trait_selection: Keep type-op region constraints in borrowck by Dnreikronos · Pull Request #161423 · rust-lang/rust

Dnreikronos · 2026-08-20T19:20:19Z

I ran into this while working on #158588. borrowck_env_fail still had a FIXME because the function body wasn't reporting the outlives error. The next solver creates the region constraint, but the canonical type-op path doesn't put it in QueryResponse. Borrowck never sees it, so the type op looks fine and the error is lost.

With -Zassumptions-on-binders, these type ops now run locally on borrowck's InferCtxt. The fast path still runs first. That leaves the constraint in the same inference context borrowck reads later.

I prefer this over adding more data to the old canonical response. The next solver already caches its work, and teaching the old query path about these constraints felt like extra machinery for something we can avoid. Running locally is pretty boring, but I think that's a good thing here. The old FIXME is now the regression test.

Keep rust-lang#158588 focused on reporting solver region constraints. The type-op behavior and its borrowck coverage now live in rust-lang#161423.

rustbot · 2026-08-20T19:33:12Z

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BoxyUwU · 2026-08-20T21:13:43Z

r? BoxyUwU

BoxyUwU · 2026-08-21T19:39:46Z

I think this is a partial revert of #160982 which we don't want. I think that teaching scrape_region_constraints to handle the new kind of constraints is exactly correct. The whole point of that function is to return the region constraints so it not doing that under -Zassumptions-on-binders seems like a clear bug to me :3

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rustbot · 2026-08-21T19:39:49Z

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

…tions_binders_diagnostics, r=BoxyUwU trait_selection: fix assumptions-on-binders diagnostics fixes rust-lang#157732 `-Zassumptions-on-binders` was losing the origin of solver region constraints. By the time regionck or borrowck reported them, every constraint received the containing item span, so diagnostics pointed at an entire function or const. Some paths were also still emitting placeholder text (`:3` and `meoow :c`). This keeps canonical solver responses and `ExternalConstraintsData` span-free, so source locations do not participate in candidate equality or caching. When a response is applied, `EvalCtxt::origin_span` is attached to each atomic solver `RegionConstraint` stored in `InferCtxt`. Late region conversion then uses the span attached to each constraint, including for ambiguity diagnostics. The affected paths now report `higher-ranked lifetime bound could not be satisfied` at the type use that introduced the failing constraint. UI coverage checks the rust-lang#157732 call-site diagnostic and the existing regionck alias-outlives case. Unit coverage checks that the spanned evaluator stays in semantic parity with the type-ir evaluator and preserves the first ambiguity origin span. The type-op behavior I ran into while working on this is split into rust-lang#161423. Future work: carry the failed outlives predicate or originating binder far enough through this path to name the exact bound that failed, rather than only pointing at its origin.

Rollup merge of #158588 - Dnreikronos:trait_selection/assumptions_binders_diagnostics, r=BoxyUwU trait_selection: fix assumptions-on-binders diagnostics fixes #157732 `-Zassumptions-on-binders` was losing the origin of solver region constraints. By the time regionck or borrowck reported them, every constraint received the containing item span, so diagnostics pointed at an entire function or const. Some paths were also still emitting placeholder text (`:3` and `meoow :c`). This keeps canonical solver responses and `ExternalConstraintsData` span-free, so source locations do not participate in candidate equality or caching. When a response is applied, `EvalCtxt::origin_span` is attached to each atomic solver `RegionConstraint` stored in `InferCtxt`. Late region conversion then uses the span attached to each constraint, including for ambiguity diagnostics. The affected paths now report `higher-ranked lifetime bound could not be satisfied` at the type use that introduced the failing constraint. UI coverage checks the #157732 call-site diagnostic and the existing regionck alias-outlives case. Unit coverage checks that the spanned evaluator stays in semantic parity with the type-ir evaluator and preserves the first ambiguity origin span. The type-op behavior I ran into while working on this is split into #161423. Future work: carry the failed outlives predicate or originating binder far enough through this path to name the exact bound that failed, rather than only pointing at its origin.

Dnreikronos · 2026-08-22T23:47:48Z

Sup,@BoxyUwU :)

Yeah, scrape_region_constraints makes more sense here. I went with the local type op first because it made the new constraints survive, but after tracing it a bit more I realized, as you said, that was just dodging the query path from #160982.

The next solver was storing the tree in the temporary query InferCtxt, but the query result only knew about the old region constraints. So when that context went away, the tree went with it. I changed QueryRegionConstraints to carry the tree too. It stays unspanned in the cached result, and the caller adds the local span when it registers it. scrape_region_constraints now returns the constraints made by that op without messing with whatever the caller already had.

I also found another place dropping the same tree during implied-bound normalization, before lexical regionck. I fixed that too and added regressions for that path and the borrowck case.

I like this version a lot more. It fixes the point where the data was lost and keeps the cache behavior from #160982. The local version worked, but mostly by going around the problem, and that felt wrong once I understood the full path.

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rustbot · 2026-08-25T19:55:21Z

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.

BoxyUwU · 2026-08-27T22:22:12Z

+
+    /// Runs `op` with an empty solver-region-constraint store, restores the
+    /// caller's constraints, and returns the constraints produced by `op`.
+    pub fn with_fresh_solver_region_constraints<R>(


why is this necessary for new style constraints but not old style?

View changes since the review

The difference is that the old obligations and assumptions are taken before the op and checked to be empty, so everything collected afterward belongs to that op. We cannot make that same check for the solver tree. Borrowck may already have constraints from an earlier type op in the same InferCtxt, and those stay there until the end of typeck. If we took the full tree afterward, this response would also contain the caller's older constraints. Clearing it first would lose them. The helper parks the old tree, captures what this op made, then restores the old one.

I do not love the extra swap, but I think this part is a real difference in how the constraints are stored and when they are consumed.

BoxyUwU · 2026-08-27T22:22:46Z

    /// discharge a requirement from another query, which is a potential problem if we did throw
    /// away these assumptions because there were no constraints.
    pub fn is_empty(&self) -> bool {
-        let QueryRegionConstraints { constraints, assumptions } = self;


the destructuring here is intentional. it means that adding new fields doesn't silently keep compiling and doing the wrong thing. please keep the let QueryRegionConstraints :3

View changes since the review

Yep, put the full destructuring back in is_empty and extend, and removed the .. from the implied-bound path too. This PR adding solver_constraints is exactly the kind of change that match was meant to catch, so keeping it exhaustive makes sense.

BoxyUwU · 2026-08-27T22:23:51Z

        &region_constraint_data,
        region_assumptions,
    );
+    region_constraints.solver_constraints = solver_constraints;


this generally feels kinda off, why doesn't this just work the same way as or region_obligations/region_assumptions?

View changes since the review

Yeah, agreed. I changed make_query_region_constraints to take the solver tree together with region_obligations and region_assumptions. The normal query path and scrape_region_constraints now build QueryRegionConstraints through the same function, and callers that cannot produce a solver tree pass the empty one.

The temporary swap is still needed so we do not mix a solver tree already registered by the caller into this op's response, but the query response side no longer bolts the field on afterward. I think this reads much better.

BoxyUwU · 2026-08-27T22:24:37Z

        // outlives bound required proving some higher-ranked coroutine obl.
-        let QueryRegionConstraints { constraints, assumptions: _ } = constraints;
+        let QueryRegionConstraints { constraints, solver_constraints, .. } = constraints;
+        if !solver_constraints.is_true() {


what's with all of these checks for !constraint.is_true() is this necessary or just a perf opt? if it's a perf opt I'd prefer we drop it.

View changes since the review

The checks do matter with the current representation, but putting them at every caller was the wrong place. true is And([]). Pushing that into the store turns it into And([And([])]); it still means true, but is_true() only recognizes the empty outer And, so an empty query starts looking nonempty.

I moved that check into the store instead. Registering a trivial tree now does nothing and does not add an undo-log entry, so the callers can register unconditionally. There is a unit test for the empty-query case too.

BoxyUwU · 2026-08-27T22:25:16Z

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rustbot · 2026-08-29T14:06:54Z

Some changes occurred to the core trait solver

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

Dnreikronos · 2026-08-29T14:12:34Z

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Dnreikronos mentioned this pull request Aug 20, 2026

trait_selection: fix assumptions-on-binders diagnostics #158588

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Dnreikronos marked this pull request as ready for review August 20, 2026 19:33

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Dnreikronos force-pushed the trait_selection/preserve_type_op_constraints branch from 65f9dd2 to 2959d13 Compare August 22, 2026 23:42

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Dnreikronos added 2 commits August 25, 2026 16:46

Carry solver region constraints through type ops

2a56108

Register solver constraints from implied bounds

dc2e6c6

Dnreikronos force-pushed the trait_selection/preserve_type_op_constraints branch from 2959d13 to dc2e6c6 Compare August 25, 2026 19:55

BoxyUwU reviewed Aug 27, 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 Aug 27, 2026

Dnreikronos added 4 commits August 28, 2026 17:44

Pass solver constraints into query responses

38a8b95

Keep query constraint handling exhaustive

69745c2

Keep implied-bound handling exhaustive

231cb46

Ignore trivial solver region constraints

96e94dd

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 Aug 29, 2026

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