Building Custom Tooling for Macros

In this lesson, you will learn how to design external CLI tools that parse your project’s source code to perform automated auditing of macro usage patterns.

Beyond the Compiler

While macros operate during compilation, they often hide complex state, invariants, or requirements that are difficult to inspect from the compiler's error messages alone. When you have a macro-heavy codebase, you often need an "auditor"—an external tool that understands the intent of your macros by scanning the project's Abstract Syntax Trees (AST) before or during the build process.

Building an Auditor

You can leverage syn and quote outside of a proc-macro context to build a standalone CLI tool. By utilizing the syn crate’s visit module, you can traverse your codebase to find specific macro invocations and validate that they comply with project-wide architectural rules.

For example, imagine you have a #[secure_gateway] macro that must always be paired with an explicit #[permission_level] attribute. You can write a small cargo-audit-macros tool:

use syn::{visit::{self, Visit}, DeriveInput};

struct MacroAuditor;

impl<'ast> Visit<'ast> for MacroAuditor {
    fn visit_item_fn(&mut self, i: &'ast syn::ItemFn) {
        let has_gateway = i.attrs.iter().any(|a| a.path().is_ident("secure_gateway"));
        let has_perm = i.attrs.iter().any(|a| a.path().is_ident("permission_level"));

        if has_gateway && !has_perm {
            panic!("Function {} uses #[secure_gateway] without #[permission_level]!", i.sig.ident);
        }
        visit::visit_item_fn(self, i);
    }
}

Integration

This auditor tool doesn't need to be part of the actual crate's compilation graph. Instead, you integrate it into your CI/CD pipeline or as a custom cargo subcommand. By running this tool over your source code, you enforce "macro invariants" that are otherwise invisible to the borrow checker. This allows you to catch architectural drift—such as developers using macros in unauthorized modules or omitting required metadata—long before the macro-expansion phase even triggers.

By moving your auditing logic into a dedicated, inspectable binary rather than embedding it deep within the macro expansion itself, you gain faster feedback loops and the ability to generate comprehensive compliance reports for your entire workspace.

Key Takeaway: External macro auditing tools leverage the syn crate to scan ASTs for architectural compliance, moving invariant enforcement out of the compiler and into your project's quality-control pipeline.