Tracking issue for Result::cloned, Result::cloned_err, Result::copied, Result::copied_err

[ksqsf]

PR

[alexcrichton]

Some unresolved questions for us to resolve while this is unstable:

• Do *_err method carry their weight?
• Is it right for cloned to work on Result<&T, E> or should it work on Result<&T, &E>?
• Should cloned be cloned_ok?

[Fishrock123]

My 2c:

cloned and copied for Result should do the respective operation to either branch, it seems to me that in most cases where you'd need it fro one you'd need it to handle both branches.

I realize this does make the copied one kinda marginal since most errors are unlikely to be Copy, but perhaps there is a workaround in the odd case you'd want to copy and yet be able exclude Err?

[spinda]

I'm in favor of cloned only cloning the Ok side, i.e., going from Result<&T, E> to Result<T, E>. This API mirrors Option::cloned, and I think it's more likely that a function of the form fn do_fallible_thing() -> Option<&T> eventually evolves into fn do_fallible_thing() -> Result<&T, SomeError> than fn do_fallible_thing() -> Result<&T, &SomeError>. It's still the possibly-present success value that you want to clone; the fact that the failure path now carries an error (versus just None) is an incidental detail.

[matklad]

Given that we already have precedent of Result::map, which applies only to Ok, and is named map rather then map_ok, I feel moderately confident that:

• cloned should only clone &T, not E
• it should be named cloned, rather than cloned_ok

Not sure about _err variants, but they are stabilizable separately. So, it seems that we potentially can write a stabilization report here and FCP?

[ksqsf]

Thanks @matklad for bringing the attention. I believe we generally agree on cloned and copied so the _err variants are left out here.

Stabilization Report

Brief summary

• Add Result::cloned() for converting Result<&T, E> and Result<&mut T, E> into Result<T, E> by cloning the contents of the Ok part.
• Add Result::copied() for converting Result<&T, E> and Result<&mut T, E> into Result<T, E> by copying the contents of the Ok part.

Test cases

• Doc tests included in the original PR (Add Result::cloned{,_err} and Result::copied{,_err} #63166).

Documentation

• Doc comments included in the original PR(Add Result::cloned{,_err} and Result::copied{,_err} #63166).

Things not covered

• The _err variants are not proposed to be stabilized.

[joshtriplett]

Shall we stabilize the cloned and copied methods on Result, per the above stabilization report?

@rfcbot merge

[rfcbot]

Team member @joshtriplett 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
• @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!

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[rfcbot]

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

[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.

This will be merged soon.