Stabilize ops::ControlFlow (just the type)

[scottmcm]

Tracking issue: #75744 (which also tracks items not closed by this PR).

With the new ? desugar implemented, it's no longer possible to mix Result and ControlFlow. (At the time of making this PR, godbolt was still on the 2021-05-01 nightly, where you can see that the mixing example compiled.) That resolves the only blocker I know of, so I'd like to propose that ControlFlow be considered for stabilization.

Its basic existence was part of rust-lang/rfcs#3058, where it got a bunch of positive comments (examples 1 2 3 4). Its use in the compiler has been well received (#78182 (comment)), and there are ecosystem updates interested in using it (rust-itertools/itertools#469 (comment), jonhoo/rust-imap#194).

As this will need an FCP, picking a libs member manually:
r? @m-ou-se

Stabilized APIs

#[derive(Debug, Clone, Copy, PartialEq)]
pub enum ControlFlow<B, C = ()> {
    /// Exit the operation without running subsequent phases.
    Break(B),
    /// Move on to the next phase of the operation as normal.
    Continue(C),
}

As well as using ? on a ControlFlow<B, _> in a function returning ControlFlow<B, _>. (Note, in particular, that there's no From::from-conversion on the Break value, the way there is for Errs.)

Existing APIs not stabilized here

All the associated methods and constants: break_value, is_continue, map_break, CONTINUE, etc.

Some of the existing methods in nightly seem reasonable, some seem like they should be removed, and some need more discussion to decide. But none of them are essential, so as in the RFC, they're all omitted from this PR.

They can be considered separately later, as further usage demonstrates which are important.

[m-ou-se]

@rfcbot merge

[rfcbot]

Team member @m-ou-se has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se
• @sfackler
• @yaahc

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[jhpratt]

To be clear, this will permit using this enum for actual control flow purposes? Or is this only stabilizing the enum itself and not its associated behavior? I think it's the former given the diff, but I'm not positive.

[scottmcm]

I'm not sure what "for actual control flow purposes" means. This would stabilize use of ? on ControlFlow, including using ControlFlow as a return type from a try_fold closure (since trait implementations cannot be private).

[jhpratt]

That's exactly what I meant. Just wanted to be positive this wasn't just stabilizing an enum for no other purpose (which would be unlike you!)

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

The RFC will be merged soon.

[m-ou-se]

@bors r+

[bors]

📌 Commit 590d452 has been approved by m-ou-se