recursive macro --- macro doesn't exist

[BurntSushi]

I believe this should output the string abc:

#![feature(macro_rules)]

macro_rules! rec(
    () => ("");
    ($a:ident) => (stringify!($a));
    ($a:ident, $($rest:tt),*) => (
        concat!(stringify!($a), rec!($($rest),*))
    );
)

fn main() {
    println!("{}", rec!(a, b, c));
}

But I get this error:

reem.rs:7:33: 7:36 error: macro undefined: 'rec'
reem.rs:7         concat!(stringify!($a), rec!($($rest),*))

Rust version:

[andrew@Liger rust] rustc --version
rustc 0.11.0-pre (6d8342f 2014-06-14 17:51:49 +0000)
host: x86_64-unknown-linux-gnu

[BurntSushi]

Note that standard recursive macros work fine, e.g.,

macro_rules! rec_works(
    () => (0);
    ($a:expr) => ($a);
    ($a:expr, $($rest:tt),*) => (
        $a + rec_works!($($rest),*)
    );
)

And rec_works!(1, 2, 3) outputs 6.

Maybe the bug here is that expanding macros within other macros is using an older environment that doesn't include the name rec?

[sfackler]

It's even worse than that. expand_expr only uses the compiler built-in macros: https://github.com/rust-lang/rust/blob/master/src/libsyntax/ext/base.rs#L421

[huonw]

I think this might be a dupe of https://github.com/rust-lang/rust/issues/12404.

[BurntSushi]

@huonw Does fixing print!(format!(...)) mean that print!(some_recursive_call!(...)) also gets fixed? I wouldn't assume so, but it might be possible depending on the implementation.

(Sorry I missed that issue. I searched for 'recursive macro' instead of 'nested macro'!)

[sfackler]

@BurntSushi both print! and concat_idents! use expand_expr, which doesn't work with non-builtin macros.

[BurntSushi]

@sfackler Right. I'm referring to the recursive aspect. Wasn't sure if that would make a difference. (I just don't know enough about how macros are expanded to know. Pure guesswork on my part.)