Implement Borrow<T> and BorrowMut<T> for Box<T: ?Sized>

[bluss]

Implement Borrow and BorrowMut for Box<T: ?Sized>

[rust-highfive]

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[bluss]

r? @aturon

This change seems logical, it allows the Box<[T]> analogue of Vec<T> among other things.

Question is of course, if it is a breaking change in any way, or if it's simple to introduce?

[alexcrichton]

I would be ok with these trait implementations, but a recent change to add a AsRef implementation to a core pointer type ended up causing a regression, so I'm somewhat hesitant about this.

I believe, however, that borrow and borrow_mut are rarer inherent methods, so I think this would be ok.

[brson]

I'll run this through crater.

Nominating since it's regression-prone, so better to get it out sooner?

[aspiwack]

I guess that if you have a x:Box<A> with A:Borrow<B>, then x.borrow() will be &A instead of &B just like in #26096 above.

At any rate, I precipitated this pull request by enquiring about Box<A> not implementing Borrow<A> on irc. In my intuition, Box and Rc are very similar, and I don't see any reason for them not to implement the same trait (apart from the obvious ones). Was that a design choice?

[bluss]

It's a breaking change but a minor change. I expected no trouble, but I understand the situation is the same. I think for our Map collections it can't break anything, it will only allow more types to use .get().

@brson Bringing out crater is great, but I didn't think this was such high stakes 😄

[alexcrichton]

triage: beta-accepted pending merging plus a crater run

[brson]

No regressions. https://gist.github.com/brson/1e14d7129c7d3c770b52

[bluss]

@brson 998 unknown crates though?

[brson]

@bluss Ugh, idk what happened. I'll run it again.

[brson]

OK, now it's run, still looks good https://gist.github.com/brson/1e14d7129c7d3c770b52

[bluss]

Yes, no impact whatsoever on crater, great!

[alexcrichton]

@bors: r+ 4fdb4cf

Thanks @brson!

[bors]

⌛ Testing commit 4fdb4cf with merge 2fbbd54...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt