fix unsoundness when compiling `[diverging_const; 0]` by WaffleLapkin · Pull Request #165416 · rust-lang/rust

WaffleLapkin · 2026-11-03T15:42:28Z

As I described in #165323 (comment), currently [diverging_const; 0] leads to usoundness because hir type check and mir building disagree on whatever diverging_const is evaluated or not. hir type check assumes that it does (and marks anything after it as unreachable) while mir building sees that the constant is repeated 0 times and decides to skip evaluating the constant, resulting in unsoundness.

This PR fixes this by doing the same kind of special case in hir type check.

Fixes (but does not close as this is a temporary workaround) #165323

Previusly we had a disagreement between hir_typeck (assuming that anything following the diverging constant is unreachable) and mir_build (assuming we don't need to evaluate constants in `[...; 0]`). This (temporarily) fixes this by making hir_typeck aware that such constants should not affect divergence / reachibility analysis

rustbot · 2026-11-03T15:42:34Z

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theemathas · 2026-11-03T15:44:34Z

Does this need a crater run, and does this need to be beta-nominated?

WaffleLapkin · 2026-11-03T15:45:26Z

Nominating for a beta backport. This fixes an unsoundness with the never type, that is currently stable in beta (and which will be stable in ~9 days).

WaffleLapkin · 2026-11-03T15:46:39Z

@theemathas I don't think this needs a crater run; this can only move code from being unsound to not compiling (and you can only write the unsound code by mentioning ! that was unstable before).

WaffleLapkin · 2026-11-03T15:47:40Z

+        let count_is_zero = {
+            if let Ok(count) = tcx.try_normalize_erasing_regions(
+                self.typing_env(self.param_env),
+                Unnormalized::new_wip(count),
+            ) {


It's unclear to me if this does anything / is needed, given the self.normalize call above. cc @khyperia

View changes since the review

yea I think just running the try_to_value method chan below would suffice, no need to try to reevaluate

oli-obk · 2026-11-03T16:02:28Z

isn't this the wrong way around? Shouldn't we instead eval the const (or at least eval it if it's uninhabited)? We put a lot of effort into making sure all runtime-reachable const blocks or uses of const items are evaluated, even if optimized out.

Edit: ah this was discussed on the issue, could have still just done that for constants of type ! without it being a breaking change and then figuring out if we can extend that to arbitrary consts without it being an issue (it shouldn't be)

oli-obk

While I would prefer to ensure the consts aren't dropped on the floor by mir building, this change is ready and can be merged now, and is strictly rejecting more code than fixing the issue properly.

@bors r+ p=1

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rust-bors · 2026-11-03T16:11:44Z

📌 Commit 23d29d9 has been tentatively approved by oli-obk

It will be put into the queue for this repository once PR CI succeeds.

JonathanBrouwer · 2026-11-03T16:20:49Z

@bors r=oli-obk force
I'll add this to the current rollup so this gets merged asap for more testing hours

rust-bors · 2026-11-03T16:20:55Z

📌 Commit 23d29d9 has been approved by oli-obk

It is now in the queue for this repository.

…ted_zero_times, r=oli-obk fix unsoundness when compiling `[diverging_const; 0]` As I described in rust-lang#165323 (comment), currently `[diverging_const; 0]` leads to usoundness because hir type check and mir building disagree on whatever `diverging_const` is evaluated or not. hir type check assumes that it does (and marks anything after it as unreachable) while mir building sees that the constant is repeated 0 times and decides to skip evaluating the constant, resulting in unsoundness. This PR fixes this by doing the same kind of special case in hir type check. Fixes rust-lang#165323

theemathas · 2026-11-03T16:56:24Z

Should #165323 be kept open after this PR merges, since it seems like the behavior isn't finalized yet?

…uwer Rollup of 21 pull requests Successful merges: - #165416 (fix unsoundness when compiling `[diverging_const; 0]`) - #149245 (Make inline attributes apply to the generated `poll` in async fn) - #163495 (Parallelize AST -> HIR lowering) - #164304 ([perf] Reuse CanonicalVarKinds if no new variables were added) - #165109 (Avoid building a region map when erasing bound regions) - #163393 (std: Use getexecpath() for current_exe() on OpenBSD) - #163625 (Add a lint helper attribute namespace ) - #164414 (Add `let_underscore_drop_guard` lint.) - #165342 (Avoid unnecessary MIR traversals in CheckPackedRef lint) - #165351 (delegation: insert probing adjustments before args check adjustments) - #165358 (Reenable some cg_gcc tests that were disabled) - #165377 (Make `no_implicit_prelude` not suppress tool attributes) - #165397 (Add codegen test for unused match arms removed after inlining) - #165398 (Rework parallel reproducibility tests) - #162608 (Avoid oscillating line wrapping due to symbol indices) - #164241 (make the safety documentation consistent in Ptr module) - #165029 (Use `MalformedLintAttributeInput` in more places) - #165203 (AIX: Implement TLS destructor guard in std and update dynamic-loading-cdylib test) - #165230 (Fix rigidity assertion ICE) - #165293 (unstable-book: update default-visibility.md to link to tracking issue) - #165374 (Refactorings in preparation for the move trait)

rust-bors · 2026-11-03T18:46:25Z

⌛ Testing commit 23d29d9 with merge 69e1d01...

Workflow: https://github.com/rust-lang/rust/actions/runs/41696987455

…mes, r=oli-obk fix unsoundness when compiling `[diverging_const; 0]` As I described in #165323 (comment), currently `[diverging_const; 0]` leads to usoundness because hir type check and mir building disagree on whatever `diverging_const` is evaluated or not. hir type check assumes that it does (and marks anything after it as unreachable) while mir building sees that the constant is repeated 0 times and decides to skip evaluating the constant, resulting in unsoundness. This PR fixes this by doing the same kind of special case in hir type check. Fixes (but does not close as this is a temporary workaround) #165323

JonathanBrouwer · 2026-11-03T18:46:56Z

💔 I suspect this PR failed tests as part of a rollup
@bors r-

After fixing the problem, consider running a try job for the failed job before re-approving.

Link to failure: #165418 (comment)

rust-bors · 2026-11-03T18:47:07Z

This pull request was unapproved.

This PR was contained in a rollup (#165418), which was unapproved.

Auto build was cancelled due to unapproval. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/41696987455

View changes since this unapproval

JonathanBrouwer · 2026-11-03T19:23:25Z

Added a commit that should fix the problem
@bors r=oli-obk
@bors try jobs=test-x86_64-gnu-llvm-22-3

rust-bors · 2026-11-03T19:23:28Z

📌 Commit b7b37d2 has been tentatively approved by oli-obk

It will be put into the queue for this repository once PR CI succeeds.

…mes, r=<try> fix unsoundness when compiling `[diverging_const; 0]` try-job: test-x86_64-gnu-llvm-22-3

JonathanBrouwer · 2026-11-03T19:42:05Z

@bors r=oli-obk force

rust-bors · 2026-11-03T19:42:08Z

📌 Commit b7b37d2 has been approved by oli-obk

It is now in the queue for this repository.

…uwer Rollup of 7 pull requests Successful merges: - #165416 (fix unsoundness when compiling `[diverging_const; 0]`) - #163217 ([perf] rustdoc: Avoid re-HIR-ty-lowering tys & const args where possible) - #165371 (Don't use `llvm_type` as the type of `static`s.) - #165354 (Remove `AbiAlign`) - #165357 (rustdoc: Fix invalid clean of paths when they end with `self`) - #165401 (Add codegen test for step_by(1) loop vectorization) - #165408 (LLVMRustWriteOutputFile: Check the result of runCodeGenPipeline)

rust-bors · 2026-11-03T21:34:58Z

☀️ Try build successful (CI)
Build commit: 105361a (105361a1d48f09da1413502159a3c5a18f512c87)
Base parent: a94dce0 (a94dce0bbb3aab81bfdcc30e8a55d7dc95bc644d)

Rollup merge of #165416 - WaffleLapkin:diverging_const_repeated_zero_times, r=oli-obk fix unsoundness when compiling `[diverging_const; 0]` As I described in #165323 (comment), currently `[diverging_const; 0]` leads to usoundness because hir type check and mir building disagree on whatever `diverging_const` is evaluated or not. hir type check assumes that it does (and marks anything after it as unreachable) while mir building sees that the constant is repeated 0 times and decides to skip evaluating the constant, resulting in unsoundness. This PR fixes this by doing the same kind of special case in hir type check. Fixes (but does not close as this is a temporary workaround) #165323

rustbot · 2026-11-04T08:43:45Z

beta backport approved as per compiler team on Zulip. A backport PR will be authored by the release team at the end of the current development cycle. Backport labels are handled by them.

…uwer Rollup of 7 pull requests Successful merges: - rust-lang/rust#165416 (fix unsoundness when compiling `[diverging_const; 0]`) - rust-lang/rust#163217 ([perf] rustdoc: Avoid re-HIR-ty-lowering tys & const args where possible) - rust-lang/rust#165371 (Don't use `llvm_type` as the type of `static`s.) - rust-lang/rust#165354 (Remove `AbiAlign`) - rust-lang/rust#165357 (rustdoc: Fix invalid clean of paths when they end with `self`) - rust-lang/rust#165401 (Add codegen test for step_by(1) loop vectorization) - rust-lang/rust#165408 (LLVMRustWriteOutputFile: Check the result of runCodeGenPipeline)

rustbot assigned wesleywiser Nov 3, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Nov 3, 2026

WaffleLapkin added A-never-type Area: `!` type. For issues not specific to `!`, use A-uninhabited-types. beta-nominated Nominated for backporting to the compiler in the beta channel. labels Nov 3, 2026

WaffleLapkin commented Nov 3, 2026

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oli-obk approved these changes Nov 3, 2026

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rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Nov 3, 2026

JonathanBrouwer mentioned this pull request Nov 3, 2026

Rollup of 21 pull requests #165418

Closed

rust-bors Bot 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-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Nov 3, 2026

Always disable backtrace in zero_diverging_consts_runnable.rs

b7b37d2

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Nov 3, 2026

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rust-bors Bot pushed a commit that referenced this pull request Nov 3, 2026

Auto merge of #165416 - WaffleLapkin:diverging_const_repeated_zero_ti…

105361a

…mes, r=<try> fix unsoundness when compiling `[diverging_const; 0]` try-job: test-x86_64-gnu-llvm-22-3

JonathanBrouwer mentioned this pull request Nov 3, 2026

Rollup of 7 pull requests #165432

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rust-bors Bot merged commit dc62573 into rust-lang:main Nov 3, 2026
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rustbot added this to the 1.101.0 milestone Nov 3, 2026

rustbot added the beta-accepted Accepted for backporting to the compiler in the beta channel. label Nov 4, 2026

JonathanBrouwer mentioned this pull request Nov 4, 2026

[beta] backports #165451

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JonathanBrouwer modified the milestones: 1.101.0, 1.100.0 Nov 4, 2026

JonathanBrouwer removed the beta-nominated Nominated for backporting to the compiler in the beta channel. label Nov 4, 2026

WaffleLapkin deleted the diverging_const_repeated_zero_times branch November 4, 2026 12:36

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