AutoValue instances should
be deeply immutable. Therefore, we recommend using immutable types for fields.
E.g., use ImmutableMap instead of Map, ImmutableSet instead of Set, etc.
Read more at:
- https://github.com/google/auto/blob/master/value/userguide/practices.md#avoid-mutable-property-types
- https://github.com/google/auto/blob/master/value/userguide/builders-howto.md#-use-a-collection-valued-property
Suppress violations by using @SuppressWarnings("AutoValueImmutableFields") on
the relevant abstract getter.