Suggestion: SyncFnOnce

[yokomizor]

Hi,

I am no sure if something like a SyncFnOnce wrapper exists and if it would even be possible to implement.
If possible, it would be nice since FnOnce only operation is call_once, which consumes self.

[rkuhn]

Sounds doable at first sight, but it makes me wonder: under which circumstances would you need this? Sync is about sending references around, and sending a reference to FnOnce isn’t useful (as you point out).

[yokomizor]

I stumbled upon this when I had a type that for other reasons had to be Sync while holding an Option<Box<dyn FnOnce() -> SyncFuture<BoxFuture<'static, SomeType>>>>. I ended up implementing the wrapper myself, but it feels like a good use case for sync_wrapper.

[rkuhn]

Sorry for the delay, other things kept me from properly thinking this through. The reasoning seems sound to me: SyncWrapper works because it only permits operations on exclusive references, so SyncFnOnce should work, too, because the only thing you can do with it requires you to (exclusively) own the value.

Would you like to try your hand at an implementation?