[https://docs.rs/mongodb/2.0.0-beta.2/mongodb/options/struct.ClientOptions.html#structfield.hosts
]
there are two problems:
1. The direct params in doc links should be directConnection. 
2. directConnection = True can not work

database link: 

mongodb://xxx:xxx@xxxx:xx/xx?replicaSet=xx&authSource=admin&readPreference=secondary&directconnection=true

the host is one of secondary node.

Error: Error { kind: ServerSelection { message: "Server selection timeout: None of the available servers suitable for criteria Predicate. Topology: { Type: Single, Servers: [...