macro_expand

A Rust library for expanding procedural macros in-place. It allows you to register custom procedural macros and apply them to Rust source code programmatically. This is particularly useful for testing procedural macros by comparing their output against expected snapshots.

Installation

Add to your Cargo.toml:

[dependencies]
macro_expand = { version = "0.1" }

Example

When developing procedural macros, it's recommended to implement your macro logic using proc_macro2 instead of directly using proc_macro. This approach enables testing without the limitations of the proc_macro API, which cannot be used outside of a procedural macro context.

use proc_macro::TokenStream;
use proc_macro2::TokenStream as TokenStream2;
use quote::quote;

// The actual proc-macro entry point
#[proc_macro_derive(MyTrait)]
pub fn derive_my_trait(tokens: TokenStream) -> TokenStream {
    derive_my_trait_inner(tokens.into()).into()
}

// The testable implementation using proc_macro2
pub(crate) fn derive_my_trait_inner(tokens: TokenStream2) -> TokenStream2 {
    let input: syn::DeriveInput = syn::parse2(tokens).unwrap();
    let input_ident = &input.ident;
    quote! {
        impl MyTrait for #input_ident {
            fn my_method(&self) {
                println!("MyTrait method called on {}", stringify!(#input_ident));
            }
        }
    }
}

#[cfg(test)]
mod test {
    use macro_expand::Context;
    use quote::quote;
    use std::fs::read_to_string;
    use super::*;

    #[test]
    fn fixtures() {
        let mut ctx = Context::new();
        // Register the inner implementation using testable implementation
        ctx.register_proc_macro_derive("MyTrait".into(), derive_my_trait_inner, vec![]);
        let input = read_to_string("fixtures/input.rs").unwrap().parse().unwrap();
        let output = ctx.transform(input);
        let expected = read_to_string("fixtures/output.rs").unwrap().parse().unwrap();
        let formatted = prettyplease::unparse(&syn::parse(output).unwrap());
        assert_eq!(formatted, expected);
    }
}