For example, the docs for this type use Vec<String, Global> rather than just Vec<String>: https://docs.rs/tokio-postgres/0.7.0/tokio_postgres/types/enum.Kind.html. The definition of the type does not specify the allocator parameter if that matters.

The same is not true for e.g. HashMap's hasher type parameter, so I'm not sure what's different in this case.