private static <T> T cast(
     String paramName, ClassObject struct, Class<T> expectedType, Location loc)
     throws EvalException {
   Object value = struct.getValue(paramName);
   return SkylarkType.cast(
       value,
       expectedType,
       loc,
       "expected %s for '%s' but got %s instead: %s",
       SkylarkType.of(expectedType),
       paramName,
       EvalUtils.getDataTypeName(value, false),
       value);
 }
  /**
   * Create a Rule Configured Target from the ruleContext and the ruleImplementation. The
   * registeredProviderTypes map indicates which keys in structs returned by skylark rules should be
   * interpreted as native TransitiveInfoProvider instances of type (map value).
   */
  public static ConfiguredTarget buildRule(
      RuleContext ruleContext,
      BaseFunction ruleImplementation,
      Map<String, Class<? extends TransitiveInfoProvider>> registeredProviderTypes)
      throws InterruptedException {
    String expectFailure = ruleContext.attributes().get("expect_failure", Type.STRING);
    try (Mutability mutability = Mutability.create("configured target")) {
      SkylarkRuleContext skylarkRuleContext = new SkylarkRuleContext(ruleContext, Kind.RULE);
      Environment env =
          Environment.builder(mutability)
              .setSkylark()
              .setCallerLabel(ruleContext.getLabel())
              .setGlobals(
                  ruleContext
                      .getRule()
                      .getRuleClassObject()
                      .getRuleDefinitionEnvironment()
                      .getGlobals())
              .setEventHandler(ruleContext.getAnalysisEnvironment().getEventHandler())
              .build(); // NB: loading phase functions are not available: this is analysis already,
      // so we do *not* setLoadingPhase().
      Object target =
          ruleImplementation.call(
              ImmutableList.<Object>of(skylarkRuleContext),
              ImmutableMap.<String, Object>of(),
              /*ast=*/ null,
              env);

      if (ruleContext.hasErrors()) {
        return null;
      } else if (!(target instanceof SkylarkClassObject)
          && target != Runtime.NONE
          && !(target instanceof Iterable)) {
        ruleContext.ruleError(
            String.format(
                "Rule should return a return a struct or a list, but got %s",
                SkylarkType.typeOf(target)));
        return null;
      } else if (!expectFailure.isEmpty()) {
        ruleContext.ruleError("Expected failure not found: " + expectFailure);
        return null;
      }
      ConfiguredTarget configuredTarget =
          createTarget(ruleContext, target, registeredProviderTypes);
      SkylarkProviderValidationUtil.checkOrphanArtifacts(ruleContext);
      return configuredTarget;
    } catch (EvalException e) {
      addRuleToStackTrace(e, ruleContext.getRule(), ruleImplementation);
      // If the error was expected, return an empty target.
      if (!expectFailure.isEmpty() && getMessageWithoutStackTrace(e).matches(expectFailure)) {
        return new com.google.devtools.build.lib.analysis.RuleConfiguredTargetBuilder(ruleContext)
            .add(RunfilesProvider.class, RunfilesProvider.EMPTY)
            .build();
      }
      ruleContext.ruleError("\n" + e.print());
      return null;
    }
  }
  private static void addOutputGroups(
      Object value, Location loc, RuleConfiguredTargetBuilder builder) throws EvalException {
    Map<String, SkylarkValue> outputGroups =
        SkylarkType.castMap(value, String.class, SkylarkValue.class, "output_groups");

    for (String outputGroup : outputGroups.keySet()) {
      SkylarkValue objects = outputGroups.get(outputGroup);
      NestedSet<Artifact> artifacts;

      String typeErrorMessage =
          "Output group '%s' is of unexpected type. "
              + "Should be list or set of Files, but got '%s' instead.";

      if (objects instanceof SkylarkList) {
        NestedSetBuilder<Artifact> nestedSetBuilder = NestedSetBuilder.stableOrder();
        for (Object o : (SkylarkList) objects) {
          if (o instanceof Artifact) {
            nestedSetBuilder.add((Artifact) o);
          } else {
            throw new EvalException(
                loc,
                String.format(
                    typeErrorMessage,
                    outputGroup,
                    "list with an element of " + EvalUtils.getDataTypeNameFromClass(o.getClass())));
          }
        }
        artifacts = nestedSetBuilder.build();
      } else {
        artifacts =
            SkylarkType.cast(
                    objects,
                    SkylarkNestedSet.class,
                    Artifact.class,
                    loc,
                    typeErrorMessage,
                    outputGroup,
                    EvalUtils.getDataTypeName(objects, true))
                .getSet(Artifact.class);
      }
      builder.addOutputGroup(outputGroup, artifacts);
    }
  }
  private static ConfiguredTarget addStructFieldsAndBuild(
      RuleContext ruleContext,
      RuleConfiguredTargetBuilder builder,
      Object target,
      Artifact executable,
      Map<String, Class<? extends TransitiveInfoProvider>> registeredProviderTypes)
      throws EvalException {
    Location loc = null;
    Runfiles statelessRunfiles = null;
    Runfiles dataRunfiles = null;
    Runfiles defaultRunfiles = null;
    if (target instanceof SkylarkClassObject) {
      SkylarkClassObject struct = (SkylarkClassObject) target;
      loc = struct.getCreationLoc();
      for (String key : struct.getKeys()) {
        if (key.equals("files")) {
          // If we specify files_to_build we don't have the executable in it by default.
          builder.setFilesToBuild(
              cast("files", struct, SkylarkNestedSet.class, Artifact.class, loc)
                  .getSet(Artifact.class));
        } else if (key.equals("runfiles")) {
          statelessRunfiles = cast("runfiles", struct, Runfiles.class, loc);
        } else if (key.equals("data_runfiles")) {
          dataRunfiles = cast("data_runfiles", struct, Runfiles.class, loc);
        } else if (key.equals("default_runfiles")) {
          defaultRunfiles = cast("default_runfiles", struct, Runfiles.class, loc);
        } else if (key.equals("output_groups")) {
          addOutputGroups(struct.getValue(key), loc, builder);
        } else if (key.equals("instrumented_files")) {
          SkylarkClassObject insStruct =
              cast("instrumented_files", struct, SkylarkClassObject.class, loc);
          Location insLoc = insStruct.getCreationLoc();
          FileTypeSet fileTypeSet = FileTypeSet.ANY_FILE;
          if (insStruct.getKeys().contains("extensions")) {
            @SuppressWarnings("unchecked")
            List<String> exts =
                cast("extensions", insStruct, SkylarkList.class, String.class, insLoc);
            if (exts.isEmpty()) {
              fileTypeSet = FileTypeSet.NO_FILE;
            } else {
              FileType[] fileTypes = new FileType[exts.size()];
              for (int i = 0; i < fileTypes.length; i++) {
                fileTypes[i] = FileType.of(exts.get(i));
              }
              fileTypeSet = FileTypeSet.of(fileTypes);
            }
          }
          List<String> dependencyAttributes = Collections.emptyList();
          if (insStruct.getKeys().contains("dependency_attributes")) {
            dependencyAttributes =
                cast("dependency_attributes", insStruct, SkylarkList.class, String.class, insLoc);
          }
          List<String> sourceAttributes = Collections.emptyList();
          if (insStruct.getKeys().contains("source_attributes")) {
            sourceAttributes =
                cast("source_attributes", insStruct, SkylarkList.class, String.class, insLoc);
          }
          InstrumentationSpec instrumentationSpec =
              new InstrumentationSpec(fileTypeSet)
                  .withSourceAttributes(sourceAttributes.toArray(new String[0]))
                  .withDependencyAttributes(dependencyAttributes.toArray(new String[0]));
          InstrumentedFilesProvider instrumentedFilesProvider =
              InstrumentedFilesCollector.collect(
                  ruleContext,
                  instrumentationSpec,
                  InstrumentedFilesCollector.NO_METADATA_COLLECTOR,
                  Collections.<Artifact>emptySet());
          builder.addProvider(InstrumentedFilesProvider.class, instrumentedFilesProvider);
        } else if (registeredProviderTypes.containsKey(key)) {
          Class<? extends TransitiveInfoProvider> providerType = registeredProviderTypes.get(key);
          TransitiveInfoProvider provider = cast(key, struct, providerType, loc);
          builder.addProvider(providerType, provider);
        } else if (key.equals("providers")) {
          Iterable iterable = cast(key, struct, Iterable.class, loc);
          for (Object o : iterable) {
            SkylarkClassObject declaredProvider =
                SkylarkType.cast(
                    o,
                    SkylarkClassObject.class,
                    loc,
                    "The value of 'providers' should be a sequence of declared providers");
            builder.addSkylarkDeclaredProvider(declaredProvider, loc);
          }
        } else if (!key.equals("executable")) {
          // We handled executable already.
          builder.addSkylarkTransitiveInfo(key, struct.getValue(key), loc);
        }
      }
    } else if (target instanceof Iterable) {
      loc = ruleContext.getRule().getRuleClassObject().getConfiguredTargetFunction().getLocation();
      for (Object o : (Iterable) target) {
        SkylarkClassObject declaredProvider =
            SkylarkType.cast(
                o,
                SkylarkClassObject.class,
                loc,
                "A return value of rule implementation function should be "
                    + "a sequence of declared providers");
        Location creationLoc = declaredProvider.getCreationLocOrNull();
        builder.addSkylarkDeclaredProvider(
            declaredProvider, creationLoc != null ? creationLoc : loc);
      }
    }

    if ((statelessRunfiles != null) && (dataRunfiles != null || defaultRunfiles != null)) {
      throw new EvalException(
          loc,
          "Cannot specify the provider 'runfiles' "
              + "together with 'data_runfiles' or 'default_runfiles'");
    }

    if (statelessRunfiles == null && dataRunfiles == null && defaultRunfiles == null) {
      // No runfiles specified, set default
      statelessRunfiles = Runfiles.EMPTY;
    }

    RunfilesProvider runfilesProvider =
        statelessRunfiles != null
            ? RunfilesProvider.simple(merge(statelessRunfiles, executable, ruleContext))
            : RunfilesProvider.withData(
                // The executable doesn't get into the default runfiles if we have runfiles states.
                // This is to keep skylark genrule consistent with the original genrule.
                defaultRunfiles != null ? defaultRunfiles : Runfiles.EMPTY,
                dataRunfiles != null ? dataRunfiles : Runfiles.EMPTY);
    builder.addProvider(RunfilesProvider.class, runfilesProvider);

    Runfiles computedDefaultRunfiles = runfilesProvider.getDefaultRunfiles();
    // This works because we only allowed to call a rule *_test iff it's a test type rule.
    boolean testRule = TargetUtils.isTestRuleName(ruleContext.getRule().getRuleClass());
    if (testRule && computedDefaultRunfiles.isEmpty()) {
      throw new EvalException(loc, "Test rules have to define runfiles");
    }
    if (executable != null || testRule) {
      RunfilesSupport runfilesSupport =
          computedDefaultRunfiles.isEmpty()
              ? null
              : RunfilesSupport.withExecutable(ruleContext, computedDefaultRunfiles, executable);
      builder.setRunfilesSupport(runfilesSupport, executable);
    }

    if (ruleContext.getRule().getRuleClassObject().isSkylarkTestable()) {
      SkylarkClassObject actions =
          ActionsProvider.create(ruleContext.getAnalysisEnvironment().getRegisteredActions());
      builder.addSkylarkDeclaredProvider(actions, loc);
    }

    try {
      return builder.build();
    } catch (IllegalArgumentException e) {
      throw new EvalException(loc, e.getMessage());
    }
  }