Guarantee that the never type `!` is zero-sized and 1-aligned. by zachs18 · Pull Request #2309 · rust-lang/reference

zachs18 · 2026-07-08T17:10:05Z

Needs T-lang FCP.

zachs18 · 2026-08-25T20:32:57Z

@rustbot label I-lang-nominated

cc @traviscross

rustbot · 2026-08-25T20:33:00Z

Error: Label I-lang-nominated can only be set by Rust team members

Please file an issue on GitHub at triagebot if there's a problem with this bot, or reach out on #triagebot on Zulip.

DanielEScherzer · 2026-09-01T18:13:06Z

 r[type.never.coercion]
 Expressions of type `!` can be coerced into any type.

+


no need for two blank lines here, one should be enough

View changes since the review

Latest push removes the extra blank line.

DanielEScherzer · 2026-09-01T18:17:08Z

 assert_eq!(0, size_of::<E4>());
 assert_eq!(0, size_of::<E5>());
 assert_eq!(0, size_of::<E6>());
 assert_eq!(0, size_of::<E7>());


there should also be an example in the test block here

Suggested change

assert_eq!(0, size_of::<!>());

View changes since the review

Latest push adds this example at the top of the list just after ()'s line, so they're still in the same order as the list in the prose.

traviscross · 2026-09-01T18:51:29Z

Thanks @zachs18.

@rfcbot fcp merge lang,opsem

rust-rfcbot · 2026-09-01T18:51:31Z

joshlf · 2026-09-01T18:57:06Z

@rfcbot reviewed

saethlin · 2026-09-01T22:05:17Z

@rfcbot reviewed

CAD97 · 2026-09-02T03:20:48Z

@rfcbot reviewed

RalfJung · 2026-09-02T06:33:44Z

@rfcbot reviewed

tmandry · 2026-09-02T19:33:17Z

@rfcbot reviewed

RalfJung · 2026-09-20T20:49:20Z

@digama0 @joshtriplett @scottmcm @nikomatsakis friendly checkbox reminder -- I think this should be fairly uncontroversial :)

rust-rfcbot · 2026-09-30T15:41:24Z

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

scottmcm · 2026-09-30T15:43:04Z

Agreed, Ralf -- I see no reason anyone could possibly want anything else 🙃
@rfcbot reviewed

tgross35 · 2026-09-30T18:26:31Z

This has tests for size. Should it also have a test for alignment? Or is that covered elsewhere?

View changes since the review

We probably should have tests for alignment, but we don't have tests for other types' alignments in the reference (including bool which is also guaranteed to have an alignment of 1 (AFAICT we don't have tests for bool's size or alignment in the reference or rust-lang/rust)), so I don't think we need to add it in this PR (especially not the "Zero-sized type" section, which is specifically about size).

I opened rust-lang/rust#163570 to add tests for ! (and bool)'s size/align to coretests.

Size-1 types cannot have any alignment other than 1 anyway.

You can add this to the glossary n-zst entry as an example of a 1-ZST, https://github.com/rust-lang/reference/blob/a286e1ecb8ce193d50056e4bb042eea9b23876ef/src/glossary.md#n-aligned-zero-sized-type-n-zst

rust-rfcbot · 2026-10-10T15:44:25Z

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

We'd referred to "the never type `!`". Mostly we should say either "the never type" or "`!`"; using parentheses sparingly, as in "the never type (`!`)", is also OK. Let's fix this and a related case.

Since we're guaranteeing the alignment of the never type, let's add it as an example of a 1-ZST in the glossary.

It might be a good idea to link to the pages for the types listed in this table, but we don't do it for the others, so let's not do it for the never type.

rustbot · 2026-10-11T09:01:36Z

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

traviscross · 2026-10-11T09:02:51Z

Thanks @zachs18.

zachs18 mentioned this pull request Jul 8, 2026

Empty repr(Rust) enums are ZSTs #2293

Merged

zachs18 force-pushed the never-1zst branch from 7c443b7 to 39d7c5a Compare July 8, 2026 17:30

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rustbot added the S-waiting-on-author Status: The marked PR is awaiting some action (such as code changes) from the PR author. label Aug 11, 2026

zachs18 force-pushed the never-1zst branch from 39d7c5a to 75131fa Compare August 25, 2026 20:24

zachs18 marked this pull request as ready for review August 25, 2026 20:25

rustbot added the S-waiting-on-review Status: The marked PR is awaiting review from a maintainer label Aug 25, 2026

traviscross added I-lang-nominated Nominated for discussion during a lang team meeting. needs-fcp I-lang-radar Items that are on lang's radar and will need eventual work or consideration. P-lang-drag-1 Lang team prioritization drag level 1. labels Aug 25, 2026

DanielEScherzer reviewed Sep 1, 2026

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traviscross added T-lang Relevant to the language team. T-opsem Team: opsem labels Sep 1, 2026

rust-rfcbot added proposed-final-comment-period disposition-merge and removed needs-fcp labels Sep 1, 2026

rust-rfcbot added final-comment-period and removed proposed-final-comment-period labels Sep 30, 2026

tmandry removed I-lang-nominated Nominated for discussion during a lang team meeting. P-lang-drag-1 Lang team prioritization drag level 1. labels Sep 30, 2026

tgross35 reviewed Sep 30, 2026

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rust-rfcbot added finished-final-comment-period to-announce and removed final-comment-period labels Oct 10, 2026

zachs18 and others added 5 commits October 10, 2026 23:56

Add layout guarantees for never type !

2368fdc

Fix spacing in never type docs, and add never type size_of example.

77b15d5

Fix mentions of the never type in the glossary

b07db38

We'd referred to "the never type `!`". Mostly we should say either "the never type" or "`!`"; using parentheses sparingly, as in "the never type (`!`)", is also OK. Let's fix this and a related case.

Add ! to list of 1-ZSTs in the glossary

fb0abea

Since we're guaranteeing the alignment of the never type, let's add it as an example of a 1-ZST in the glossary.

Remove link from ! row in primitive size table

956d1bd

It might be a good idea to link to the pages for the types listed in this table, but we don't do it for the others, so let's not do it for the never type.

traviscross force-pushed the never-1zst branch from c0374f2 to 956d1bd Compare October 11, 2026 09:01

traviscross approved these changes Oct 11, 2026

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traviscross enabled auto-merge October 11, 2026 09:02

traviscross added this pull request to the merge queue Oct 11, 2026

Merged via the queue into rust-lang:master with commit 5baa209 Oct 11, 2026
8 checks passed

rustbot removed the S-waiting-on-review Status: The marked PR is awaiting review from a maintainer label Oct 11, 2026

		r[type.never.coercion]
		Expressions of type `!` can be coerced into any type.

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