Check closure's constness validity in the constness query by oli-obk · Pull Request #155772 · rust-lang/rust

oli-obk · 2026-04-25T11:09:51Z

instead of checking during ast lowering, where it's not easily possible to obtain all the right information in time. While lowering an assoc item we don't know if the parent was a const trait or a const impl. Tracing this information is quite annoying, and complicates a lot of code, which checking here after the fact is trivial.

rustbot · 2026-04-25T11:09:55Z

Some changes occurred to the CTFE machinery

cc @RalfJung, @lcnr

rustbot · 2026-04-25T11:09:57Z

r? @mati865

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oli-obk · 2026-04-25T11:10:22Z

   |
 LL |     (const || { (()).foo() })();
-   |      ^^^^^
+   |      ^^^^^^^^


The only downside of this PR is that we highlight more than the const part of the closure now

View changes since the review

fee1-dead

r=me after

View changes since this review

fee1-dead · 2026-04-25T12:31:06Z

+        Node::Expr(e) if let ExprKind::Closure(c) = e.kind => {
+            match c.constness {
+                Constness::Const => if tcx.hir_body_const_context(tcx.local_parent(def_id)).is_none() {
+                    tcx.dcx().span_err(tcx.def_span(def_id), "cannot use `const` closures outside of const contexts");


A hacky way could be to use https://doc.rust-lang.org/stable/nightly-rustc/rustc_span/source_map/struct.SourceMap.html#method.span_until_whitespace since we always know that const is the first keyword (for now).

But this is not necessary

fee1-dead · 2026-04-25T12:31:39Z

+            match c.constness {
+                Constness::Const => if tcx.hir_body_const_context(tcx.local_parent(def_id)).is_none() {


I would use if let Constness::Const = c.constness && tcx.hir_body_const_context(tcx.local_parent(def_id)).is_none() here

rustbot · 2026-04-25T12:31:55Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

oli-obk · 2026-04-25T12:53:40Z

@bors r=fee1-dead rollup

rust-bors · 2026-04-25T12:53:43Z

📌 Commit 415454f has been approved by fee1-dead

It is now in the queue for this repository.

oli-obk · 2026-04-25T18:56:39Z

@bors r=fee1-dead rollup

rust-bors · 2026-04-25T18:56:41Z

📌 Commit ae91b21 has been approved by fee1-dead

It is now in the queue for this repository.

… r=fee1-dead Check closure's constness validity in the constness query fixes rust-lang#155584 instead of checking during ast lowering, where it's not easily possible to obtain all the right information in time. While lowering an assoc item we don't know if the parent was a const trait or a const impl. Tracing this information is quite annoying, and complicates a lot of code, which checking here after the fact is trivial.

JonathanBrouwer · 2026-04-25T20:30:24Z

@bors r-
#155791 (comment)

rust-bors · 2026-04-25T20:30:30Z

This pull request was unapproved.

This PR was contained in a rollup (#155791), which was unapproved.

View changes since this unapproval

instead of during ast lowering, where it's not easily possible to obtain all the right information in time

rustbot · 2026-04-27T05:45:43Z

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.

oli-obk · 2026-04-27T05:45:47Z

@bors r=fee1-dead rollup

rust-bors · 2026-04-27T05:45:50Z

📌 Commit 8e1c34f has been approved by fee1-dead

It is now in the queue for this repository.

rust-bors · 2026-04-27T05:55:07Z

⌛ Testing commit 8e1c34f with merge 7a841ad...

Workflow: https://github.com/rust-lang/rust/actions/runs/24978916136

… r=fee1-dead Check closure's constness validity in the constness query fixes rust-lang#155584 instead of checking during ast lowering, where it's not easily possible to obtain all the right information in time. While lowering an assoc item we don't know if the parent was a const trait or a const impl. Tracing this information is quite annoying, and complicates a lot of code, which checking here after the fact is trivial.

Check closure's constness validity in the constness query fixes #155584 instead of checking during ast lowering, where it's not easily possible to obtain all the right information in time. While lowering an assoc item we don't know if the parent was a const trait or a const impl. Tracing this information is quite annoying, and complicates a lot of code, which checking here after the fact is trivial.

jhpratt · 2026-04-27T05:55:35Z

@bors yield

to encompassing rollup

rust-bors · 2026-04-27T05:55:38Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/24978916136

The next pull request likely to be tested is #155857.

Rollup of 2 pull requests Successful merges: - #155772 (Check closure's constness validity in the constness query) - #155839 (Remove unnecessary requires_lto: false for eBPF)

rust-bors · 2026-04-27T07:47:37Z

⌛ Testing commit 8e1c34f with merge 786c9b5...

Workflow: https://github.com/rust-lang/rust/actions/runs/24982905857

Check closure's constness validity in the constness query fixes #155584 instead of checking during ast lowering, where it's not easily possible to obtain all the right information in time. While lowering an assoc item we don't know if the parent was a const trait or a const impl. Tracing this information is quite annoying, and complicates a lot of code, which checking here after the fact is trivial.

jhpratt · 2026-04-27T07:49:16Z

once again after a spurious failure

@bors yield

rust-bors · 2026-04-27T07:49:20Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/24982905857

The next pull request likely to be tested is #155863.

Rollup of 3 pull requests Successful merges: - #155772 (Check closure's constness validity in the constness query) - #155833 (CI: rfl: move job forward to Linux v7.0) - #155839 (Remove unnecessary requires_lto: false for eBPF)

Rollup merge of #155772 - oli-obk:const-closures-everywhere, r=fee1-dead Check closure's constness validity in the constness query fixes #155584 instead of checking during ast lowering, where it's not easily possible to obtain all the right information in time. While lowering an assoc item we don't know if the parent was a const trait or a const impl. Tracing this information is quite annoying, and complicates a lot of code, which checking here after the fact is trivial.

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 Apr 25, 2026

rustbot assigned mati865 Apr 25, 2026

oli-obk commented Apr 25, 2026

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fee1-dead requested changes Apr 25, 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 Apr 25, 2026

fee1-dead assigned fee1-dead and unassigned mati865 Apr 25, 2026

oli-obk force-pushed the const-closures-everywhere branch from ee5b899 to 415454f Compare April 25, 2026 12:53

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fmease added the PG-const-traits Project group: Const traits label Apr 25, 2026

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oli-obk force-pushed the const-closures-everywhere branch from 415454f to ae91b21 Compare April 25, 2026 18:56

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JonathanBrouwer mentioned this pull request Apr 25, 2026

Rollup of 4 pull requests #155791

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 Apr 25, 2026

oli-obk added 2 commits April 27, 2026 07:43

Add regression test

2b2f28d

Check closure's constness validity in the constness query

8e1c34f

instead of during ast lowering, where it's not easily possible to obtain all the right information in time

oli-obk force-pushed the const-closures-everywhere branch from ae91b21 to 8e1c34f Compare April 27, 2026 05:45

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 Apr 27, 2026

jhpratt mentioned this pull request Apr 27, 2026

Rollup of 2 pull requests #155857

Closed

jhpratt mentioned this pull request Apr 27, 2026

Rollup of 3 pull requests #155863

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rust-bors Bot merged commit 6897624 into rust-lang:main Apr 27, 2026
11 of 12 checks passed

rustbot added this to the 1.97.0 milestone Apr 27, 2026

		match c.constness {
		Constness::Const => if tcx.hir_body_const_context(tcx.local_parent(def_id)).is_none() {

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