Remove `MaybeUninit::uninit_array()` and replace it with inline const blocks. #125082

kpreid · 2024-05-13T15:54:27Z

[This PR originally contained the changes in #125995 too. See edit history for the original PR description.]

The documentation of MaybeUninit::uninit_array() says:

Note: in a future Rust version this method may become unnecessary when Rust allows inline const expressions. The example below could then use let mut buf = [const { MaybeUninit::<u8>::uninit() }; 32];.

The PR adding it also said: #65580 (comment)

if it’s stabilized soon enough maybe it’s not worth having a standard library method that will be replaceable with let buffer = [MaybeUninit::<T>::uninit(); $N];

That time has come to pass — inline const expressions are stable — so MaybeUninit::uninit_array() is now unnecessary. The only remaining question is whether it is an important enough convenience to keep it around.

I believe it is net good to remove this function, on the principle that it is better to compose two orthogonal features (MaybeUninit and array construction) than to have a specific function for the specific combination, now that that is possible.

rustbot · 2024-05-13T15:54:35Z

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ChayimFriedman2 · 2024-05-13T17:20:16Z

I still prefer MaybeUninit::uninit_array() to [const { MaybeUninit::uninit() }; N], it's shorter and more readable IMHO.

workingjubilee · 2024-05-14T20:34:40Z

The examples and the compiler code should probably still prefer the stable option.

Technically the compared-against example was [MaybeUninit::uninit(); N], which implies that the array initializer is a const context.

Nilstrieb · 2024-06-04T19:07:30Z

this fell down into the "i hope to get to this eventually but will probably never lol" part of my notification queue and i forgot to look at it again, sorry.
you have my approval for the first two commits, but I'd like a libs-api opinion on the last one. you can split out the first two into a new PR and i'll approve that, or you can keep them all like this if you're okay with letting the first two wait longer (as it will probably take a bit).
r? libs-api

kpreid · 2024-06-04T21:18:24Z

Split out the first two parts into #125995. I'll re-summarize this PR once that's merged.

…trieb Use inline const blocks to create arrays of `MaybeUninit`. This PR contains 2 changes enabled by the fact that [`inline_const` is now stable](rust-lang#104087), and was split out of rust-lang#125082. 1. Use inline const instead of `unsafe` to construct arrays in `MaybeUninit` examples. Rationale: Demonstrate good practice of avoiding `unsafe` code where it is not strictly necessary. 4. Use inline const instead of `unsafe` to implement `MaybeUninit::uninit_array()`. This is arguably giving the compiler more work to do, in exchange for eliminating just one single internal unsafe block, so it's less certain that this is good on net. r? `@Nilstrieb`

Rollup merge of rust-lang#125995 - kpreid:const-uninit-stable, r=Nilstrieb Use inline const blocks to create arrays of `MaybeUninit`. This PR contains 2 changes enabled by the fact that [`inline_const` is now stable](rust-lang#104087), and was split out of rust-lang#125082. 1. Use inline const instead of `unsafe` to construct arrays in `MaybeUninit` examples. Rationale: Demonstrate good practice of avoiding `unsafe` code where it is not strictly necessary. 4. Use inline const instead of `unsafe` to implement `MaybeUninit::uninit_array()`. This is arguably giving the compiler more work to do, in exchange for eliminating just one single internal unsafe block, so it's less certain that this is good on net. r? `@Nilstrieb`

bors · 2024-06-05T14:28:23Z

☔ The latest upstream changes (presumably #126016) made this pull request unmergeable. Please resolve the merge conflicts.

kpreid · 2024-06-05T20:04:41Z

This PR now consists solely of removing MaybeUninit::uninit_array().

compiler/rustc_data_structures/src/sip128.rs

library/core/src/fmt/float.rs

library/core/src/mem/maybe_uninit.rs

dtolnay

Thanks!

I am attempting to start a T-libs-api FCP for this in #96097 (comment).

https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra/topic/rfcbot.20asleep

Amanieu · 2024-06-11T17:09:02Z

We discussed this in the @rust-lang/libs-api meeting today. We are happy to remove uninit_array, but would like to keep it unstable to see if there is demand for a shorter-to-write version of the const block variant.

As such we would like to:

Point people to the const block syntax in the docs for uninit_array.
If after 2-3 releases of having stabilized const blocks nobody has commented in the tracking issue for uninit_array then just remove it.

kpreid · 2024-06-20T22:22:07Z

@Amanieu What should be done with the usage of uninit_array()? In other words, should this PR be just closed, or should it be trimmed down to only remove usage and not remove the function?

bors · 2024-06-22T19:00:56Z

☔ The latest upstream changes (presumably #116113) made this pull request unmergeable. Please resolve the merge conflicts.

clarfonthey · 2024-06-22T22:01:49Z

Presumably the answer would be deprecating the method and pointing to the tracking issue to get more feedback?

I still think that the correct answer would be the ability to index into MaybeUninit<[T; N]> directly to get MaybeUninit<T> references, but every time I bring this up folks don't seem to keen on the idea.

dtolnay · 2024-06-24T06:46:33Z

What should be done with the usage of uninit_array()? In other words, should this PR be just closed, or should it be trimmed down to only remove usage and not remove the function?

I would like to accept all the other changes in this PR other than the deletion of the function. I think we should go ahead and use our own recommended replacement in rustc that we are recommending everyone outside rustc to use.

I would also accept a #[deprecated = "..."] attribute on uninit_array with guidance in the deprecation message to use [MaybeUninit::uninit(); N] or [const { MaybeUninit::uninit() }; N] instead.

I still think that the correct answer would be the ability to index into MaybeUninit<[T; N]> directly to get MaybeUninit<T> references, but every time I bring this up folks don't seem too keen on the idea.

@clarfonthey do you have a link to this getting shot down? I am on board with this Index and IndexMut impl being added.

clarfonthey · 2024-06-24T14:08:30Z

I don't have any specific examples of an Index impl being shot down, although I can go and take a look later today to find some of the specific discussions I've been thinking of.

Most of them concern specifically impls for MaybeUninit<[T]> and the existence of that type in general, which has mostly been met with apprehension. It would require dedicated compiler support to make that work, since unsized unions aren't a thing right now.

Of course, for arrays, that restriction doesn't apply.

This is possible now that inline const blocks are stable; the idea was even mentioned as an alternative when `uninit_array()` was added: <rust-lang#65580 (comment)> > if it’s stabilized soon enough maybe it’s not worth having a > standard library method that will be replaceable with > `let buffer = [MaybeUninit::<T>::uninit(); $N];` Const array repetition and inline const blocks are now stable (in the next release), so that circumstance has come to pass, and we no longer have reason to want `uninit_array()` other than convenience. Therefore, let’s evaluate the inconvenience by not using `uninit_array()` in the standard library, before potentially deleting it entirely.

kpreid · 2024-06-24T17:30:24Z

I would like to accept all the other changes in this PR other than the deletion of the function.

Updated.

kpreid · 2024-06-24T17:30:52Z

@rustbot ready

@rustbot label -O-windows

dtolnay · 2024-06-24T17:41:53Z

@bors r+

bors · 2024-06-24T17:41:56Z

📌 Commit 13fca73 has been approved by dtolnay

It is now in the queue for this repository.

…mpiler-errors Rollup of 11 pull requests Successful merges: - rust-lang#124460 (Show notice about "never used" of Debug for enum) - rust-lang#124712 (Deprecate no-op codegen option `-Cinline-threshold=...`) - rust-lang#125082 (Remove `MaybeUninit::uninit_array()` and replace it with inline const blocks.) - rust-lang#125575 (SmartPointer derive-macro) - rust-lang#126413 (compiletest: make the crash test error message abit more informative) - rust-lang#126673 (Ensure we don't accidentally succeed when we want to report an error) - rust-lang#126682 (coverage: Overhaul validation of the `#[coverage(..)]` attribute) - rust-lang#126899 (Suggest inline const blocks for array initialization) - rust-lang#126904 (Small fixme in core now that NonZero is generic) - rust-lang#126909 (add `@kobzol` to bootstrap team for triagebot) - rust-lang#126911 (Split the lifetimes of `MirBorrowckCtxt`) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of rust-lang#125082 - kpreid:const-uninit, r=dtolnay Remove `MaybeUninit::uninit_array()` and replace it with inline const blocks. \[This PR originally contained the changes in rust-lang#125995 too. See edit history for the original PR description.] The documentation of `MaybeUninit::uninit_array()` says: > Note: in a future Rust version this method may become unnecessary when Rust allows [inline const expressions](rust-lang#76001). The example below could then use `let mut buf = [const { MaybeUninit::<u8>::uninit() }; 32];`. The PR adding it also said: <rust-lang#65580 (comment)> > if it’s stabilized soon enough maybe it’s not worth having a standard library method that will be replaceable with `let buffer = [MaybeUninit::<T>::uninit(); $N];` That time has come to pass — inline const expressions are stable — so `MaybeUninit::uninit_array()` is now unnecessary. The only remaining question is whether it is an important enough *convenience* to keep it around. I believe it is net good to remove this function, on the principle that it is better to compose two orthogonal features (`MaybeUninit` and array construction) than to have a specific function for the specific combination, now that that is possible.

rustbot assigned Nilstrieb May 13, 2024

rustbot added the T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. label Jun 4, 2024

rustbot assigned dtolnay and unassigned Nilstrieb Jun 4, 2024

kpreid mentioned this pull request Jun 4, 2024

Use inline const blocks to create arrays of MaybeUninit. #125995

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kpreid force-pushed the const-uninit branch from 5b157b5 to d1c71d6 Compare June 5, 2024 19:57

kpreid changed the title ~~Use inline const blocks to create arrays of MaybeUninit; remove uninit_array().~~ Remove MaybeUninit::uninit_array() and replace it with inline const blocks. Jun 5, 2024

dtolnay mentioned this pull request Jun 5, 2024

Tracking Issue for MaybeUninit methods for arrays #96097

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Rollup of 11 pull requests #126914

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Remove `MaybeUninit::uninit_array()` and replace it with inline const blocks. #125082

Remove `MaybeUninit::uninit_array()` and replace it with inline const blocks. #125082

kpreid commented May 13, 2024 •

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bors commented Jun 24, 2024

Remove MaybeUninit::uninit_array() and replace it with inline const blocks. #125082

Remove MaybeUninit::uninit_array() and replace it with inline const blocks. #125082

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kpreid commented May 13, 2024 • edited Loading

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Remove `MaybeUninit::uninit_array()` and replace it with inline const blocks. #125082

Remove `MaybeUninit::uninit_array()` and replace it with inline const blocks. #125082

kpreid commented May 13, 2024 •

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