How to Write a Function-Like Procedural Macro in Rust

You write a function-like procedural macro by defining a proc-macro crate with a single function annotated with #[proc_macro] that takes a TokenStream and returns a Result<TokenStream>.

// In a new crate with `proc-macro = true` in Cargo.toml
use proc_macro::TokenStream;

#[proc_macro]
pub fn my_macro(input: TokenStream) -> TokenStream {
    // Transform input tokens here
    input
}

Add the macro crate as a dependency in your main project's Cargo.toml and call it like a function: my_macro!();.

A function-like procedural macro is a special piece of code that runs while your program is being built. It takes the code you write as input, transforms it into new code, and hands it back to the compiler to be compiled. Think of it as a custom code generator that lets you write less repetitive code by automating the creation of boilerplate.