Add coherency check for query cycle breaking by zetanumbers · Pull Request #153953 · rust-lang/rust

zetanumbers · 2026-03-16T12:51:16Z

With debug_assertions compare if generated CycleErrors differ between single-threaded and multi-threaded versions of query cycle breaking code and panic if so.

I reasonably assume multi-threaded cycle breaking prioritizes breaking single-threaded cycles. I've made find_cycle_in_stack to be a fallible function, always finding single-threaded cycle if there is any relative to the query argument.

There are also some changes to Symbol's Debug and Display impls to make those panic less.

Split from #153185.

r? @nnethercote

cc @Zoxc

…king

zetanumbers · 2026-03-16T13:03:02Z

I got 3 errors while running parallel-rustc tests:

zetanumbers · 2026-03-16T13:03:31Z

tests/ui/parallel-rustc/cycle_crash-issue-135870.rs

Cycle diff

5,24d4
<                 tagged_key: check_type_wf(
<                     (),
<                 ),
<                 dep_kind: check_type_wf,
<                 def_id: None,
<             },
<             span: Span {
<                 lo: BytePos(
<                     0,
<                 ),
<                 hi: BytePos(
<                     0,
<                 ),
<                 ctxt: #0,
<             },
<         },
<     ),
<     cycle: [
<         Spanned {
<             node: QueryStackFrame {
42a23,24
>     ),
>     cycle: [

Panic message

CycleError coherency check failed:
expected: CycleError {
    usage: Some(
        Spanned {
            node: QueryStackFrame {
                tagged_key: analysis(
                    (),
                ),
                dep_kind: analysis,
                def_id: None,
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
    ),
    cycle: [
        Spanned {
            node: QueryStackFrame {
                tagged_key: eval_to_const_value_raw(
                    PseudoCanonicalInput {
                        typing_env: TypingEnv {
                            typing_mode: PostAnalysis,
                            param_env: ParamEnv {
                                caller_bounds: [],
                            },
                        },
                        value: GlobalId {
                            instance: Instance {
                                def: Item(
                                    DefId(0:3 ~ 2802[4f9a]::2801),
                                ),
                                args: [],
                            },
                            promoted: None,
                        },
                    },
                ),
                dep_kind: eval_to_const_value_raw,
                def_id: None,
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
        Spanned {
            node: QueryStackFrame {
                tagged_key: trivial_const(
                    DefId(0:3 ~ 2802[4f9a]::2801),
                ),
                dep_kind: trivial_const,
                def_id: Some(
                    DefId(0:3 ~ 2802[4f9a]::2801),
                ),
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
        Spanned {
            node: QueryStackFrame {
                tagged_key: mir_built(
                    DefId(0:3 ~ 2802[4f9a]::2801),
                ),
                dep_kind: mir_built,
                def_id: Some(
                    DefId(0:3 ~ 2802[4f9a]::2801),
                ),
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
    ],
}
got: CycleError {
    usage: Some(
        Spanned {
            node: QueryStackFrame {
                tagged_key: eval_to_const_value_raw(
                    PseudoCanonicalInput {
                        typing_env: TypingEnv {
                            typing_mode: PostAnalysis,
                            param_env: ParamEnv {
                                caller_bounds: [],
                            },
                        },
                        value: GlobalId {
                            instance: Instance {
                                def: Item(
                                    DefId(0:3 ~ 2802[4f9a]::2801),
                                ),
                                args: [],
                            },
                            promoted: None,
                        },
                    },
                ),
                dep_kind: eval_to_const_value_raw,
                def_id: None,
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
    ),
    cycle: [
        Spanned {
            node: QueryStackFrame {
                tagged_key: trivial_const(
                    DefId(0:3 ~ 2802[4f9a]::2801),
                ),
                dep_kind: trivial_const,
                def_id: Some(
                    DefId(0:3 ~ 2802[4f9a]::2801),
                ),
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
        Spanned {
            node: QueryStackFrame {
                tagged_key: mir_built(
                    DefId(0:3 ~ 2802[4f9a]::2801),
                ),
                dep_kind: mir_built,
                def_id: Some(
                    DefId(0:3 ~ 2802[4f9a]::2801),
                ),
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
    ],
}

zetanumbers · 2026-03-16T13:03:40Z

tests/ui/parallel-rustc/ty-variance-issue-124423.rs

Cycle diff

5,24d4
<                 tagged_key: analysis(
<                     (),
<                 ),
<                 dep_kind: analysis,
<                 def_id: None,
<             },
<             span: Span {
<                 lo: BytePos(
<                     0,
<                 ),
<                 hi: BytePos(
<                     0,
<                 ),
<                 ctxt: #0,
<             },
<         },
<     ),
<     cycle: [
<         Spanned {
<             node: QueryStackFrame {
56a37,38
>     ),
>     cycle: [

Panic message

CycleError coherency check failed:
expected: CycleError {
    usage: Some(
        Spanned {
            node: QueryStackFrame {
                tagged_key: check_type_wf(
                    (),
                ),
                dep_kind: check_type_wf,
                def_id: None,
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
    ),
    cycle: [
        Spanned {
            node: QueryStackFrame {
                tagged_key: check_well_formed(
                    DefId(0:4 ~ 2812[5a0e]::2801),
                ),
                dep_kind: check_well_formed,
                def_id: Some(
                    DefId(0:4 ~ 2812[5a0e]::2801),
                ),
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
        Spanned {
            node: QueryStackFrame {
                tagged_key: fn_sig(
                    DefId(0:4 ~ 2812[5a0e]::2801),
                ),
                dep_kind: fn_sig,
                def_id: Some(
                    DefId(0:4 ~ 2812[5a0e]::2801),
                ),
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
        Spanned {
            node: QueryStackFrame {
                tagged_key: typeck(
                    DefId(0:4 ~ 2812[5a0e]::2801),
                ),
                dep_kind: typeck,
                def_id: Some(
                    DefId(0:4 ~ 2812[5a0e]::2801),
                ),
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
        Spanned {
            node: QueryStackFrame {
                tagged_key: variances_of(
                    DefId(0:22 ~ 2812[5a0e]::2223),
                ),
                dep_kind: variances_of,
                def_id: Some(
                    DefId(0:22 ~ 2812[5a0e]::2223),
                ),
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
        Spanned {
            node: QueryStackFrame {
                tagged_key: crate_variances(
                    (),
                ),
                dep_kind: crate_variances,
                def_id: None,
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
    ],
}
got: CycleError {
    usage: Some(
        Spanned {
            node: QueryStackFrame {
                tagged_key: check_well_formed(
                    DefId(0:4 ~ 2812[5a0e]::2801),
                ),
                dep_kind: check_well_formed,
                def_id: Some(
                    DefId(0:4 ~ 2812[5a0e]::2801),
                ),
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
    ),
    cycle: [
        Spanned {
            node: QueryStackFrame {
                tagged_key: fn_sig(
                    DefId(0:4 ~ 2812[5a0e]::2801),
                ),
                dep_kind: fn_sig,
                def_id: Some(
                    DefId(0:4 ~ 2812[5a0e]::2801),
                ),
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
        Spanned {
            node: QueryStackFrame {
                tagged_key: typeck(
                    DefId(0:4 ~ 2812[5a0e]::2801),
                ),
                dep_kind: typeck,
                def_id: Some(
                    DefId(0:4 ~ 2812[5a0e]::2801),
                ),
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
        Spanned {
            node: QueryStackFrame {
                tagged_key: variances_of(
                    DefId(0:22 ~ 2812[5a0e]::2223),
                ),
                dep_kind: variances_of,
                def_id: Some(
                    DefId(0:22 ~ 2812[5a0e]::2223),
                ),
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
        Spanned {
            node: QueryStackFrame {
                tagged_key: crate_variances(
                    (),
                ),
                dep_kind: crate_variances,
                def_id: None,
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
    ],
}

zetanumbers · 2026-03-16T13:06:11Z

tests/ui/parallel-rustc/ty-variance-issue-124423.rs

Cycle diff

5,24d4
<                 tagged_key: check_type_wf(
<                     (),
<                 ),
<                 dep_kind: check_type_wf,
<                 def_id: None,
<             },
<             span: Span {
<                 lo: BytePos(
<                     0,
<                 ),
<                 hi: BytePos(
<                     0,
<                 ),
<                 ctxt: #0,
<             },
<         },
<     ),
<     cycle: [
<         Spanned {
<             node: QueryStackFrame {
42a23,24
>     ),
>     cycle: [

Panic message

CycleError coherency check failed:
expected: CycleError {
    usage: Some(
        Spanned {
            node: QueryStackFrame {
                tagged_key: check_type_wf(
                    (),
                ),
                dep_kind: check_type_wf,
                def_id: None,
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
    ),
    cycle: [
        Spanned {
            node: QueryStackFrame {
                tagged_key: check_well_formed(
                    DefId(0:4 ~ 2806[4b15]::2801),
                ),
                dep_kind: check_well_formed,
                def_id: Some(
                    DefId(0:4 ~ 2806[4b15]::2801),
                ),
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
        Spanned {
            node: QueryStackFrame {
                tagged_key: fn_sig(
                    DefId(0:4 ~ 2806[4b15]::2801),
                ),
                dep_kind: fn_sig,
                def_id: Some(
                    DefId(0:4 ~ 2806[4b15]::2801),
                ),
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
        Spanned {
            node: QueryStackFrame {
                tagged_key: typeck(
                    DefId(0:4 ~ 2806[4b15]::2801),
                ),
                dep_kind: typeck,
                def_id: Some(
                    DefId(0:4 ~ 2806[4b15]::2801),
                ),
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
        Spanned {
            node: QueryStackFrame {
                tagged_key: variances_of(
                    DefId(0:10 ~ 2806[4b15]::2223),
                ),
                dep_kind: variances_of,
                def_id: Some(
                    DefId(0:10 ~ 2806[4b15]::2223),
                ),
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
        Spanned {
            node: QueryStackFrame {
                tagged_key: crate_variances(
                    (),
                ),
                dep_kind: crate_variances,
                def_id: None,
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
    ],
}
got: CycleError {
    usage: Some(
        Spanned {
            node: QueryStackFrame {
                tagged_key: check_well_formed(
                    DefId(0:4 ~ 2806[4b15]::2801),
                ),
                dep_kind: check_well_formed,
                def_id: Some(
                    DefId(0:4 ~ 2806[4b15]::2801),
                ),
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
    ),
    cycle: [
        Spanned {
            node: QueryStackFrame {
                tagged_key: fn_sig(
                    DefId(0:4 ~ 2806[4b15]::2801),
                ),
                dep_kind: fn_sig,
                def_id: Some(
                    DefId(0:4 ~ 2806[4b15]::2801),
                ),
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
        Spanned {
            node: QueryStackFrame {
                tagged_key: typeck(
                    DefId(0:4 ~ 2806[4b15]::2801),
                ),
                dep_kind: typeck,
                def_id: Some(
                    DefId(0:4 ~ 2806[4b15]::2801),
                ),
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
        Spanned {
            node: QueryStackFrame {
                tagged_key: variances_of(
                    DefId(0:10 ~ 2806[4b15]::2223),
                ),
                dep_kind: variances_of,
                def_id: Some(
                    DefId(0:10 ~ 2806[4b15]::2223),
                ),
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
        Spanned {
            node: QueryStackFrame {
                tagged_key: crate_variances(
                    (),
                ),
                dep_kind: crate_variances,
                def_id: None,
            },
            span: Span {
                lo: BytePos(
                    0,
                ),
                hi: BytePos(
                    0,
                ),
                ctxt: #0,
            },
        },
    ],
}

zetanumbers · 2026-03-16T13:10:30Z

These 3 similar mismatches leads me to believe current query cycle breaking code is inconsistent with the single-threaded implementation. In particular, multi-threaded cycle usage query is actually the first query in the cycle for single-threaded version.

~~I propose we fix the multi-threaded version. I have explicitly addressed this issue in #153185. However I think @Zoxc might have a strong opinion on how to better fix this.~~

UPDATE: This was issue with coherency check implementation

nnethercote

Thanks for splitting this off from #153185, that makes reviewing much easier. A few comments below.

View changes since this review

nnethercote · 2026-03-17T05:51:04Z

compiler/rustc_span/src/symbol.rs

+        if let Some(s) = self.opt_as_str() {
+            fmt::Debug::fmt(s, f)
+        } else {
+            fmt::Debug::fmt(&self.0, f)


This will just print a number, e.g. 123. Printing Symbol(123) would be much better.

Symbols should never be used in contexts in which this path can be triggered, it's better to continue panicking here.
@zetanumbers In which specific context was this encountered?

Consult #t-compiler/parallel-rustc > Getting rid of the cycle detection thread

Consult #t-compiler/parallel-rustc > Getting rid of the cycle detection thread

That thread doesn't have anything answering my question.

This context is the cycle detection thread

Could you add a comment explaining this use?
Using Symbols when the interner is dead is like using dangling pointers, suspicious at least.
If the cycle detection thread is the only such context and it's removed, then it would be better to remove this logic as well.

nnethercote · 2026-03-17T05:51:10Z

compiler/rustc_span/src/symbol.rs

+        if let Some(s) = self.opt_as_str() {
+            fmt::Display::fmt(s, f)
+        } else {
+            fmt::Debug::fmt(&self.0, f)


nnethercote · 2026-03-17T05:55:03Z

compiler/rustc_query_impl/src/job.rs

+        {
+            if error != expected {
+                panic!(
+                    "CycleError coherency check failed:\n  expected: {expected:#?}\n  got: {error:#?}"


Instead of "expected" and "got", would it be better to indicate the single-threaded result and the multi-threaded result? It's conceivable that the single-threaded one might be wrong.

Also, does "coherency" have a particular meaning. It makes me think of trait coherency, which is unrelated. "Consistency" might be better, it doesn't have a Rust-specific meaning, e.g. "inconsistent query cycle detection: ...".

nnethercote · 2026-03-17T05:58:12Z

compiler/rustc_query_impl/src/job.rs

                .collect(),
        };

+        if cfg!(debug_assertions)


There are no added comments in this commit. An explanatory comment here would be useful. Something like "compare the single-threaded cycle detection against the multi-threaded cycle detection".

nnethercote · 2026-03-17T06:05:08Z

Also, presumably we can't land this PR as is due to the test failures. I assume the idea is to show the existing bug, and that prompts other fixes?

petrochenkov · 2026-03-17T13:37:39Z

compiler/rustc_span/src/symbol.rs

 impl fmt::Debug for Symbol {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt::Debug::fmt(self.as_str(), f)
+        if let Some(s) = self.opt_as_str() {


Suggested change

if let Some(s) = self.opt_as_str() {

if are_session_globals_set() {

Together with a comment #153953 (comment) explaining why it's acceptable.
opt_as_str can be removed, it shouldn't be a generally available method.

Zoxc · 2026-03-17T14:07:30Z

The main problem with this PR would be that it tests for a property we do not have, which means it's effectively just adding unnecessary panics.

It's also unclear to me if the use of find_cycle_in_stack here results in cycle equivalent to an execution on a single thread, given that it makes assumptions that doesn't hold for the calls here.

rust-bors · 2026-03-18T07:51:52Z

☔ The latest upstream changes (presumably #154032) made this pull request unmergeable. Please resolve the merge conflicts.

zetanumbers · 2026-03-18T09:12:02Z

I ground my changes in improvements to observable consistency between single-threaded and multi-threaded runs of "cycle detected" tests for which I compare those runs' outputs. It's WIP right now, so this isn't yet the final version.

The main problem with this PR would be that it tests for a property we do not have, which means it's effectively just adding unnecessary panics.

Why don't we have this property? Isn't it possible to add such a property? Or are you saying we shouldn't try doing this change?

It's also unclear to me if the use of find_cycle_in_stack here results in cycle equivalent to an execution on a single thread, given that it makes assumptions that doesn't hold for the calls here.

Could you elaborate on which assumptions you are talking about?

Add coherency check for multi-threaded and single-threaded cycle brea…

7cec0df

…king

rustbot assigned nnethercote Mar 16, 2026

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nnethercote reviewed Mar 17, 2026

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nnethercote 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 Mar 17, 2026

Fix cycle coherency check and cycle inconsistency

dec769a

petrochenkov reviewed Mar 17, 2026

View reviewed changes

	if let Some(s) = self.opt_as_str() {
	if are_session_globals_set() {

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