@Test
  public void customExtensionsCanBeIsolating() {
    AutoValueExtension isolatingExtension = new IsolatingExtension();
    assertThat(isolatingExtension.incrementalType((ProcessingEnvironment) null))
        .isEqualTo(IncrementalExtensionType.ISOLATING);
    ImmutableList<AutoValueExtension> extensions =
        ImmutableList.<AutoValueExtension>builder()
            .addAll(
                AutoValueProcessor.extensionsFromLoader(AutoValueProcessor.class.getClassLoader()))
            .add(isolatingExtension)
            .build();

    AutoValueProcessor processor = new AutoValueProcessor(extensions);
    assertThat(processor.getSupportedOptions())
        .contains(IncrementalAnnotationProcessorType.ISOLATING.getProcessorOption());
  }
Tests of Gradle incrementality in the presence of extensions. See <a
href="https://docs.gradle.org/5.0/userguide/java_plugin.html#sec:incremental_annotation_processing">
incremental annotation processing</a> in the Gradle user guide.
value/src/test/java/com/google/auto/value/processor/IncrementalExtensionTest.java