Unsized temporary is not an implementation error

crates/hir-ty/src/consteval/tests.rs

@@ -2066,3 +2066,22 @@ fn type_error() {
         |e| matches!(e, ConstEvalError::MirLowerError(MirLowerError::TypeMismatch(_))),
     );
 }
+
+#[test]
+fn unsized_local() {
+    check_fail(
+        r#"
+    //- minicore: coerce_unsized, index, slice
+    const fn x() -> SomeUnknownTypeThatDereferenceToSlice {
+        SomeUnknownTypeThatDereferenceToSlice
+    }
+
+    const GOAL: u16 = {
+        let y = x();
+        let z: &[u16] = &y;
+        z[1]
+    };
+    "#,
+        |e| matches!(e, ConstEvalError::MirLowerError(MirLowerError::UnsizedTemporary(_))),
+    );
+}

crates/hir-ty/src/mir/lower.rs

@@ -75,6 +75,7 @@ pub enum MirLowerError {
     RecordLiteralWithoutPath,
     UnresolvedMethod(String),
     UnresolvedField,
+    UnsizedTemporary(Ty),
     MissingFunctionDefinition,
     TypeMismatch(TypeMismatch),
     /// This should be never happen. Type mismatch should catch everything.
@@ -108,6 +109,7 @@ pub fn pretty_print(
                 }
             }
             MirLowerError::LayoutError(_)
+            | MirLowerError::UnsizedTemporary(_)
             | MirLowerError::IncompleteExpr
             | MirLowerError::UnaccessableLocal
             | MirLowerError::TraitFunctionDefinition(_, _)
@@ -199,7 +201,7 @@ fn new(
 
     fn temp(&mut self, ty: Ty) -> Result<LocalId> {
         if matches!(ty.kind(Interner), TyKind::Slice(_) | TyKind::Dyn(_)) {
-            implementation_error!("unsized temporaries");
+            return Err(MirLowerError::UnsizedTemporary(ty));
         }
         Ok(self.result.locals.alloc(Local { ty }))
     }