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

WaffleLapkin · 2026-04-18T22:13:46Z

View all comments

This PR:

stabilizes the never type (!!!)
sets the never type fallback to ! on all editions (breaking change, see crater report analysis and refresher on never type fallback)
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:

Stabilization plan FCP:

Edition 2024: Make ! fall back to ! #123508

Lint bump:

Report never type lints in dependencies #141937

Rust 2024 edition change:

rustbot · 2026-04-18T22:13:54Z

⚠️ Warning ⚠️

Some commits in this PR modify submodules.

If this was not intentional, see I changed a submodule on accident in the rustc dev guide.

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 ↩

rust-rfcbot · 2026-06-11T00:42:18Z

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

theemathas · 2026-06-12T04:04:45Z

-            // Only suggest the `never_type` if the feature is enabled
-            && cx.tcx.features().never_type()


I think this Clippy lint should not fire if the project have an MSRV that supports Rust before this stabilization.

Also, I'm not sure I agree with current lint description which suggests writing a struct Thing(pub !);

View changes since the review

Checking MSRV requires knowing in which release the never type will be merged, so I'll leave this as a follow-up.

Don't really agree with the lint either, but that's unrelated to this PR.

I decided to ask on zulip: https://rust-lang.zulipchat.com/#narrow/channel/257328-clippy/topic/The.20empty_enums.20lint.20and.20the.20never.20type/near/602492777

ehuss · 2026-06-12T16:20:01Z

If we're changing this in this PR, then I think these docs need updating.

View changes since the review

ehuss · 2026-06-12T16:34:21Z

I'm unclear if the Infallible changes will also be made in this PR at the same time?

Either way, I recommend addressing the docs here saying never is unstable.

traviscross · 2026-06-12T16:41:27Z

I'm unclear if the Infallible changes will also be made in this PR at the same time?

@WaffleLapkin had mentioned wanting to land that in a separate PR. We do need to ensure these land in the same release, of course.

ehuss · 2026-06-12T17:26:31Z

What's the story with never_type_fallback_flowing_into_unsafe? I'm guessing you want to keep it for now? And then make it a hard error in the future?

If you want to keep it, I suggest updating it so that it is FutureReleaseSemanticsChange and to update its documentation so that it is updated for the never stabilization.

traviscross · 2026-06-12T19:09:37Z

What's the story with never_type_fallback_flowing_into_unsafe? I'm guessing you want to keep it for now? And then make it a hard error in the future?

Probably I wouldn't expect us to make this a hard error (the semantics are clear and making it a hard error would not simplify the language definition). I'm happy with it at deny-by-default for now. Looking at it:

    pub NEVER_TYPE_FALLBACK_FLOWING_INTO_UNSAFE,
    Deny,
    "never type fallback affecting unsafe function calls",
    @future_incompatible = FutureIncompatibleInfo {
        reason: fcw!(EditionAndFutureReleaseSemanticsChange 2024 "never-type-fallback"),
        report_in_deps: true,
    };
    @edition Edition2024 => Deny;
    report_in_external_macro

Probably I'd expect we want to remove the FCW- and edition-related bits and make this a plain deny-by-default lint — once this PR goes in, it will no longer be linting about anything happening in the future.

tbu- · 2026-06-13T21:13:09Z

Probably I'd expect we want to remove the FCW- and edition-related bits and make this a plain deny-by-default lint

This would mean it'd not show up for dependencies, which was useful.

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 · 2026-06-16T20:20:28Z

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.

rust-log-analyzer · 2026-06-16T20:26:38Z

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

fmt: checked 6917 files
tidy check
tidy [rustdoc_json (src)]: `rustdoc-json-types` modified, checking format version
tidy: Skipping binary file check, read-only filesystem
tidy [alphabetical (compiler)]: /checkout/compiler/rustc_ast_ir/src/lib.rs:9: line not in alphabetical order (tip: use --bless to sort this list)
tidy [alphabetical (compiler)]: FAIL
removing old virtual environment
creating virtual environment at '/checkout/obj/build/venv' using 'python3.10' and 'venv'
creating virtual environment at '/checkout/obj/build/venv' using 'python3.10' and 'virtualenv'
Requirement already satisfied: pip in ./build/venv/lib/python3.10/site-packages (26.1.1)
Collecting pip
---
info: ✓ ES-Check passed! All files are ES10 compatible.
typechecking javascript files
tidy: The following check failed: alphabetical (compiler)
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy --root-path=/checkout --cargo-path=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo --output-dir=/checkout/obj/build --concurrency=4 --npm-path=/node/bin/yarn --ci=true --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1624:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1511:29

--- BACKTRACE vvv
   0: <bootstrap::utils::exec::DeferredCommand>::finish_process
             at /checkout/src/bootstrap/src/utils/exec.rs:939:17
   1: <bootstrap::utils::exec::DeferredCommand>::wait_for_output::<&bootstrap::utils::exec::ExecutionContext>
             at /checkout/src/bootstrap/src/utils/exec.rs:831:21
   2: <bootstrap::utils::exec::ExecutionContext>::run
             at /checkout/src/bootstrap/src/utils/exec.rs:741:45
   3: <bootstrap::utils::exec::BootstrapCommand>::run::<&bootstrap::core::builder::Builder>
             at /checkout/src/bootstrap/src/utils/exec.rs:339:27
   4: <bootstrap::core::build_steps::test::Tidy as bootstrap::core::builder::Step>::run
             at /checkout/src/bootstrap/src/core/build_steps/test.rs:1511:29
   5: <bootstrap::core::builder::Builder>::ensure::<bootstrap::core::build_steps::test::Tidy>
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1596:36
   6: <bootstrap::core::build_steps::test::Tidy as bootstrap::core::builder::Step>::make_run
             at /checkout/src/bootstrap/src/core/build_steps/test.rs:1433:21
   7: <bootstrap::core::builder::StepDescription>::maybe_run
             at /checkout/src/bootstrap/src/core/builder/mod.rs:476:13
   8: bootstrap::core::builder::cli_paths::match_paths_to_steps_and_run
             at /checkout/src/bootstrap/src/core/builder/cli_paths.rs:232:18
   9: <bootstrap::core::builder::Builder>::run_step_descriptions
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1139:9
  10: <bootstrap::core::builder::Builder>::execute_cli
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1118:14
  11: <bootstrap::Build>::build
             at /checkout/src/bootstrap/src/lib.rs:803:25
  12: bootstrap::main
             at /checkout/src/bootstrap/src/bin/main.rs:130:11
  13: <fn() as core::ops::function::FnOnce<()>>::call_once
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/core/src/ops/function.rs:250:5
  14: std::sys::backtrace::__rust_begin_short_backtrace::<fn(), ()>
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/sys/backtrace.rs:166:18
  15: std::rt::lang_start::<()>::{closure#0}
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/rt.rs:206:18
  16: <&dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe as core::ops::function::FnOnce<()>>::call_once
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/core/src/ops/function.rs:287:21
  17: std::panicking::catch_unwind::do_call::<&dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe, i32>
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/panicking.rs:581:40
  18: std::panicking::catch_unwind::<i32, &dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe>
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/panicking.rs:544:19
  19: std::panic::catch_unwind::<&dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe, i32>
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/panic.rs:359:14
  20: std::rt::lang_start_internal::{closure#0}
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/rt.rs:175:24
  21: std::panicking::catch_unwind::do_call::<std::rt::lang_start_internal::{closure#0}, isize>
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/panicking.rs:581:40
---
  28: __libc_start_main
  29: _start


Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:02:44
  local time: Tue Jun 16 20:26:26 UTC 2026
  network time: Tue, 16 Jun 2026 20:26:26 GMT
##[error]Process completed with exit code 1.
##[group]Run echo "disk usage:"

This comment was marked as off-topic.

Sign in to view

rustbot assigned TaKO8Ki Apr 18, 2026

This comment was marked as off-topic.

Sign in to view

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

This comment has been minimized.

Sign in to view

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

This comment has been minimized.

Sign in to view

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

theemathas reviewed Jun 12, 2026

View reviewed changes

mkrasnitski mentioned this pull request Jun 12, 2026

Update wording for ! (never type) docs #157823

Open

ehuss reviewed Jun 12, 2026

View reviewed changes

This was referenced Jun 12, 2026

Update never-type-fallback for never type stabilization rust-lang/edition-guide#384

Draft

Update for stabilization of the never type rust-lang/reference#2283

Open

WaffleLapkin force-pushed the stabilize-never-type branch from 02de51f to 5675e12 Compare June 16, 2026 17:12

This comment has been minimized.

Sign in to view

WaffleLapkin added 13 commits June 16, 2026 22:18

Set never type fallback to always be !

2db9efe

stabilize the never type (!)

d4256a7

fixup clippy::empty_enums

9535220

remove #![feature(never_type)] from tests

fdbb4b1

remove #![feature(never_type)] from library

5be2ceb

remove #![feature(never_type)] from tools

34d7aeb

bless tests

22d64cd

bless mir-opt tests

f09fc72

update tests which depended on never fallback to ()

7fa243a

remove dependency_on_unit_never_type_fallback lint

4ecbae4

remove stray stderr files

963658b

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

update edition-guide submodule to unfuck tests there

8b0d675

WaffleLapkin force-pushed the stabilize-never-type branch from 5675e12 to 8b0d675 Compare June 16, 2026 20:20

		// Only suggest the `never_type` if the feature is enabled
		&& cx.tcx.features().never_type()

Uh oh!

Conversation

WaffleLapkin commented Apr 18, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

This comment was marked as off-topic.

This comment was marked as off-topic.

rustbot commented Apr 18, 2026

Uh oh!

WaffleLapkin commented Apr 18, 2026

Uh oh!

This comment has been minimized.

This comment has been minimized.

rust-bors Bot commented Apr 19, 2026

Uh oh!

WaffleLapkin commented Apr 19, 2026

Uh oh!

craterbot commented Apr 19, 2026

Uh oh!

craterbot commented Apr 20, 2026

Uh oh!

craterbot commented Apr 20, 2026

Footnotes

Uh oh!

rust-rfcbot commented Jun 11, 2026

Uh oh!

theemathas Jun 12, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

WaffleLapkin Jun 12, 2026

Choose a reason for hiding this comment

Uh oh!

theemathas Jun 12, 2026

Choose a reason for hiding this comment

Uh oh!

ehuss Jun 12, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

ehuss commented Jun 12, 2026

Uh oh!

traviscross commented Jun 12, 2026

Uh oh!

ehuss commented Jun 12, 2026

Uh oh!

traviscross commented Jun 12, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

tbu- commented Jun 13, 2026

Uh oh!

This comment has been minimized.

This comment has been minimized.

rustbot commented Jun 16, 2026

Uh oh!

rust-log-analyzer commented Jun 16, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

19 participants

WaffleLapkin commented Apr 18, 2026 •

edited by rustbot

Loading

theemathas Jun 12, 2026 •

edited by rustbot

Loading

ehuss Jun 12, 2026 •

edited by rustbot

Loading

traviscross commented Jun 12, 2026 •

edited

Loading