Improve error message when parent is not a Starlark rule

PiperOrigin-RevId: 690771161
Change-Id: Iec8807ba88da6f17ad6e5a67177b47ba859e7b90

src/main/java/com/google/devtools/build/lib/analysis/starlark/StarlarkRuleClassFunctions.java

diff --git a/src/main/java/com/google/devtools/build/lib/analysis/starlark/StarlarkRuleClassFunctions.java b/src/main/java/com/google/devtools/build/lib/analysis/starlark/StarlarkRuleClassFunctions.java
index 31e2755..1ce27a9 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/starlark/StarlarkRuleClassFunctions.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/starlark/StarlarkRuleClassFunctions.java
@@ -457,7 +457,9 @@
       test = testUnchecked == Starlark.UNBOUND ? false : (Boolean) testUnchecked;
     } else {
       failIf(
-          !(parentUnchecked instanceof StarlarkRuleFunction), "Parent needs to be a Starlark rule");
+          !(parentUnchecked instanceof StarlarkRuleFunction),
+          "Parent needs to be a Starlark rule, was %s",
+          Starlark.type(parentUnchecked));
       // Assuming parent is already exported.
       failIf(
           ((StarlarkRuleFunction) parentUnchecked).ruleClass == null,