README for this exercise.
Chris Biscardi: [0:00] Macros2 has a very similar setup to macros1, where we have a main function with a body that calls our macro and a macro_rules definition that defines my_macro with a println. The help message from the Rust compiler asks us if we've added the macro_use on the module import. It warns us because we have the unused macros warning turned on, which is on by default.
[0:19] However, macros need to be defined before they're used. If we switch the order of the main function body and the macro, our test work. This ordering for macros needing to be defined before they're used is different than most other things in Rust.