Avoid infinite recursion when trying to build valtrees for self-referential consts by sjwang05 · Pull Request #158553 · rust-lang/rust

sjwang05 · 2026-06-29T09:17:23Z

This PR adds cycle detection to const_to_valtree_inner by using a tricolor DFS, so self-referential statics in pats don't cause valtree construction to recurse infinitely. Instead, we now return a new ValTreeCreationError::CyclicConst, similar to the current InvalidConst and NodesOverflow variants.

fixes #144719

rustbot · 2026-06-29T09:17:27Z

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

rustbot · 2026-06-29T09:17:29Z

r? @petrochenkov

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petrochenkov · 2026-06-29T12:08:33Z

r? ctfe

rustbot · 2026-06-29T12:08:37Z

Failed to set assignee to ctfe: invalid assignee

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petrochenkov · 2026-06-29T12:08:43Z

r? const-eval

rustbot · 2026-06-29T12:08:47Z

Failed to set assignee to const-eval: invalid assignee

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petrochenkov · 2026-06-29T12:09:03Z

r? @RalfJung for review or further reassignment

rustbot · 2026-06-29T12:09:09Z

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RalfJung · 2026-06-29T12:25:55Z

r? @oli-obk
or maybe @lcnr ?

oli-obk

I don't think this actually solves the issue. It solves it for small loops, but if the chain is deep enough, we'll just end up in the same situation again:

#[derive(PartialEq)]
struct Thing(&'static Thing);

static A: Thing = Thing(&B);
static B: Thing = Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&Thing(&A))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))));
const C: &Thing = &A;

fn main() {
    if let C = C {}
    //~^ ERROR constant C cannot be used as pattern
}

or possibly a few more nested ones.

Or maybe the following works?

#[derive(PartialEq, Copy, Clone)]
struct Thing(&'static Thing);

static A: Thing = Thing(B[0]);
const N: usize = 100_000_000;
static B: [&Thing; N] = {
    let mut x = [&Thing(&A); N];
    let mut i = 0;
    while i < N - 1 {
        x[i] = &x[i + 1];
        i += 1;
    }
    x
};
const C: &Thing = A;

fn main() {
    if let C = C {}
    //~^ ERROR constant C cannot be used as pattern
}

View changes since this review

rustbot · 2026-07-02T21:04:00Z

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

sjwang05 · 2026-07-03T07:31:15Z

The second example overflowed the stack, even with N = 8000, so I wrapped the recursion in ensure_sufficient_stack, which seems to have solved the issue, though I'm not sure if that's the best approach, or if rewriting the entire function to be iterative would be better here.

I tested it up to N = 50000 without issue, but anything past that took far too long to get any results.

The first case also errors properly, and when I tried increasing the nesting to several thousand, both the current nightly and this stage1 crash during name resolution.

I added a test for the second case which sets RUST_MIN_STACK to 3MB, so N can stay relatively small and the test doesn't take forever to complete.

@rustbot ready

oli-obk · 2026-07-03T07:57:45Z

both the current nightly and this stage1 crash during name resolution.

🤣

oli-obk · 2026-07-06T14:40:14Z

or if rewriting the entire function to be iterative would be better here.

another alternative is to do no tracking at all and just use the recursion limit. But if it works well, let's go with this for now.

@bors try @rust-timer queue

let's make sure this common code path doesn't regress too much

Avoid infinite recursion when trying to build valtrees for self-referential consts

rust-bors · 2026-07-06T16:48:46Z

☀️ Try build successful (CI)
Build commit: 5da228f (5da228f755a19e6d66549a30a446be06f9b99bb1)
Base parent: 3c00c96 (3c00c96d3af4d5b5e101e56cc161a608b21366ee)

rust-timer · 2026-07-06T17:56:45Z

Finished benchmarking commit (5da228f): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

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This perf run didn't have relevant results for this metric.

Max RSS (memory usage)

Results (primary -0.6%, secondary 2.2%)

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	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.2%	[1.7%, 2.5%]	3
Improvements ✅ (primary)	-0.6%	[-0.6%, -0.6%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.6%	[-0.6%, -0.6%]	1

Cycles

Results (primary -2.5%)

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	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.5%	[-2.5%, -2.5%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.5%	[-2.5%, -2.5%]	1

Binary size

Results (secondary 0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Bootstrap: 491.018s -> 494.091s (0.63%)
Artifact size: 388.45 MiB -> 388.99 MiB (0.14%)

oli-obk · 2026-07-06T19:53:32Z

@bors r+ rollup

rust-bors · 2026-07-06T19:53:35Z

📌 Commit c61c52f has been approved by oli-obk

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 6. This pull request will be tested once the tree is reopened.

Avoid infinite recursion when trying to build valtrees for self-referential consts This PR adds cycle detection to `const_to_valtree_inner` by using a tricolor DFS, so self-referential statics in pats don't cause valtree construction to recurse infinitely. Instead, we now return a new `ValTreeCreationError::CyclicConst`, similar to the current `InvalidConst` and `NodesOverflow` variants. fixes rust-lang#144719

Rollup of 13 pull requests Successful merges: - #158478 (Use correct typing mode in mir building for the next solver) - #158796 (Distinguish null reference production from null pointer dereference in UB checks) - #158821 (add regression test for late-bound type method probe ICE) - #158245 (Update the rustc_perf submodule) - #158346 (Prevent rustdoc auto-trait ICEs with `-Znext-solver=globally`) - #158536 (Instatiate binder instead of skipping when suggesting function arg error) - #158553 (Avoid infinite recursion when trying to build valtrees for self-referential consts) - #158679 (Document blocking guarantees for `std::sync`) - #158826 (Reorganize `tests/ui/issues` [19/N]) - #158860 (Delete `impl_def_id` field from `ImplHeader`) - #158867 (rewrite `Align::max_for_target` in a more obvious way) - #158878 (borrowck: Inline `free_region_constraint_info()` to simplify the code) - #158881 (Update mdbook to 0.5.4)

Rollup merge of #158553 - sjwang05:issue-144719, r=oli-obk Avoid infinite recursion when trying to build valtrees for self-referential consts This PR adds cycle detection to `const_to_valtree_inner` by using a tricolor DFS, so self-referential statics in pats don't cause valtree construction to recurse infinitely. Instead, we now return a new `ValTreeCreationError::CyclicConst`, similar to the current `InvalidConst` and `NodesOverflow` variants. fixes #144719

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 Jun 29, 2026

rustbot assigned petrochenkov Jun 29, 2026

sjwang05 changed the title ~~Avoid infinite recursion when building valtrees for self-referential consts~~ Avoid infinite recursion when trying to build valtrees for self-referential consts Jun 29, 2026

rustbot assigned RalfJung and unassigned petrochenkov Jun 29, 2026

rustbot assigned oli-obk and unassigned RalfJung Jun 29, 2026

oli-obk requested changes Jul 2, 2026

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Comment thread compiler/rustc_const_eval/src/const_eval/valtrees.rs Outdated

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 Jul 2, 2026

detect cycles during valtree construction for self-referential consts

c61c52f

sjwang05 force-pushed the issue-144719 branch from f216552 to c61c52f Compare July 3, 2026 06:27

rustbot 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 Jul 3, 2026

oli-obk approved these changes Jul 6, 2026

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rust-bors Bot pushed a commit that referenced this pull request Jul 6, 2026

Auto merge of #158553 - sjwang05:issue-144719, r=<try>

5da228f

Avoid infinite recursion when trying to build valtrees for self-referential consts

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 6, 2026

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 Jul 6, 2026

JonathanBrouwer mentioned this pull request Jul 6, 2026

Rollup of 7 pull requests #158884

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jhpratt mentioned this pull request Jul 7, 2026

Rollup of 13 pull requests #158891

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rust-bors Bot merged commit ce811cd into rust-lang:main Jul 7, 2026
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rustbot added this to the 1.99.0 milestone Jul 7, 2026

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rust-timer commented Jul 6, 2026

Overall result: no relevant changes - no action needed

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