The collections returned by the protobuf API are unmodifiable: attempts to modify them will result in an UnsupportedOperationException.

The protobuf collections are not implemented using Guava‘s immutable collections, so copying them as a Guava collection isn’t free.

This check suggests omitting copies of unmodifiable protobuf collections as Guava immutable collections in places where a Guava immutable collection is not required, for example to replace this:

List<String> foos(MyProtoMessage message) {
  return ImmutableList.copyOf(message.getFoos());
}

with this:

List<String> foos(MyProtoMessage message) {
  return message.getFoos();
}

Note that the check will not report diagnostics if the result of the copy is being used somewhere that requires a Guava Immutable collection, for example it is returned from the current method, or passed to an API that expects a Guava immutable collection:

ImmutableList<String> foos(MyProtoMessage message) {
  return ImmutableList.copyOf(message.getFoos());
}

The check will suggest refactoring local variables, if the local variable is never used in a context that requires a Guava immutable collection. Using Guava's collections has some benefit in this case, since it makes the immutability of the collection clear and can enable other compile-time static analysis for accidental attempts to modify the collection. But this value is limited if it is only used as a local variable that never escapes the current method, and there is some runtime cost from making the copy.