cargo-expand works well, but it does not respect hygiene when expanding the macros. As a result, the expanded code may behave diffentently. Here's an example:

fn f() -> i32 {
    let x = 1;

    macro_rules! first_x {
        () => { x }
    }

    let x = 2;

    x + first_x!()
}

In this example, the x coming from the expansion has call-site hygiene. At least, it should resolve to the x that is defined in the first statement of the function. The f function should return 3.

cargo-hexpand uses the rustc internal API in order to detect shadowing that alter the program behavior and rename rename the variables so that no semantic change occurs. This leads to the following expansion:

use std::prelude::rust_2021::*;
extern crate std;
fn hygiene_test() -> i32 {
    let x = 1;

    macro_rules! first_x {
        () => {
            x
        };
    }

    let x_0 = 2;

    x_0 + x
}

The second x was renamed to x_0, so that no shadowing occurs. f returns 3 as well. Great!

TODO

(nightly, rustc-dev, ...)