How to write a Rust syntax extension

While I was working on my ggp-rs project last week, I was having some trouble tracking down a strange bug that was happening. The relevant code was related to the unification, substitution, and general statement proving algorithms, which is a non-trivial piece of code to write, read, and debug. I started to put println! statements in various functions, sometimes just to see if the function was entered, and sometimes to see the value of some variables. After spending about half an hour on this bug, I got fed up with the code, took a step back, and started thinking of an easier, more structured approach to debugging the code. I realized that putting print statements in the code to trace execution is a common debugging practice used by myself and other developers all the time, and it might be possible to make this more convenient with the help of Rust’s syntax extensions/compiler plugins.