`-Znext-solver` Prevent committing unfulfilled unsized coercion by ShoyuVanilla · Pull Request #152444 · rust-lang/rust

ShoyuVanilla · 2026-02-10T16:30:28Z

Fixes rust-lang/trait-system-refactor-initiative#266

r? lcnr

rustbot · 2026-02-10T16:30:32Z

Some changes occurred to the core trait solver

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

ShoyuVanilla · 2026-02-10T16:31:40Z

tests/ui/traits/next-solver/coercion/unfulfilled-unsize-coercion-recursion-limit.rs

+            .add(0.0)
+            .add(0.0)
+            .add(0.0)
+            .add(0.0)


Added some more .add calls from adwin's original code to make it sure this exceeds the default recursion limit (10)

lcnr · 2026-02-10T17:50:52Z

compiler/rustc_trait_selection/src/solve/inspect/analyse.rs

        if self.depth < visitor.config().max_depth {
            try_visit!(visitor.visit_goal(self));
+        } else {
+            return visitor.on_recursion_limit();
        }

        V::Result::output()


that can be simplified

lcnr · 2026-02-10T17:56:58Z

compiler/rustc_hir_typeck/src/coercion.rs

+    fn on_recursion_limit(&mut self) -> Self::Result {
+        // This prevents accidentally committing unfulfilled unsized coercions while trying to find
+        // the error source for diagnostics.
+        // See https://github.com/rust-lang/trait-system-refactor-initiative/issues/266.


i think this breaks the following snippet if we encounter it in the Certainty::Maybe recursion

struct W<T: ?Sized>(T); type Four<T: ?Sized> = W<W<W<W<T>>>>; type Sixteen<T: ?Sized> = Four<Four<Four<Four<T>>>>; fn ret<T>(x: T) -> Sixteen<T> { todo!(); } fn please_coerce() { let mut y = Default::default(); let x = ret(y); let _: &Sixteen<dyn Send> = &x; y = 1u32; }

Oops, I missed the case that we could visit nested goal here 😅

rust/compiler/rustc_hir_typeck/src/coercion.rs

Lines 2012 to 2015 in 7b25457

} else if let Some(cand) = goal.unique_applicable_candidate()

&& cand.shallow_certainty() == Certainty::Yes

{

cand.visit_nested_no_probe(self)

rustbot · 2026-02-11T06:27:29Z

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

comment nit, then r=me

View changes since this review

lcnr · 2026-02-12T11:59:02Z

compiler/rustc_hir_typeck/src/coercion.rs

 struct CoerceVisitor<'a, 'tcx> {
    fcx: &'a FnCtxt<'a, 'tcx>,
    span: Span,
+    // Whether the coercion is impossible and just continuing to visit to locate the error source.


Suggested change

// Whether the coercion is impossible and just continuing to visit to locate the error source.

/// Whether the coercion is impossible. If so we someties still try to

/// coerce in these cases to emit better errors. This impacts changes the

/// behavior when hitting the recursion limit.

lcnr · 2026-02-12T11:59:55Z

@bors delegate+

rust-bors · 2026-02-12T11:59:58Z

✌️ @ShoyuVanilla, you can now approve this pull request!

If @lcnr told you to "r=me" after making some further change, then please make that change and post @bors r=lcnr.

ShoyuVanilla · 2026-02-12T17:24:17Z

@bors r=lcnr rollup

rust-bors · 2026-02-12T17:24:20Z

📌 Commit 891acbd has been approved by lcnr

It is now in the queue for this repository.

…it, r=lcnr `-Znext-solver` Prevent committing unfulfilled unsized coercion Fixes rust-lang/trait-system-refactor-initiative#266 r? lcnr

…uwer Rollup of 11 pull requests Successful merges: - #146901 (Support AVRTiny devices in AVR inline assembly) - #150988 (Improve code suggestion for incorrect macro_rules! usage) - #152422 (Change query proc macro to be more rust-analyzer friendly) - #152496 (Fix multi-cgu+debug builds using autodiff by delaying autodiff till lto) - #152520 (Don't use `DepContext` in `rustc_middle::traits::cache`) - #152528 (Support serializing CodegenContext) - #152082 (Move tests) - #152444 (`-Znext-solver` Prevent committing unfulfilled unsized coercion) - #152486 (remove redundant backchain attribute in codegen) - #152529 (sparc64: enable abi compatibility test) - #152548 (reject inline const patterns pre-expansion)

…it, r=lcnr `-Znext-solver` Prevent committing unfulfilled unsized coercion Fixes rust-lang/trait-system-refactor-initiative#266 r? lcnr

Rollup of 18 pull requests Successful merges: - #150551 (Compute localized outlives constraints lazily) - #150752 (Update libc to v0.2.181) - #150988 (Improve code suggestion for incorrect macro_rules! usage) - #152422 (Change query proc macro to be more rust-analyzer friendly) - #152496 (Fix multi-cgu+debug builds using autodiff by delaying autodiff till lto) - #152514 (Collect active query jobs into struct `QueryJobMap`) - #152520 (Don't use `DepContext` in `rustc_middle::traits::cache`) - #152528 (Support serializing CodegenContext) - #152082 (Move tests) - #152232 (Add must_use for FileTimes) - #152329 (Simplify parallel! macro) - #152444 (`-Znext-solver` Prevent committing unfulfilled unsized coercion) - #152486 (remove redundant backchain attribute in codegen) - #152519 (Fix feature gating for new `try bikeshed` expressions) - #152529 (sparc64: enable abi compatibility test) - #152548 (reject inline const patterns pre-expansion) - #152550 (Port #[prelude_import] to the attribute parser) - #152552 (Add 2048-bit HvxVectorPair support to Hexagon SIMD ABI checks)

Rollup of 17 pull requests Successful merges: - #150551 (Compute localized outlives constraints lazily) - #150988 (Improve code suggestion for incorrect macro_rules! usage) - #152422 (Change query proc macro to be more rust-analyzer friendly) - #152496 (Fix multi-cgu+debug builds using autodiff by delaying autodiff till lto) - #152514 (Collect active query jobs into struct `QueryJobMap`) - #152520 (Don't use `DepContext` in `rustc_middle::traits::cache`) - #152528 (Support serializing CodegenContext) - #152082 (Move tests) - #152232 (Add must_use for FileTimes) - #152329 (Simplify parallel! macro) - #152444 (`-Znext-solver` Prevent committing unfulfilled unsized coercion) - #152486 (remove redundant backchain attribute in codegen) - #152519 (Fix feature gating for new `try bikeshed` expressions) - #152529 (sparc64: enable abi compatibility test) - #152548 (reject inline const patterns pre-expansion) - #152550 (Port #[prelude_import] to the attribute parser) - #152552 (Add 2048-bit HvxVectorPair support to Hexagon SIMD ABI checks) Failed merges: - #152515 (Extract `DepKindVTable` constructors to their own module)

Rollup merge of #152444 - ShoyuVanilla:unsized-recursion-limit, r=lcnr `-Znext-solver` Prevent committing unfulfilled unsized coercion Fixes rust-lang/trait-system-refactor-initiative#266 r? lcnr

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 Feb 10, 2026

rustbot assigned lcnr Feb 10, 2026

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ShoyuVanilla force-pushed the unsized-recursion-limit branch from 1751d10 to e93b68c Compare February 10, 2026 16:40

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ShoyuVanilla force-pushed the unsized-recursion-limit branch from e93b68c to 96d2318 Compare February 11, 2026 06:27

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ShoyuVanilla force-pushed the unsized-recursion-limit branch from 96d2318 to 069c78f Compare February 12, 2026 15:38

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-Znext-solver Prevent committing unfulfilled unsized coercion

891acbd

ShoyuVanilla force-pushed the unsized-recursion-limit branch from 069c78f to 891acbd Compare February 12, 2026 15:57

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 Feb 12, 2026

JonathanBrouwer mentioned this pull request Feb 12, 2026

Rollup of 11 pull requests #152551

Closed

Zalathar mentioned this pull request Feb 13, 2026

Rollup of 18 pull requests #152560

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Zalathar mentioned this pull request Feb 13, 2026

Rollup of 17 pull requests #152562

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rust-bors bot merged commit 0c0af5c into rust-lang:main Feb 13, 2026
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rustbot added this to the 1.95.0 milestone Feb 13, 2026

ShoyuVanilla deleted the unsized-recursion-limit branch February 13, 2026 14:57

	} else if let Some(cand) = goal.unique_applicable_candidate()
	&& cand.shallow_certainty() == Certainty::Yes
	{
	cand.visit_nested_no_probe(self)

-    // Whether the coercion is impossible and just continuing to visit to locate the error source.
+    /// Whether the coercion is impossible. If so we someties still try to
+    /// coerce in these cases to emit better errors. This impacts changes the
+    /// behavior when hitting the recursion limit.

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