Tracking issue for std::sync::Once::is_completed

[SimonSapin]

Implemented in #53027

I propose to expose a way to check if a Once instance is initialized.

I need it in once_cell. OnceCell is effetively a pair of (Once, UnsafeCell<Option<T>>), which can set the T only once. Because I can't check if Once is initialized, I am forced to add an indirection and check the value of ptr instead:

https://github.com/matklad/once_cell/blob/8127a81976c3f2f4c0860562c3f14647ebc025c0/src/lib.rs#L423-L429

https://github.com/matklad/once_cell/blob/8127a81976c3f2f4c0860562c3f14647ebc025c0/src/lib.rs#L457-L461

The parking_lot's version of Once exposes the state as an enum: https://docs.rs/parking_lot/0.6.3/parking_lot/struct.Once.html#method.state.

I suggest, for now, just to add a simple bool function: this fits my use-case perfectly, exposes less implementation details, and is forward-compatible with more fine-grained state checking.

[nagisa]

The documentation needs to be made slightly more strict.

For example:

It is also possible that immediately after is_completed returns false, some other thread finishes executing call_once.

Should be replaced with

This function returning false does not mean that Once has not been executed.

The distinction is important, because at the exact moment this function returns Once might already be executed/resolved, without it necessarily happening “immediately after”.

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To avoid people shotgunning their feet off, I would suggest to add some guidance on how this should be used. For example, doing something useful based on this function returning a false is somewhat likely to be a bug. And yet, examples do not really result in such an impression.

[SimonSapin]

CC @matklad for nagisa’s comment above.

[mjbshaw]

It's been 9 months since this was implemented. Can this be stabilized now?

[pyrrho]

Is there anything preventing this method from being stabilized? It looks like there was some bikesheding in #53027 but it's now been just past a year since initial adoption.

[matklad]

Note that I am personally no longer directly interested in having this method: in once_cell, I ended-up copy-pasting the whole of Once anyway. That said, this does look like straightforward addition, which allows to do more with the thing, without actually expanding it's surface area greatly.