Change:
#![feature(extract_if)] #![feature(if_let_guard)] #![feature(let_chains)] <del> #![feature(option_take_if)] </del> #![feature(never_type)] #![feature(type_alias_impl_trait)] #![recursion_limit = "512"] // For rustdoc

Discussion:
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Is there a reason to have both and when we already have in ?

To be honest, personally I can't think of a concrete reason at the moment. has both () and I used it as impl orientation. The question would then also be which name to keep: or . A suffixed method without the corresponding not-suffixed one seems odd, but and a in the callback might be unexpected after using ? I am also not sure about all the stuff which I copied over from the impl. edit: I am totally fine with only and
Vec::retain is a mistake and should not be followed. I prefer keep 'retain' only with '&mut' signature.​
Done :)
Do you think there are further changes needed?
Signature looks good now, can't say anything about trait bounds though.
Hi! Any progress on this unstable-feature? It's not that big a feature and I'd like to use it so much that I'd like to move forward with the stabilization process if there are no major issues. Any concerns come to mind?
Hi I can't get a comment on whether it's ready to be stabilized, so I had to ping a member of the libs team! I'd be happy to have them review it.
The implementation is straightforward, but API goes to a different team -- anyone from want to comment?
I think this is a generally useful utility function that can make some use cases easier to write & more readable. fcp merge
Team member has proposed to merge this. The next step is review by the rest of the tagged team members: [x] [ ] [x] [ ] [x] 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 for info about what commands tagged team members can give me.
What about ? Like but for :
I guess replaceif would be useful as well. It seems consistent that and be provided, just as and are provided respectively.
+1 for I've encountered several occasions recently when the implementation of for an combinator would have been much more concise and readable in these terms, to the point where I actually did write a crate-local trait. Glad to see it proposed in an unstable feature!

This may be starting to cross the "out-of-scope event horizon", but, similarly, I would like to see . The pattern of has a habit of cropping up independently in other places. The really could be and the could be . For instance, when conditionally mutating a shared object behind an , I'd rather not and potentially force a copy just to make the check to see if I have to. Or, when using one-or-more-layers of to avoid "ambiguous interacting sets of " or something. I want to do something in the case, but only when a predicate, but I need to pass on an , the else is always going to be , the calculation-after-the-predicate also returns an already, and I'd rather not break out of my method chain. Or (and I'm not joking), a function that returns an iterator in an which itself yields items in s and loops until exhausted then loops to call the original function again and loops until exhausted but breaks the loop and returns the of any of the should that happen. (That may sound insane, but, due to external constraints, I'm reluctantly rather confident that this is actually the most elegant/optimal it can get...) I'm a big fan of and . I will concede that it's possible to abuse/overuse them, but in some circumstances it really is the cleanest and most readable way to express what's happening. I think nicely rounds out the family. : Eager : Lazy : Lazy-but-could-be- You can sort of do this today, with but is silly, unsightly, and noise. The pattern could then turn into could then be expressible as and So, in a sense, may actually be a more fundamental construct than . So if gets in the language, I think it makes sense that would also.
Actually, I think may be the wrong semantics. closure takes an argument. It's a mapping from the type parameter of some type to . That doesn't really seem analogous to what's going on here at all. I guess got me thinking in the direction, and has a method called with a function parameter that returns self-but-with-a-different-parameter, so I it seemed like the first obvious choice. The implementation looks like Looks rather more like the of , so... maybe ?
:bell: This is now entering its final comment period, as per the .

:bell:
The final comment period, with a disposition to merge, as per the , 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. This will be merged soon.