Let destructors move out of class fields if it's the last use.

[bblum]

Currently writing taskgroup code, where the destructor needs to pass some exclusive ARCs around, which are noncopyable. This is only possible since every function that I pass them to takes them by reference - but maybe somebody will need to destruct a class by passing a noncopyable thing in move mode.

[eholk]

I usually use the option dance here. In a few places recently, I've written a box class that tries to do some of the option dance for you. I think there was an example of this in the ICFP code.

[nikomatsakis]

I would prefer not to make liveness more complex by supporting this case. However, we should fairly trivially be able to support something like:

let {f, g, h, _} = self

for destructors or any other case that takes owns self.

[bblum]

If non-destructor code destructures self, does it bypass the destructor?

[nikomatsakis]

Interesting question. I think it would have to be illegal in that event to destructure a class with a dtor?

[bblum]

not sure if this is worth supporting. it doesn't add any expressiveness power if you can just use the option dance.