stabilize never type by WaffleLapkin · Pull Request #155499 · rust-lang/rust

WaffleLapkin · 2026-04-18T22:13:46Z

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This PR

stabilizes the never type (aka !) (!!!)
sets the never type fallback to ! on all editions (breaking change, see crater report analysis and refresher on never type fallback)
makes Infallible an alias to !
removes dependency_on_unit_never_type_fallback lint (there is no more never type fallback to () so this lint can't be triggered)

Cat

Tracking

FCPs

Edition 2024: Make ! fall back to ! #123508 (comment) (stabilization plan, T-lang)
stabilize never type #155499 (comment) (never type stabilization modulo bugs, T-lang)
Make Infallible = ! #155924 (comment) (make Infallible = !, T-libs)

Related changes

Lint bump: Report never type lints in dependencies #141937
Rust 2024 edition change:
- Tracking Issue for Rust 2024: Make ! fall back to ! #123748
- Edition 2024: Make ! fall back to ! #123508
Various changes to lessen the effect of the breaking changes:
Never type documentation changes: rewrite never type documentation #158370 (blocked on this pr)

Experiments

Fallback change and never type stabilization:
- Run: stabilize never type #155499 (comment)
- Analysis: stabilize never type #155499 (comment)
- Status of backports: stabilize never type #155499 (comment)
Fallback change, never type stabilization and making Infallible = !: [experiment][1] mew #155500
Fallback change, never type stabilization, making Infallible = !, and removing reservation impl: [experiment][2] meow mrrrumph #155501
Never type stabilization and Infallible = ! change, without fallback change: [experiment][3] meowww #155657

Closes #35121
Closes #148922

WaffleLapkin · 2026-04-18T22:23:36Z

@bors try

[experiment][0] nya

rust-bors · 2026-04-19T00:36:04Z

☀️ Try build successful (CI)
Build commit: 1c4199d (1c4199da70f816f2239f41c55168ff75a3663718, parent: b2f1ccf524a3a4cf9c34545167cc23b659cf1cbd)

WaffleLapkin · 2026-04-19T01:07:46Z

@craterbot check p=1 crates=https://crater-reports.s3.amazonaws.com/pr-154887/retry-regressed-list.txt

craterbot · 2026-04-19T01:07:50Z

👌 Experiment pr-155499 created and queued.
🤖 Automatically detected try build 1c4199d
⚠️ Try build based on commit 2895d33, but latest commit is 3e8df8d. Did you forget to make a new try build?
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2026-04-20T04:32:44Z

🚧 Experiment pr-155499 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2026-04-20T08:37:49Z

🎉 Experiment pr-155499 is completed!
📊 3277 regressed and 0 fixed (9024 total)
📊 1149 spurious results on the retry-regressed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-155499/retry-regressed-list.txt ↩

The reason `rustc_ast_ir` and `rustc_pattern_analysis` have nested `cfg_attr`s is so that they don't mention `cfg(bootstrap)` when nightly or rustc cargo feature turned on. If you don't do it like that, they stop compiling on stable (or rather, they emit unexpected cfg warnings...).

??? why wasn't this detected by compiletest ???

rustbot · 2026-08-19T14:35:35Z

This PR was rebased onto a different main 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.

JonathanBrouwer · 2026-08-20T20:42:50Z

+   |
+LL ~     panic!();
+LL +     let _data: [!; 0] = [];
+   |


Hmmm I wonder how this changed

View changes since the review

Oh the suggestion was previously not made because ! was unnameable, that is very cool. This suggestion is no longer unfixable so we can move it to the normal zero_repeat_side_effects.rs test

JonathanBrouwer · 2026-08-20T20:51:25Z

Trying commonly failed jobs
@bors try jobs=dist-various-1,test-various,x86_64-gnu-aux,x86_64-gnu-llvm-21-3,x86_64-msvc-1,aarch64-apple-1,aarch64-apple-2,x86_64-mingw-1,i686-msvc-1,i686-msvc-2

stabilize never type try-job: dist-various-1 try-job: test-various try-job: x86_64-gnu-aux try-job: x86_64-gnu-llvm-21-3 try-job: x86_64-msvc-1 try-job: aarch64-apple-1 try-job: aarch64-apple-2 try-job: x86_64-mingw-1 try-job: i686-msvc-1 try-job: i686-msvc-2

JonathanBrouwer

Reviewed the entire diff now :)

View changes since this review

rust-bors · 2026-08-21T00:01:50Z

☀️ Try build successful (CI)
Build commit: 565d1d2 (565d1d2f5710463a3a055f4d2d767dcc75e78a0f)
Base parent: 8925ea3 (8925ea358a0f265ca61026aadc7ecc506c545cbe)

rust-bors · 2026-08-21T02:32:11Z

☔ The latest upstream changes (presumably #161043) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

tshepang · 2026-08-21T11:12:12Z

 //
-//@ revisions: e2021 e2024
-//@[e2024] edition: 2024
+//@ edition: 2018..2024


looks like this test ran on edition 2015, even though the revision name implied (before this change) that 2021 is the lowest edition that is applicable

View changes since the review

tshepang · 2026-08-21T11:24:11Z

@@ -1,5 +1,4 @@
-//@ revisions: e2015 e2024
-//@[e2024] edition:2024
+//@ edition: 2015..2024


this test will accidentally be missed when the next edition happens
we can use 2015.. to say all-of-em

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WaffleLapkin marked this pull request as draft April 18, 2026 22:14

rustbot 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-review Status: Awaiting review from the assignee but also interested parties. labels Apr 18, 2026

WaffleLapkin unassigned TaKO8Ki Apr 18, 2026

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rust-bors Bot pushed a commit that referenced this pull request Apr 18, 2026

Auto merge of #155499 - WaffleLapkin:stabilize-never-type, r=<try>

1c4199d

[experiment][0] nya

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WaffleLapkin force-pushed the stabilize-never-type branch from 2895d33 to 3e8df8d Compare April 19, 2026 01:05

craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Apr 19, 2026

WaffleLapkin added 14 commits August 19, 2026 16:30

remove #![feature(never_type)] from tests

58fcec2

remove #![feature(never_type)] from library

f42429a

remove #![feature(never_type)] from tools

39dec73

bless tests

aaafa9e

bless mir-opt tests

bf0800b

update tests which depended on never fallback to ()

65963cd

remove dependency_on_unit_never_type_fallback lint

4b87bb8

remove stray stderr files

e71bafa

??? why wasn't this detected by compiletest ???

make never_type_fallback_flowing_into_unsafe a non-FCW lint

95a0b68

Make Infallible = !

288eadd

remove unreachable code

8e6f818

fixup tests to account for Infallible = !

c5013a1

bless mir-opt tests

fc2e81c

WaffleLapkin force-pushed the stabilize-never-type branch from 6f4ed82 to 66d5e72 Compare August 19, 2026 14:35

WaffleLapkin added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Aug 19, 2026

WaffleLapkin force-pushed the stabilize-never-type branch from 66d5e72 to fc2e81c Compare August 19, 2026 14:39

JonathanBrouwer reviewed Aug 20, 2026

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rustbot 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-review Status: Awaiting review from the assignee but also interested parties. labels Aug 20, 2026

tshepang reviewed Aug 21, 2026

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