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I didn't do anything to bring back Option::as_slice and Result::as_slice because x.map_or(&[], ref_slice) is almost as short and just as easy to understand.

Maybe it's just me, but, looking at the name alone, it's hard to tell what slice::ref_slice actually does.
Perhaps slice::from_ref would sound better? Or slice::single_ref? 🚲

I'm not particularly opposed to bringing this back, but I do agree with @stjepang's naming concern. We may want to land these as unstable initially, though I don't really know what good that would do in this specific case.

@rfcbot fcp merge

I've added the waiting-on-crater label for now so that if we decide to go ahead with this we do a crater run -- I think it's worth doing that just in case others have implemented ref_slice (which could play into the naming bikeshed, too, so probably best after fcp).

Team member @sfackler has proposed to merge this. The next step is review by the rest of the tagged teams:

• @BurntSushi
• @Kimundi
• @alexcrichton
• @aturon
• @dtolnay
• @sfackler

No concerns currently listed.

Once these reviewers reach consensus, 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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These are free functions, so I don't think there's as much of a concern with collisions as you'd have with new methods.

Since the naming is uncertain and insta-stable is proposed anyway, could they just be

impl<'a, T> From<&'a T> for &'a [T] { ... }
impl<'a, T> From<&'a mut T> for &'a mut [T] { ... }

Personally, I think slice::from_ref is an obviously better name than slice::ref_slice. If I thought about naming at all (I did not, I just copied them from old rustc sources) then I would have chosen that too.

A From impl seems more disruptive to me.

I'd personally prefer to initially add these as unstable (but perhaps push on stabilization soon after landing), and I agree with @whitequark that a name like from_ref may be a bit nicer!

Also strongly agree with from_ref. My first guess as to what slice::ref_slice would do was take a slice of Ts and convert it to a slice of refs of T.

Already renamed it, just not reflected in the PR.

ping @BurntSushi waiting for your ticky box here!

My crate that provides these functions also provides an Option variant: https://github.com/steveklabnik/ref_slice/blob/master/src/lib.rs

I'm not sure if they're worth bringing in too, figured I'd bring it up.

@steveklabnik #45306 (comment)

I'm going to go ahead and check @BurntSushi's checkbox as this is just unstable anyway.

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

@bors: r+

📌 Commit 8431811 has been approved by alexcrichton

@bors r-

If the final FCP checkbox expects "this is just unstable anyway" the current code certainly isn't unstable.

Oops sorry, thanks for catching that @kennytm!

Can you please guide me on the stability markers here?

@whitequark

1. File a tracking issue

2. Replace those #[stable] attributes with

   #[unstable(feature = "from_ref", issue = "45678")]

3. Write an unstable-book entry in src/doc/unstable-book/src/library-features/from-ref.md if you like.

Tracking issue filed as #45703, no unstable book entry since it seems far too trivial.

Stability annotation updated.

Thanks! It should be ready to go after fixing the build failure.

[00:02:34] error: use of unstable library feature 'from_ref' (see issue #45703)
[00:02:34]    --> /checkout/src/liballoc/slice.rs:123:23
[00:02:34]     |
[00:02:34] 123 | pub use core::slice::{from_ref, from_ref_mut};
[00:02:34]     |                       ^^^^^^^^
[00:02:34]     |
[00:02:34]     = help: add #![feature(from_ref)] to the crate attributes to enable
[00:02:34] 
[00:02:34] error: use of unstable library feature 'from_ref' (see issue #45703)
[00:02:34]    --> /checkout/src/liballoc/slice.rs:123:33
[00:02:34]     |
[00:02:34] 123 | pub use core::slice::{from_ref, from_ref_mut};
[00:02:34]     |                                 ^^^^^^^^^^^^
[00:02:34]     |
[00:02:34]     = help: add #![feature(from_ref)] to the crate attributes to enable

Updated.

@bors r=alexcrichton

📌 Commit 1cc88be has been approved by alexcrichton

⌛ Testing commit 1cc88be with merge d5b69d4...

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing d5b69d4 to master...