Make Function Env immutable

[MarkMcCaskey]

Alternative / related to #1625

Resolves #1612

Review

• Add a short description of the the change to the CHANGELOG.md file

[MarkMcCaskey]

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[Hywan]

I agree Env must be immutable, and let the user use interior mutability (with Cell-like types) if necessary. Thanks for addressing that!

[MarkMcCaskey]

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[nlewycky]

Optional: really wish we didn't have to do this. Deprecated ought to be built on top of the API. Or put another way, anything deprecated does should be supported by our real API as something that other library consumer may also want to do.

Not that I'm offering an alternative. IIUC we considered wrapping it in a Mutex and rejected that?

[MarkMcCaskey]

Not that I'm offering an alternative. IIUC we considered wrapping it in a Mutex and rejected that?

That was a different PR, if it were as simple as just conditionally putting something behind a mutex, then I agree. I tried 2-3 times to make it work without editing the API and this is the only way I've been able to make it work.

I agree that it kind of sucks, but I figure the deprecated API won't be around forever so 🤷

[nlewycky]

IMO given how all the parts deprecated needs are now unsafe and documented as non-stable, you could remove the deprecated feature. A pro of this is that we don't have two possible builds of lib/api, with deprecated enabled and disabled. Maybe that doesn't matter.

[syrusakbary]

Looks great to merge, once the "deprecated" comment from Nick is addressed :)

[MarkMcCaskey]

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