Add a test for subrule bug that permits a subrule to call another

The current code assumes a subrule is always called from a rule/aspect implementation and does not check if we're evaluating a subrule. So the call works as long the called subrule was declared on the rule/aspect.

PiperOrigin-RevId: 580145481
Change-Id: I1e2fcf44b3b6245589dddc91c524cffc473ca186

src/test/java/com/google/devtools/build/lib/analysis/starlark/StarlarkSubruleTest.java

@@ -150,6 +150,38 @@ public void testSubrule_aspectMustDeclareSubrule() throws Exception {
                 + " declare '_my_subrule' in 'subrules'");
   }
 
+  // this is a bug that must be fixed
+  @Test
+  public void testSubruleCallingSibling_succeeds() throws Exception {
+    scratch.file(
+        "subrule_testing/myrule.bzl",
+        "def _subrule1_impl(ctx):",
+        "  return 'result from subrule1'",
+        "_my_subrule1 = subrule(implementation = _subrule1_impl)",
+        "",
+        "def _subrule2_impl(ctx):",
+        "  return _my_subrule1()",
+        "_my_subrule2 = subrule(implementation = _subrule2_impl)",
+        "",
+        "MyInfo=provider()",
+        "def _rule_impl(ctx):",
+        "  res = _my_subrule2()",
+        "  return [MyInfo(result = res)]",
+        "",
+        "my_rule = rule(_rule_impl, subrules = [_my_subrule2, _my_subrule1])");
+    scratch.file(
+        "subrule_testing/BUILD",
+        //
+        "load('myrule.bzl', 'my_rule')",
+        "my_rule(name = 'foo')");
+
+    StructImpl provider =
+        getProvider("//subrule_testing:foo", "//subrule_testing:myrule.bzl", "MyInfo");
+
+    assertThat(provider).isNotNull();
+    assertThat(provider.getValue("result")).isEqualTo("result from subrule1");
+  }
+
   @Test
   public void testSubrule_implementationMustAcceptSubruleContext() throws Exception {
     scratch.file(