expr metavariable drops commas

[robertbastian]

When matching an expression in a macro_rules, trailing commas in argument lists and initializers disappear.

macro_rules! compare {
    ($expr: expr) => {
        assert_eq!(stringify!($expr), "Foo { bar: 32, }");
    };
}

compare!(Foo { foo: 32, }); // fails
              // this ^ comma disappears

Playground

[robertbastian]

(this is not an issue with stringify, I'm actually encountering this when I pass the expression into the quote macro)

[sffc]

To be clear, the use case here is to check an arbitrary TokenStream, such as one generated by a derive macro, against code in stringify!($expr). I understand that the stringified form of TokenStream is not stable, but two TokenStreams should have the same ToString.

[fmease]

As already mentioned in this thread, the output of stringify! is not stable. Closing as wontfix / works as intended.

To be clear, the use case here is to check an arbitrary TokenStream, such as one generated by a derive macro, against code in stringify!($expr). I understand that the stringified form of TokenStream is not stable, but two TokenStreams should have the same ToString.

Please provide a concrete reproducer for this if you think it doesn't directly relate to stringify.

[robertbastian]

I did mention that this is not an issue with the stringify! macro, but with macro variables. Here's a reproduction using quote!: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=c168c46ac64ef031f0f9b3c271aa63ab