trait solver: account for universes from replace_bound_vars by Dnreikronos · Pull Request #158362 · rust-lang/rust

Dnreikronos · 2026-06-24T15:19:03Z

The ICE was not really because eager placeholder handling looked at too many universes. Boxy was right: with -Zassumptions-on-binders, if the new solver creates a placeholder universe, that universe needs an assumptions entry.

The bad path is FindParamInClause entering a binder through replace_bound_vars. BoundVarReplacer materializes a placeholder universe for the escaping bound vars, but nothing records placeholder assumptions for it. Later eager placeholder handling walks all non-input universes, which imo is the right behavior, and get_placeholder_assumptions hits the missing entry.

This moves the empty-assumptions bookkeeping into EvalCtxt::replace_bound_vars instead of keeping it local to FindParamInClause. The helper snapshots the universe slots before replacement and inserts Assumptions::empty() for any slot that got filled.

There is still a FIXME there because idk that empty assumptions is the final shape irl. replace_bound_vars does not have the param-env context to compute proper assumptions. But for this PR, imo this is the least weird local fix: it keeps the eager pass looking at all non-input universes and fixes the missing bookkeeping where the universe is created.

The repro is covered by a UI test. It reports the overflow diagnostic instead of ICEing.

rustbot · 2026-06-24T15:19:08Z

r? @JonathanBrouwer

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JonathanBrouwer · 2026-06-24T15:27:15Z

r? types

jackh726 · 2026-06-25T03:18:34Z

I'm not entirely sure if this is the right fix (I would actually expect that self.delegate.universe() to return the proper universe), but this is also quite new code. So going to reassign to Boxy.

r? @BoxyUwU

Dnreikronos · 2026-06-25T13:27:40Z

yeah, i think you're right that this probably is not the ideal long-term shape.

what this pr does is more of a local bound: eager placeholder handling only walks universes the current solver region constraint can actually name. imo that part still makes sense to me. self.delegate.universe() is just the highest universe the infcx has created, and solver work can bump that even if the final constraint no longer mentions that universe. then we ask get_placeholder_assumptions for a universe that is not relevant to the constraint and may not have an entry, which is the unwrap in the repro.

so yeah, idk if i would call this the best fix in the abstract. long term i'd expect us to have a cleaner invariant between created universes and placeholder assumptions, or maybe an explicit set of universes that eager handling should process.

but for this pr, i still prefer this local fix over making missing assumptions silently mean ambiguity. if the constraint cannot name the universe, i don't think this pass should care about it.

so this still lgtm to me as a short term fix, with the caveat that self.delegate.universe() being too global here is probably worth documenting.

JorgeCepeda · 2026-06-26T00:22:01Z

Offtopic: what does ltm mean?

Dnreikronos · 2026-06-26T00:25:41Z

Offtopic: what does ltm mean?

long term.
i just edited to be easier to understand, my bad....

BoxyUwU · 2026-06-26T11:23:10Z

this in theory makes sense as an optimization but it shouldn't make anything stop ICEing. We should not be entering binders inside of the trait solver without accounting for their assumptions 🤔

long term i'd expect us to have a cleaner invariant between created universes and placeholder assumptions, or maybe an explicit set of universes that eager handling should process.

the set of universes it should handle is all of them (other than input universes). can you look into where/why we're entering a binder and not inserting assumptions for it

Dnreikronos · 2026-06-27T01:29:39Z

this in theory makes sense as an optimization but it shouldn't make anything stop ICEing. We should not be entering binders inside of the trait solver without accounting for their assumptions 🤔

long term i'd expect us to have a cleaner invariant between created universes and placeholder assumptions, or maybe an explicit set of universes that eager handling should process.

the set of universes it should handle is all of them (other than input universes). can you look into where/why we're entering a binder and not inserting assumptions for it

yeah, you were right. i looked into where the unaccounted binder entry was happening, and it seems like the issue was in findparaminclause.

the bad path was basically: findparaminclause walks into a binder, then replace_bound_vars materializes a placeholder universe through boundvarreplacer, but that universe never got an entry in the placeholder assumptions map. later eager placeholder handling still walks all non-input universes, as it should, and then get_placeholder_assumptions hits the missing entry and ices.

i changed the patch so eager handling keeps using all non-input universes. instead, when findparaminclause calls replace_bound_vars, it tracks any previously-empty universe slot that got filled and registers empty placeholder assumptions for that universe. imo this is a better shape than bounding eager handling by the current constraint, since it fixes the missing bookkeeping directly and does not change the global/non-global classification by bailing on binders.

i also added the issue repro as a ui test. with this version it no longer ices and reports the overflow diagnostic instead.

pushed this version for you to validate. idk if long-term we want boundvarreplacer to report newly-created universes more explicitly, but for this pr i think this keeps the fix small and matches the existing empty-assumptions pattern. lgtm to me, but i'd like your take on whether empty assumptions is the right local accounting here.

BoxyUwU

thx for looking into that, yeah this makes total sense to me ✨ I think this is the right fix though would like to do something a little more general (see comment). can you update your title/description now that the approach has been changed

View changes since this review

BoxyUwU · 2026-07-14T19:44:04Z

+{
+    fn replace_bound_vars<T: TypeFoldable<I>>(&mut self, value: T) -> T {
+        let old_universes = self.universes.clone();
+        let value = self.ecx.replace_bound_vars(value, &mut self.universes);


I think slightly more generally than FindParamInClause, any part of the new solver which calls replace_bound_vars is just wrong under -Zassumptions-on-binders 🤔

I think ideally we would update replace_bound_vars to, when it creates new universes, also insert empty assumptions (and add an associated fixme to fix that in the long term :3)

yeah, makes sense. i changed this to be less FindParamInClause-specific.

replace_bound_vars now snapshots the universe slots before calling BoundVarReplacer, then inserts Assumptions::empty() for any slot that got materialized. FindParamInClause is back to just calling ecx.replace_bound_vars, so btw this should cover the other new-solver callers of that helper too.

imo this is a better shape than the previous patch. idk if empty assumptions is what we want irl, since replace_bound_vars still does not know enough to compute the real assumptions, so i left a FIXME there as you requested. ltm if you'd rather push this closer to BoundVarReplacer, but i think this is the right small step for this PR.

fyi i also updated the title/description so they no longer talk about limiting eager placeholder handling.

rustbot · 2026-07-16T00:46:38Z

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.

rust-log-analyzer · 2026-07-16T01:22:42Z

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

---- [ui] tests/ui/assumptions_on_binders/placeholder-assumptions-issue-157840.rs stdout ----
Saved the actual stderr to `/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/assumptions_on_binders/placeholder-assumptions-issue-157840/placeholder-assumptions-issue-157840.stderr`
diff of stderr:

- error[E0275]: overflow evaluating the requirement `(): Trait<<T as Proj<'a>>::Assoc>`
+ error[E0277]: the trait bound `(): Trait<fn(for<'a> fn(<T as Proj<'a>>::Assoc))>` is not satisfied
2   --> $DIR/placeholder-assumptions-issue-157840.rs:12:9
3    |
4 LL |     (): Trait<<T as Proj<'a>>::Assoc>,

-    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Trait<fn(for<'a> fn(<T as Proj<'a>>::Assoc))>` is not implemented for `()`
6    |
-    = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`placeholder_assumptions_issue_157840`)
+ help: consider extending the `where` clause, but there might be an alternative better way to express this requirement
+    |
+ LL |     (): Trait<<T as Proj<'a>>::Assoc>, (): Trait<fn(for<'a> fn(<T as Proj<'a>>::Assoc))>
+    |                                        +++++++++++++++++++++++++++++++++++++++++++++++++
8 
9 error: aborting due to 1 previous error
10 

---
To only update this specific test, also pass `--test-args assumptions_on_binders/placeholder-assumptions-issue-157840.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/assumptions_on_binders/placeholder-assumptions-issue-157840.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/assumptions_on_binders/placeholder-assumptions-issue-157840" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers" "-Znext-solver=globally" "-Zassumptions-on-binders"
stdout: none
--- stderr -------------------------------
error[E0277]: the trait bound `(): Trait<fn(for<'a> fn(<T as Proj<'a>>::Assoc))>` is not satisfied
##[error]  --> /checkout/tests/ui/assumptions_on_binders/placeholder-assumptions-issue-157840.rs:12:9
   |
LL |     (): Trait<<T as Proj<'a>>::Assoc>,
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Trait<fn(for<'a> fn(<T as Proj<'a>>::Assoc))>` is not implemented for `()`
   |
help: consider extending the `where` clause, but there might be an alternative better way to express this requirement
   |
LL |     (): Trait<<T as Proj<'a>>::Assoc>, (): Trait<fn(for<'a> fn(<T as Proj<'a>>::Assoc))>
   |                                        +++++++++++++++++++++++++++++++++++++++++++++++++

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0277`.

rustbot assigned JonathanBrouwer Jun 24, 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. WG-trait-system-refactor The Rustc Trait System Refactor Initiative (-Znext-solver) labels Jun 24, 2026

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rustbot added the T-types Relevant to the types team, which will review and decide on the PR/issue. label Jun 24, 2026

rustbot assigned jackh726 and unassigned JonathanBrouwer Jun 24, 2026

Dnreikronos force-pushed the trait_solver/placeholder_universe_bounds branch from 9cc11df to 697770e Compare June 24, 2026 15:28

Dnreikronos mentioned this pull request Jun 24, 2026

[ICE]: assumptions on binders: called Option::unwrap() on a None value context.rs #157840

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rustbot assigned BoxyUwU and unassigned jackh726 Jun 25, 2026

BoxyUwU mentioned this pull request Jul 13, 2026

Assumptions on Binders rust-lang/rust-project-goals#621

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BoxyUwU reviewed Jul 14, 2026

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Dnreikronos changed the title ~~trait solver: limit eager placeholder handling~~ trait solver: record assumptions for replaced bound vars Jul 15, 2026

Dnreikronos changed the title ~~trait solver: record assumptions for replaced bound vars~~ trait solver: account for universes from replace_bound_vars Jul 15, 2026

Dnreikronos requested a review from BoxyUwU July 15, 2026 19:57

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Dnreikronos added 4 commits July 15, 2026 21:18

Limit eager placeholder handling to constraint universes

691b93c

Add coverage for placeholder assumption ICE

6e902ec

Account for placeholder universes in param env classification

4183d5d

Move placeholder assumptions into bound replacement

0985b02

Track placeholder assumptions for created universes

25bd5cc

Dnreikronos force-pushed the trait_solver/placeholder_universe_bounds branch from ad0815f to 25bd5cc Compare July 16, 2026 00:46

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