Can a procedural macro be debugged as a function?

Question

I'm trying to debug a complex procedural macro in a library I am using.

Since I cannot use a debugger with macros and various macro expansion tools have proved useless here, I'm looking for an alternative.

Can a procedural macro be run like a function with proper debugging? I imagine storing the resultant proc_macro::TokenStream in a variable.

Answer 1

The proc-macro2 crate is a drop-in replacement for proc_macro except that it is available outside of macros - which makes it testable. Its types are all convertible to and from the proc_macro types and have identical methods.

The usual pattern for writing a nontrivial macro is to use proc_macro only for the entry point, and use proc-macro2 for all of the real work:

extern crate proc_macro;
use proc_macro2::TokenStream;

#[proc_macro]
pub fn my_macro(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
    let output = transform_stream(TokenStream::from(input));
    proc_macro::TokenStream::from(output)
}

// A testable function!
fn transform_stream(input: TokenStream) -> TokenStream {
    // actual work goes here
}

It's common to import items from proc-macro2 so they can be used unqualified, and just used fully qualified names for proc_macro, since the only time you will use it is in the entry point. It's also usual to put the core components in a separate library crate, which does not have a dependency on proc_macro.

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In tests, you can create a TokenStream from a string:

use std::str::FromStr;
let ts = TokenStream::from_str("fn foo() {}").unwrap();