Approaches to get a destructor when `main` finishes.

[dpc]

Hi, I have a problem that I wish I could rely somehow on destruction of lazy_static to call drop on what's inside it. I know it might be impossible, but I guess it's worth asking. Suggestions for workaround etc. are welcome. I was eg. thinking about using a Weak inside lazy_static, but then I do have to have a strong reference somewhere in the main thread.

[Kimundi]

Rust as a language has basically made the design decision to not have custom user code running before or after main(), so if a static variable contains a type with a destructor, that destructor will just never be called. As far as I'm aware, there is no good way to have a automatic destruction anyway, so lazy_static just mirrors that semantic.

However, the crate could provide something like lazy_static::destroy() for manually destroying a static. The issue there is just that the internally used std::Once has no way to be "reset", so someone who is more knowledgeable about thread synchronisation primitives than I would have to write a custom implementation for that (which would also be useful for #60).

[Kimundi]

One possible workaround you could do is to have the lazy_static be of a Mutex<Option<T>> or similar, and then destroy the T by overwriting the Option with a None

[dpc]

For synchronization I'm quite sure to resort to ArcCell from crossbeam or what you have mentioned. The API compilcations are main concern.

[KodrAus]

It's been a while now, so I'll go ahead and close this issue as a design decision we inherit from Rust upstream. Please feel free to re-open though if there's anything more you'd like to add!

[jtmoon79]

FYI this affects crate tempfile.