public IncrementalExtensionType incrementalType(ProcessingEnvironment processingEnvironment) {
    return IncrementalExtensionType.UNKNOWN;
  }
Determines the incremental type of this Extension.

<p>The {@link ProcessingEnvironment} can be used, among other things, to obtain the processor
options, using {@link ProcessingEnvironment#getOptions()}.

<p>The actual incremental type of the AutoValue processor as a whole will be the loosest
incremental types of the Extensions present in the annotation processor path. The default
returned value is {@link IncrementalExtensionType#UNKNOWN}, which will disable incremental
annotation processing entirely.
value/src/main/java/com/google/auto/value/extension/AutoValueExtension.java