2018: #[macro_export(local_inner_macros)] breaks recursion in submodules

[durka]

I tried adding #[macro_export(local_inner_macros)] to make a macro usable outside my crate, but it broke usability inside the crate (like for tests). The code:

#[macro_export(local_inner_macros)]
macro_rules! foo {
    () => { bar!(); };
}

macro_rules! bar {
    () => {}
}

mod tests {
    foo!();
}

This works in 2015, and it works in 2018 if you remove (local_inner_macros), but with both it doesn't. This seems like a problem for the migration path.

[dtolnay]

This is being fixed in #52234.

I have a similar problem. In crossbeam-channel I put #[macro_export(local_inner_macros)] above the select macro:

#[macro_export(local_inner_macros)]
macro_rules! select {
    // ...
}

Then, in another crate I invoke the macro like this:

select! { default => {} }

This works on stable Rust, but doesn't compile on nightly anymore:

error[E0433]: failed to resolve. Could not find `unreachable` in `$crate`
  --> examples/csp.rs:40:5
   |
40 |     select! { default => {} }
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^ Could not find `unreachable` in `$crate`
   |
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

It's complaining about this invocation of unreachable!():

#[macro_export(local_inner_macros)]
macro_rules! select {
    // ...

    (@codegen_finalize
        $token:ident
        $index:ident
        $selected:ident
        $handles:ident
        ()
        ()
        ()
    ) => {
        unreachable!("internal error in crossbeam-channel")
    };

    // ...
}

If I change this to std::unreachable!, it will compile on nightly but not on stable anymore. Ugh. :(

Is there anything I can do to make this compile on both stable and nightly?

[dtolnay]

unreachable! is not a local macro defined in crossbeam so I believe the behavior is correct.

You can re-export unreachable like this:

#[macro_export]
#[doc(hidden)]
macro_rules! crossbeam_unreachable {
    ($($args:tt)*) => {
        unreachable! { $($args)* }
    };
}

Works perfectly, thank you!