See https://github.com/mongodb/mongo-rust-driver/issues/475 for more details.

This could possibly create tricky situations however. For example, imagine a user parses a URI like "mongodb://user:pass@localhost/myauthdb" into a ClientOptions, and then sets the default_database on the options to be something else. The one in the credential will still be "myauthdb", which could be surprising. That being said, this case is fairly contrived and could even be considered expected/desired behavior.