fix: use sub-unification roots in type_var_is_sized - #23464
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ChayimFriedman2 merged 1 commit intoOct 4, 2026
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When checking whether an inference variable is Sized (used during unsize coercions), ype_var_is_sized previously compared equality-only roots via oot_var. This meant that a Sized obligation on a variable related through subtyping (e.g. from Box::new(y) unsized to Box<dyn Send>) was missed, causing spurious type mismatches. Thread a UseSubtyping flag (modeled on rust-lang/rust#157820) through obligations_for_self_ty, predicate_has_self_ty, and ype_matches_expected_vid. ype_var_is_sized passes Yes (since sizedness is preserved under subtyping), while all closure callers pass No. Fixes rust-lang#23114
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Part of #23114. This fixes a false positive
expected i32, found dyn Senddiagnostic when an inference variable is related to aSizedobligation through subtyping.Previously, sizedness checks only considered inference variables connected through the regular unification table. This could miss obligations connected through subtyping. The fix introduces
UseSubtypingand uses the sub-unification table when checkingSizedobligations. Other callers keep the existing equality-based behavior.This follows the corresponding rustc change in rust-lang/rust#157820,
where
FnCtxt::type_var_is_sized(compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs) usesUseSubtyping::Yes, together with the subtype-aware obligation matching incompiler/rustc_hir_typeck/src/fn_ctxt/inspect_obligations.rs. Inrust-analyzer, the corresponding logic is in
InferenceTable::obligations_for_self_tyandtype_matches_expected_vid.Adds one regression test,
coerce_unsize_infer_var_via_default_and_box, based on the first snippet in the issue. It failed before this change and passes now.The second snippet in the issue is not fixed by this PR. It reports the same error with and without this change, so it appears to be a separate problem.