Here is a link to the code at this point in time. https://github.com/JeffMurray/must/tree/29fd8652674a1b1009d7a168fc544c830fdce87c/src I am getting the error when compiling par.rs, but I am guessing the opaque trait the compiler is referring to is ParFitable in fit.rs. Thanks for making Rust, and sorry if I am making a noob mistake :)