The logging section of the 0.6 docs ( http://static.rust-lang.org/doc/0.6/rust.html#logging-system ) doesn't specify how you log messages. This link is broken: http://static.rust-lang.org/doc/0.6/rust.html#log-expressions It is not broken in the 0.5 docs, but I believe the API has changed. http://static.rust-lang.org/doc/0.5/rust.html#log-expressions