Docs: slice elements are equidistant #64703

llogiq · 2019-09-23T07:34:51Z

Recently, someone asked why [char] and str are not interchangeable, and I explained that in a slice, the elements must be laid out equidistantly, whereas the chars in a str are stored compactly regardless their size. However I couldn't find this documented anywhere, so here's a small addition of this fact.

rust-highfive · 2019-09-23T07:34:54Z

r? @rkruppe

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gnzlbg · 2019-09-23T09:04:31Z

the elements must be laid out equidistantly

I understood this from the word "contiguous". Maybe instead of introducing a new word ("equidistantly"), it might be more helpful to explain in the body what contiguous means, e.g.,

/// "Contiguous" means that there are no gaps between the elements, that is, 
/// the `i`-th slice field is located at offset `i * size_of::<T>()`.

but maybe trying to word that in a beginner-friendly way.

sinkuu · 2019-09-23T09:41:31Z

"constant stride" is the word used in the Unsafe Code Guidelines. (Also, it's possible that stride == size does not hold in the future Rust versions, according to this page.)

hanna-kruppe · 2019-09-27T15:29:07Z

I am not a fan of the UCG trying to accommodate hypothetical future "stride != size" changes (see discussion in rust-lang/unsafe-code-guidelines#176) so I'm not thrilled to see it make its way into some[1] libstd docs. Can we make the intended clarification without opening this can of worms?

Perhaps something like @gnzlbg propoed earlier, but avoid mentioning size_of directly? Perhaps:

/// "Contiguous" means that there are no gaps between the elements, so
/// every element is the same distance from its neighbors.

_{[1] Some... but not all! This is one of my complaints about it in the the UCG too, that the carefully couched language is not applied consistently enough to actually make a dent in future compatibility.}

gnzlbg · 2019-09-27T17:40:17Z

That part of that page of the UCGs is non-normative and quite controversial so it will probably be removed as soon as someone sends a PR for it. Beyond what @rkruppe said, putting anything about that page into the docs would require Lang team approval because it introduces the concept of a stride in Rust, which AFAIK is not mentioned anywhere else.

llogiq · 2019-09-27T17:46:00Z

Ok, I removed the "stride" part as per @rkruppe's request. r?

hanna-kruppe · 2019-09-27T17:50:46Z

Thanks!

@bors r+ rollup

bors · 2019-09-27T17:50:47Z

📌 Commit 6ccb7ae has been approved by rkruppe

llogiq · 2019-09-27T18:22:24Z

Thanks @rkruppe!

petertodd · 2019-09-27T23:37:08Z

src/libstd/primitive_docs.rs

@@ -566,7 +566,9 @@ mod prim_array { }
 #[doc(alias = "[")]
 #[doc(alias = "]")]
 #[doc(alias = "[]")]
-/// A dynamically-sized view into a contiguous sequence, `[T]`.
+/// A dynamically-sized view into a contiguous sequence, `[T]`. Contiguous here
+/// means that elements are layed out so that every element is the same


s/layed out/laid out/

Had to look this one up myself: https://writingexplained.org/laid-out-or-layed-out

…, r=rkruppe Docs: slice elements are equidistant Recently, someone asked why `[char]` and `str` are not interchangeable, and I explained that in a slice, the elements must be laid out equidistantly, whereas the chars in a `str` are stored compactly regardless their size. However I couldn't find this documented anywhere, so here's a small addition of this fact.

@ghost

Rollup of 14 pull requests Successful merges: - #63492 (Remove redundancy from the implementation of C variadics.) - #64703 (Docs: slice elements are equidistant) - #64745 (Include message on tests that should panic but do not) - #64781 (Remove stray references to the old global tcx) - #64794 (Remove unused DepTrackingMap) - #64802 (Account for tail expressions when pointing at return type) - #64809 (hir: Disallow `target_feature` on constants) - #64815 (Fix div_duration() marked as stable by mistake) - #64818 (update rtpSpawn's parameters type(It's prototype has been updated in libc)) - #64830 (Thou shallt not `.abort_if_errors()`) - #64836 (Stabilize map_get_key_value feature) - #64845 (pin.rs: fix links to primitives in documentation) - #64847 (Upgrade env_logger to 0.7) - #64851 (Add mailmap entry for Dustin Bensing by request) Failed merges: - #64824 (No StableHasherResult everywhere) r? @ghost

…, r=rkruppe Docs: slice elements are equidistant Recently, someone asked why `[char]` and `str` are not interchangeable, and I explained that in a slice, the elements must be laid out equidistantly, whereas the chars in a `str` are stored compactly regardless their size. However I couldn't find this documented anywhere, so here's a small addition of this fact.

@ghost

Rollup of 14 pull requests Successful merges: - #64703 (Docs: slice elements are equidistant) - #64745 (Include message on tests that should panic but do not) - #64781 (Remove stray references to the old global tcx) - #64794 (Remove unused DepTrackingMap) - #64802 (Account for tail expressions when pointing at return type) - #64809 (hir: Disallow `target_feature` on constants) - #64815 (Fix div_duration() marked as stable by mistake) - #64818 (update rtpSpawn's parameters type(It's prototype has been updated in libc)) - #64830 (Thou shallt not `.abort_if_errors()`) - #64836 (Stabilize map_get_key_value feature) - #64845 (pin.rs: fix links to primitives in documentation) - #64847 (Upgrade env_logger to 0.7) - #64851 (Add mailmap entry for Dustin Bensing by request) - #64859 (check_match: improve diagnostics for `let A = 2;` with `const A: i32 = 3`) Failed merges: r? @ghost

llogiq · 2019-09-28T07:13:15Z

Thanks @petertodd for the correction! @rkruppe can you reapprove? Sorry @Centril if this interferes with your roll-up.

Centril · 2019-09-28T07:18:13Z

(Noted also on Discord):

Force pushing to a PR in a rollup has no effect on the rollup (which should not be cancelled unless there's a serious problem in the PR (a typo does not qualify)). You will need to rebase atop upstream-master after the rollup has been merged.

bors · 2019-09-28T07:37:35Z

☔ The latest upstream changes (presumably #64864) made this pull request unmergeable. Please resolve the merge conflicts.

Slice docs: fix typo With rust-lang#64703, I introduced a typo. Here is the fix. Sorry for the inconvenience.

rust-highfive assigned hanna-kruppe Sep 23, 2019

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Sep 23, 2019

llogiq force-pushed the slices-elems-are-equidistant branch from 4fc66db to 156f686 Compare September 24, 2019 19:33

llogiq force-pushed the slices-elems-are-equidistant branch from 156f686 to 6ccb7ae Compare September 27, 2019 17:44

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 27, 2019

petertodd reviewed Sep 27, 2019

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Centril mentioned this pull request Sep 28, 2019

Rollup of 14 pull requests #64860

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Centril mentioned this pull request Sep 28, 2019

Rollup of 14 pull requests #64864

Merged

Docs: slice elements are equidistant

410ce86

llogiq force-pushed the slices-elems-are-equidistant branch from 6ccb7ae to 410ce86 Compare September 28, 2019 07:07

bors added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Sep 28, 2019

llogiq mentioned this pull request Sep 28, 2019

Slice docs: fix typo #64880

Merged

Centril added a commit to Centril/rust that referenced this pull request Sep 28, 2019

Rollup merge of rust-lang#64880 - llogiq:slice-docs, r=Centril

4652671

Slice docs: fix typo With rust-lang#64703, I introduced a typo. Here is the fix. Sorry for the inconvenience.

hanna-kruppe closed this Sep 28, 2019

nikomatsakis mentioned this pull request Feb 14, 2020

ICE on beta and nightly as result of use of yield in combination with macro call in async block #67158

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Docs: slice elements are equidistant #64703

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Docs: slice elements are equidistant #64703

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