Document array expression with a const. #914

ehuss · 2020-12-22T13:49:34Z

I did not include any details about drop behavior with [a; 0], as it seems like rust-lang/rust#74836 is unresolved, though I can add that if desired. (I'm not sure if that would go in the array-expr chapter, or the destructor chapter.)

ehuss · 2020-12-22T13:49:42Z

@RalfJung Sorry, bugging you to review another one.

RalfJung · 2020-12-26T09:54:32Z

This sounds correct. However, do you want to mention the subtleties around drop? See rust-lang/rust#79270 (comment) and the surrounding discussion.

ehuss · 2020-12-26T17:37:00Z

Hm, yea, as mentioned in the description, I intentionally left that out because I was uncertain what to say. Since rust-lang/rust#74836 is unresolved, should it document that [non_const_expr; 0] evaluates once and then drops, and then include a note that this is not working as intended? Or just leave that part out completely and only say [CONST; 0] does not evaluate CONST (it seems a little weird to say that without contrasting it with the non-const example). Also, would you recommend putting that in the Destructors chapter, the Array Expression chapter, the Constant Evaluation chapter, or maybe in multiple locations?

Havvy · 2020-12-26T18:32:34Z

I would document it how it is now ([CONST; 0]) does not evaluate CONST), but include a warning directly after contrasting this behavior with the non-const example.

RalfJung · 2020-12-26T18:52:27Z

I would document it how it is now ([CONST; 0]) does not evaluate CONST), but include a warning directly after contrasting this behavior with the non-const example.

Sounds good.

As to where to put this... I have little idea about the structure of the reference, so I'll leave that up to you. I would not document the same thing in multiple places, but adding cross-references might be a good idea.

ehuss · 2020-12-27T19:54:07Z

OK, I included a note on the [a; 0] behavior. I waffled a little on whether or not to note the bug. We talked about the policy a little bit in #599, and it seemed like the consensus is that it doesn't hurt to include the occasional note about a bug. I can remove it whenever rust-lang/rust#74836 is closed.

RalfJung · 2020-12-27T22:53:43Z

src/expressions/array-expr.md

 greater than 1 then this requires that the type of `a` is
 [`Copy`](../special-types-and-traits.md#copy), or `a` must be a path to a
-constant item.
+constant item. When `b` evaluates to 0, the expression `a` is evaluated once


"the expression a is evaluated once" is always true for non-constant-items. So I think it is a bit strange to make 0 sound like a special case here.

I'd describe this (probably in a separate paragraph) as:

for constant items, the expression is instantiated b times. (So if b == 0, it is not instantiated at all.)

for other a, the expression is evaluated exactly once (and then the result is copied as needed).

Oh, that makes sense and is much clearer to me now. Thanks for your patience!

RalfJung · 2020-12-28T16:54:05Z

LGTM. :)

Havvy

r+ after switching the note to a warning.

Havvy · 2020-12-29T05:18:41Z

src/expressions/array-expr.md

+are not a constant item, it is evaluated exactly once, and then the result is
+copied `b` times.
+
+> **Note:** In the case where `b` is 0, and `a` is a non-constant item, there


Can you use the warning for this?

<div class="warning"> Warning: This is an example warning. </div>

Havvy · 2020-12-29T19:06:04Z

And merged. 💟 Thanks!

Update books ## nomicon 2 commits in a5a48441d411f61556b57d762b03d6874afe575d..a8584998eacdea7106a1dfafcbf6c1c06fcdf925 2020-12-06 10:39:41 +0900 to 2021-01-06 12:49:49 -0500 - Update vector code examples - Remove outdated information about `jemalloc` ## reference 13 commits in b278478b766178491a8b6f67afa4bcd6b64d977a..50af691f838937c300b47812d0507c6d88c14f97 2020-12-21 18:18:03 -0800 to 2021-01-12 21:19:20 -0800 - Update grammar for parser unification. (rust-lang/reference#927) - Define constraining an implementation (rust-lang/reference#928) - Document extra behavior of #[no_mangle] (rust-lang/reference#930) - Add a float examle without a `.`. (rust-lang/reference#929) - Add more details about const generics. (rust-lang/reference#921) - Fix footnotes. (rust-lang/reference#926) - Add "Logic errors" as behavior not considered unsafe (rust-lang/reference#919) - Update grammar for order of parameters/arguments. (rust-lang/reference#920) - Fix formatting in the tuple section (rust-lang/reference#923) - document const generics (rust-lang/reference#901) - Update mdbook (rust-lang/reference#918) - linkage.md: update link to FFI section of the Book. (rust-lang/reference#917) - Document array expression with a const. (rust-lang/reference#914) ## book 8 commits in 5bb44f8b5b0aa105c8b22602e9b18800484afa21..ac57a0ddd23d173b26731ccf939f3ba729753275 2020-12-18 20:07:31 -0500 to 2021-01-09 14:18:45 -0500 - Update version of mdbook we're testing with to 0.4.5 (rust-lang/book#2561) - Fix grammar in ch13-01-closures.md (rust-lang/book#2534) - Merge remote-tracking branch 'origin/pr/2527' - Clarify code example ch6.3 (rust-lang/book#2485) - Fix link added in rust-lang/book#2495 to be relative and at the bottom - Merge remote-tracking branch 'origin/pr/2495' - Update output to match the updated poem punctuation - Fix rust-lang/book#2539 - Remove fancy apostrophes from poem for Windows ## rust-by-example 3 commits in 1cce0737d6a7d3ceafb139b4a206861fb1dcb2ab..03e23af01f0b4f83a3a513da280e1ca92587f2ec 2020-12-21 17:36:29 -0300 to 2021-01-09 10:20:28 -0300 - Replace for loop with iteration (rust-lang/rust-by-example#1404) - Update mdbook (rust-lang/rust-by-example#1402) - Add note for match guards to include catch-all (rust-lang/rust-by-example#1401) ## embedded-book 1 commits in ba34b8a968f9531d38c4dc4411d5568b7c076bfe..ceec19e873be87c6ee5666b030c6bb612f889a96 2020-11-17 00:20:43 +0000 to 2021-01-03 13:13:10 +0000 - book.toml: add link to GitHub repo (rust-embedded/book#276)

RalfJung reviewed Dec 27, 2020

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ehuss added 3 commits December 28, 2020 08:17

Move links to bottom of the file.

8027bc8

Document array expression with a const.

e031b14

Document [a; 0] behavior.

a2e5a14

ehuss force-pushed the array-const branch from 3d5a774 to a2e5a14 Compare December 28, 2020 16:23

Havvy requested changes Dec 29, 2020

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Switch note to warning.

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Havvy approved these changes Dec 29, 2020

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Havvy merged commit 6d6debc into rust-lang:master Dec 29, 2020

ehuss mentioned this pull request Jan 13, 2021

Update books rust-lang/rust#80973

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Document array expression with a const. #914

Document array expression with a const. #914

ehuss commented Dec 22, 2020

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RalfJung commented Dec 26, 2020

ehuss commented Dec 26, 2020

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ehuss commented Dec 27, 2020

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Havvy commented Dec 29, 2020

Document array expression with a const. #914

Document array expression with a const. #914

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