infer_clone/infer/documentation/issues/CHECKERS_IMMUTABLE_CAST.md

This error type is reported in Java. It fires when an immutable collection is returned from a method whose type is mutable.

  public List<String> getSomeList() {
    ImmutableList<String> l = foo(...);
    return l;
  }

This can lead to a runtime error if users of getSomeList try to modify the list e.g. by adding elements.

Action: you can change the return type to be immutable, or make a copy of the collection so that it can be modified.