Code review request of procedural macro

I just published a new crate here (github). In summary, it is a crate to facilitate a new way of handling errors in Rust, but before I start advertising it as such I'd like to get feedback from someone who is more experienced with procedural macros than I am (this is my first such macro). Detailed information about what the crate is is available on Github and crates.io (and docs.rs once it gets around to generating documentation for my cratee).