Your first macro

Let's start from the basics: you'll write a macro that does nothing. It just re-emits the code that's been annotated with the macro, unchanged.
This will give you a chance to get familiar with the overall setup before moving on to more complex endeavors.

proc-macro = true

You can't define a procedural macro in a "normal" library crate.
They need to be in a separate crate, with a Cargo.toml that includes this key:

[lib]
proc-macro = true

That key tells cargo that this crate contains procedural macros and it should be compiled accordingly.

#[proc_macro_attribute]

There are various kinds of procedural macros:

• Function-like macros. Their invocation looks like a function call (e.g. println!).
• Derive macros. They're specified inside a derive attribute (e.g. #[derive(Debug)]).
• Attribute procedural macros. They're applied to items as attributes (e.g. #[tokio::test]).

For a test macro, we need an attribute procedural macro.
As you've learned in the intro, it's a function that's annotated with #[proc_macro_attribute]:

#![allow(unused)]
fn main() {
use proc_macro::TokenStream;

#[proc_macro_attribute]
pub fn my_attribute_macro(args: TokenStream, item: TokenStream) -> TokenStream {
    // [...]
}
}

The proc_macro crate is distributed as part of the Rust toolchain, just like the standard library, std.

Exercise

The exercise for this section is located in 08_macros/01_no_op_macro