be more permissive wrt overflow and and improve diagnostics by adwinwhite · Pull Request #161341 · rust-lang/rust

adwinwhite · 2026-08-19T04:50:24Z

This builds on #160632

Previously we only showed a single root goal for the FCW. It was difficult to find out how the goal overflowed.
We display a proving chain now which should help users identify relevant types or auto traits.

This will affect perf for crates emitting the FCW. E.g. calimero-store goes from 4.7s -> 5.7s in local testing since it emits thousands of FCWs internally. The FCW is a mitigation of future hard error and authors are expected to resolve it so it's probably acceptable. It doesn't affect crates without the FCW.

r? lcnr

rustbot · 2026-08-20T08:35:06Z

Some changes occurred to the core trait solver

cc @rust-lang/initiative-trait-system-refactor

lcnr · 2026-08-21T07:01:06Z

-   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "16"]` attribute to your crate (`fcw_on_normalization`)
-   = help: or consider adding a manual `impl` of auto traits like `Send` for intermediate types, if auto traits are involved
+   = note: which requires <W<W<W<W<W<W<W<W<W<()>>>>>>>>> as HasAssoc>::Assoc == ()
+   = note: which requires <W<W<W<W<W<W<W<W<()>>>>>>>> as HasAssoc>::Assoc == ()


why does this stop after 2 goals? that's surprising

View changes since the review

oh, that's because Projection goals don't add their nested NormalizesTo goal as a nested goal for proof trees 🤔 that sucks

see https://rust-lang.zulipchat.com/#narrow/channel/364551-t-types.2Ftrait-system-refactor/topic/lcnr.20overflow.20lint.20fun/with/618110720

lcnr · 2026-08-21T07:01:34Z

+    }
+
+    fn visit_goal(&mut self, goal: &inspect::InspectGoal<'_, 'tcx>) -> Self::Result {
+        self.predicates.push(goal.goal().predicate);


does this currently print both NormalizesTo and the Projection goal? if so, we should probably filter the NormalizesTo goal here :>

View changes since the review

rustbot · 2026-08-22T17:04:11Z

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.

lcnr · 2026-08-22T17:25:40Z

first three commits were approved by adwin in #160632, r=me on the second 2 with my applied nits in the last commit, which feel small enough to not require an additional review

wrt to perf

Similar to #160254, the current perf collection won't be affected by this as they don't overflow(maybe except typenum)?

@bors r=lcnr,adwinwhite rollup

rust-bors · 2026-08-22T17:25:44Z

📌 Commit 3f63986 has been approved by lcnr,adwinwhite

It is now in the queue for this repository.

lcnr · 2026-08-22T18:12:28Z

@bors r-

rust-bors · 2026-08-22T18:12:32Z

This pull request was unapproved.

View changes since this unapproval

lcnr · 2026-08-22T18:30:20Z

@bors r=lcnr,adwinwhite rollup

rust-bors · 2026-08-22T18:30:22Z

📌 Commit a868158 has been approved by lcnr,adwinwhite

It is now in the queue for this repository.

rust-bors · 2026-08-22T18:43:23Z

⚠️ A new commit fbc16357117040b3aa3270b8890fe5758f1ff0f9 was pushed.

This pull request was unapproved.

lcnr · 2026-08-22T18:49:39Z

having the same branch for both this and #161552 really is not ideal

@bors r=lcnr,adwinwhite rollup

rust-bors · 2026-08-22T18:49:42Z

📌 Commit 3f63986 has been approved by lcnr,adwinwhite

It is now in the queue for this repository.

…cnr,adwinwhite be more permissive wrt overflow and and improve diagnostics This builds on rust-lang#160632 Previously we only showed a single root goal for the FCW. It was difficult to find out how the goal overflowed. We display a proving chain now which should help users identify relevant types or auto traits. This will affect perf for crates emitting the FCW. E.g. `calimero-store` goes from 4.7s -> 5.7s in local testing since it emits thousands of FCWs internally. The FCW is a mitigation of future hard error and authors are expected to resolve it so it's probably acceptable. It doesn't affect crates without the FCW. r? lcnr

…uwer Rollup of 7 pull requests Successful merges: - #157513 (Reject non-constructor self types in const-arg tuple-call lowering) - #159887 (compiletest: forward disable-minification from bootstrap) - #160949 (Add floating point inline ASM support for SPARC) - #156160 (feat: add symmetric PartialEq impls for Vec, &[T], &mut [T] versus Cow<'_, [T]>) - #160302 (target_features: sse (or at least avx2) is incompatible with soft-float ABI) - #160914 (Derive `GenericTypeVisitable` for `RegionConstraint` _correctly_) - #161341 (be more permissive wrt overflow and and improve diagnostics)

…cnr,adwinwhite be more permissive wrt overflow and and improve diagnostics This builds on rust-lang#160632 Previously we only showed a single root goal for the FCW. It was difficult to find out how the goal overflowed. We display a proving chain now which should help users identify relevant types or auto traits. This will affect perf for crates emitting the FCW. E.g. `calimero-store` goes from 4.7s -> 5.7s in local testing since it emits thousands of FCWs internally. The FCW is a mitigation of future hard error and authors are expected to resolve it so it's probably acceptable. It doesn't affect crates without the FCW. r? lcnr

rustbot assigned lcnr Aug 19, 2026

lcnr reviewed Aug 20, 2026

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adwinwhite force-pushed the overflow-fcw-chain branch from 858bfc1 to 0d21359 Compare August 20, 2026 08:05

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adwinwhite force-pushed the overflow-fcw-chain branch 2 times, most recently from 61aecad to ce9f31f Compare August 20, 2026 08:25

adwinwhite marked this pull request as ready for review August 20, 2026 08:35

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lcnr mentioned this pull request Aug 21, 2026

overflow -> ambig: trigger recursion_depth_exceeding_limit #160632

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Comment thread compiler/rustc_type_ir/src/search_graph/mod.rs Outdated

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lcnr mentioned this pull request Aug 22, 2026

bevy_render 0.19.1 doesn't compile on current Rust nightly due to -Znext-solver enabled by default bevyengine/bevy#25511

Open

lcnr changed the title ~~Show the nested goal chain for the next solver overflow FCW~~ be more permissive wrt overflow and and improve diagnostics Aug 22, 2026

lcnr and others added 6 commits August 22, 2026 18:11

overflow FCW also trigger for overflow -> ambig

7b72b73

only emit a single recursion_depth_exceeding_limit

89c2f92

move fcw to SolverDelegate, resolve infer vars

d3bde3c

add required_depth to InspectGoal

c0683f6

visit nested goal with largest required depth

be2ff7e

apply nits

3f63986

lcnr force-pushed the overflow-fcw-chain branch from ce9f31f to 3f63986 Compare August 22, 2026 17:04

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lcnr mentioned this pull request Aug 22, 2026

also trigger overflow FCW when going from overflow -> error #161552

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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 Aug 22, 2026

lcnr force-pushed the overflow-fcw-chain branch from 3f63986 to a868158 Compare August 22, 2026 18:20

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lcnr force-pushed the overflow-fcw-chain branch from a868158 to fbc1635 Compare August 22, 2026 18:43

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lcnr force-pushed the overflow-fcw-chain branch from fbc1635 to 3f63986 Compare August 22, 2026 18:49

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JonathanBrouwer mentioned this pull request Aug 22, 2026

Rollup of 7 pull requests #161559

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jhpratt mentioned this pull request Aug 22, 2026

Rollup of 8 pull requests #161566

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