Bring back slice::ref_slice as slice::from_ref.

[whitequark]

These functions were deprecated and removed in 1.5, but such simple
functionality shouldn't require using unsafe code, and it isn't
cluttering libstd too much.

The original removal was quite contentious (see #27774), since then
we've had precedent for including such nuggets of functionality (see rust-lang/rfcs#1789),
and @nikomatsakis has provided a lot of use cases in rust-lang/rfcs#1789 (comment).
Hence this PR.

I'm not too sure what to do with stability, feel free to correct me.
It seems pointless to go through stabilization for these functions though.

cc @aturon

[rust-highfive]

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[whitequark]

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.

[ghost]

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? 🚲

[sfackler]

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

[Mark-Simulacrum]

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).

[rfcbot]

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!

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

[sfackler]

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.

[scottmcm]

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

impl<'a,T>From<&'aT>for&'a[T]{ ... }impl<'a,T>From<&'amutT>for&'amut[T]{ ... }

[whitequark]

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.

[alexcrichton]

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!

[Ixrec]

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.

[whitequark]

Already renamed it, just not reflected in the PR.

[carols10cents]

ping @BurntSushi waiting for your ticky box here!

[steveklabnik]

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.

[whitequark]

@steveklabnik#45306 (comment)

[alexcrichton]

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

[rfcbot]

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

[alexcrichton]

@bors: r+

[bors]

📌 Commit 8431811 has been approved by alexcrichton

[kennytm]

I don't think we intend to insta-stable these?

[kennytm]

@bors r-

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

[alexcrichton]

Oops sorry, thanks for catching that @kennytm!

[whitequark]

Can you please guide me on the stability markers here?

[kennytm]

@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.

[whitequark]

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

[whitequark]

Stability annotation updated.

[kennytm]

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

[whitequark]

Updated.

[steveklabnik]

Library stuff doesn't need an unstable book entry as the api docs are docs; the unstable book pages get generated automatically.

…

On Wed, Nov 1, 2017 at 6:22 PM, whitequark ***@***.***> wrote: Updated. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#45306 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AABsikXJgsaCpcQfEEdjztR6V-nnUeB7ks5syO8KgaJpZM4P5v2u> .

[kennytm]

@bors r=alexcrichton

[bors]

📌 Commit 1cc88be has been approved by alexcrichton

[bors]

⌛ Testing commit 1cc88be with merge d5b69d4...

[bors]

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