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stabilize never type - #155499

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stabilize never type#155499
WaffleLapkin wants to merge 19 commits into
rust-lang:mainfrom
WaffleLapkin:stabilize-never-type

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@WaffleLapkin WaffleLapkin commented Apr 18, 2026 •
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Closes #35121
Closes #148922

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@rustbot rustbot added A-rustdoc-js Area: Rustdoc's JS front-end A-rustdoc-json Area: Rustdoc JSON backend A-rustdoc-search Area: Rustdoc's search feature S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-clippy Relevant to the Clippy team. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. T-rust-analyzer Relevant to the rust-analyzer team, which will review and decide on the PR/issue. T-rustdoc Relevant to the rustdoc team, which will review and decide on the PR/issue. T-rustdoc-frontend Relevant to the rustdoc-frontend team, which will review and decide on the web UI/UX output. labels Apr 18, 2026
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WaffleLapkin marked this pull request as draft April 18, 2026 22:14
@rustbot 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
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@bors try

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rust-bors Bot pushed a commit that referenced this pull request Apr 18, 2026
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Auto merge of #155499 - WaffleLapkin:stabilize-never-type, r=<try>
1c4199d 
[experiment][0] nya
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☀️ Try build successful (CI)
Build commit: 1c4199d (1c4199da70f816f2239f41c55168ff75a3663718, parent: b2f1ccf524a3a4cf9c34545167cc23b659cf1cbd)

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

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

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👌 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 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
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🚧 Experiment pr-155499 is now running

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

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🎉 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

Footnotes

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

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@JonathanBrouwer JonathanBrouwer added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Aug 18, 2026
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JonathanBrouwer requested changes Aug 18, 2026
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Not finished with review yet, but this is the first part

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Comment thread compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs
}

fn never_type_behavior(tcx: TyCtxt<'_>) -> (DivergingFallbackBehavior, DivergingBlockBehavior) {
// FIXME(waffle): rip out the whole system which allows you to choose never type fallback

@JonathanBrouwer JonathanBrouwer Aug 18, 2026

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It would be nice to do this immediately after this PR lands, so that this is not forgotten. Rust analyzer seems to have its own copy of this enum, funnily enough.
Can we somehow make sure this is not forgotten?

@WaffleLapkin WaffleLapkin Aug 19, 2026

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I already have a commit for it in my local checkout, so I'm unlikely to forget:
jj commit (empty) TODO: rip out the never type fallback system, to make it simpler

Comment thread compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs
// For backwards compatibility reasons, we fall back to
// `()` until we the diverging default is changed.
EarlyBinder::bind(tcx, tcx.types.unit)
EarlyBinder::bind(tcx, tcx.types.never)

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This change looks right to me, but I feel like I don't have enough type system knowledge to be 100% sure about this change.

I wonder why this wasn't affected by the rustc_never_type_options attribute, was that an accident?

@WaffleLapkin WaffleLapkin Aug 19, 2026

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IIRC this wasn't affected by the attribute on purpose, but I don't remember the details.

Comment thread library/core/src/primitive_docs.rs
Comment thread library/core/src/primitive_docs.rs
Comment thread src/doc/edition-guide
Comment thread library/std/src/process.rs
Comment thread compiler/rustc_lint_defs/src/builtin.rs Outdated
@rustbot 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 18, 2026
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@WaffleLapkin
Remove From<!> for T *reservation* impl
158e13d
@WaffleLapkin
bless ui tests
7c696e5
@WaffleLapkin
Set never type fallback to always be !
1b2826d
@WaffleLapkin
stabilize the never type (!)
8a92d57
@WaffleLapkin
fixup clippy::empty_enums
21d61d5
@WaffleLapkin
remove #![feature(never_type)] from the compiler
2b86b05 
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...).
@WaffleLapkin
remove #![feature(never_type)] from tests
58fcec2
@WaffleLapkin
remove #![feature(never_type)] from library
f42429a
@WaffleLapkin
remove #![feature(never_type)] from tools
39dec73
@WaffleLapkin
bless tests
aaafa9e
@WaffleLapkin
bless mir-opt tests
bf0800b
@WaffleLapkin
update tests which depended on never fallback to ()
65963cd
@WaffleLapkin
remove dependency_on_unit_never_type_fallback lint
4b87bb8
@WaffleLapkin
remove stray stderr files
e71bafa 
??? why wasn't this detected by compiletest ???
@WaffleLapkin
make never_type_fallback_flowing_into_unsafe a non-FCW lint
95a0b68
@WaffleLapkin
Make Infallible = !
288eadd
@WaffleLapkin
remove unreachable code
8e6f818
@WaffleLapkin
fixup tests to account for Infallible = !
c5013a1
@WaffleLapkin
bless mir-opt tests
fc2e81c
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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.

@WaffleLapkin 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
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Successfully merging this pull request may close these issues.

Tracking Issue for never type fallback change Tracking issue for promoting ! to a type (RFC 1216)