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Fix ICE when projection error reporting sees opaque alias terms #151923

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rust-bors merged 1 commit into from
Feb 2, 2026
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Fix ICE when projection error reporting sees opaque alias terms #151923

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2 changes: 2 additions & 0 deletions compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
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@fmease fmease Feb 1, 2026 •
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The fix is correct; the existing logic in report_projection_error "can only deal with" / "is mostly geared to handle" projection mismatches, not arbitrary aliases mismatches.

Still, we currently map unsolved NormalizesTo and AliasRelate goals to FulfillmentErrorCode::Project (in fn fulfillment_error_for_no_solution).

Long term, we should probably rename+generalize FulfillmentErrorCode::Project to account for all AliasTerms and extend downstream code to handle all alias terms better (e.g., give more suggestions).

Original file line number Diff line number Diff line change
Expand Up @@ -1607,6 +1607,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
};

if let Some(lhs) = lhs.to_alias_term()
&& let ty::AliasTermKind::ProjectionTy | ty::AliasTermKind::ProjectionConst = lhs.kind(self.tcx)
&& let Some((better_type_err, expected_term)) =
derive_better_type_error(lhs, rhs)
{
Expand All @@ -1615,6 +1616,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
better_type_err,
)
} else if let Some(rhs) = rhs.to_alias_term()
&& let ty::AliasTermKind::ProjectionTy | ty::AliasTermKind::ProjectionConst = rhs.kind(self.tcx)
&& let Some((better_type_err, expected_term)) =
derive_better_type_error(rhs, lhs)
{
Expand Down
10 changes: 10 additions & 0 deletions tests/ui/type-alias-impl-trait/opaque-alias-relate-issue-151331.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,10 @@
//@ compile-flags: -Znext-solver=globally
#![feature(type_alias_impl_trait)]

type Foo = Vec<impl Send>;

#[define_opaque(Foo)]
fn make_foo() -> Foo {}
//~^ ERROR type mismatch resolving

fn main() {}
9 changes: 9 additions & 0 deletions tests/ui/type-alias-impl-trait/opaque-alias-relate-issue-151331.stderr
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Original file line number Diff line number Diff line change
@@ -0,0 +1,9 @@
error[E0271]: type mismatch resolving `Foo == ()`
--> $DIR/opaque-alias-relate-issue-151331.rs:7:18
|
LL | fn make_foo() -> Foo {}
| ^^^ types differ

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0271`.
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