Procedural macros take a list of tokens, and you give a list of tokens back, and they're then compiled.

https://doc.rust-lang.org/stable/book/ch19-06-macros.html#ho... (starts with derive, then moves on to attributes. Function-like ones aren't stable yet)

IMO the fully general version of that is too flexible to be maintainable; it's better to have more constrained/standardised functionality that lets you solve the problems you'd need it for. Generally there are two kinds of things that you would want to be doing at compile time: producing code to handle the fields of a structure in a generic way (which you can do with frunk), or producing delegating wrappers to handle cross-cutting concerns (which you can do up to a point via the combination of built in delegation and first-class functions, though the lack of HKT does make some cases impossible).