Add missing const _IS_BIASED in test/mpsc.rs

[rikkaka]

When I clone this repo to local and run cargo test, the compilation fails with "cannot find value _IS_BIASED in this scope" after the expansion of macro select.

flume/tests/mpsc.rs

Lines 174 to 187 in fcf3849

	macro_rules! select {
	(
	$($name:pat = $rx:ident.$meth:ident() => $code:expr),+
	) => ({
	crossbeam_channel_internal! {
	$(
	recv(($rx).inner) -> res => {
	let $name = res.map_err(|_| ::std::sync::mpsc::RecvError);
	$code
	}
	)+
	}
	})
	}

What's more, I forked this repo and tried github workflows, which failed due to same reason.

I find it uses a macro export from crossbeam-channel. flume specifies crossbeam-channel = "0.5.5" in [dev-dependencies]. However, crossbeam-channel of version 0.5.5 has been yanked, so actually the newest version 0.5.13 is used. In the new version, a new value _IS_BIASED is used in macro crossbeam_channel_internal, resulting in the error above.

I find two use cases of crossbeam_channel_internal in crossbeam-channel here:

macro_rules! select {
    ($($tokens:tt)*) => {
        {
            const _IS_BIASED: bool = false;


            $crate::crossbeam_channel_internal!(
                $($tokens)*
            )
        }
    };
}
#[macro_export]
macro_rules! select_biased {
    ($($tokens:tt)*) => {
        {
            const _IS_BIASED: bool = true;

            $crate::crossbeam_channel_internal!(
                $($tokens)*
            )
        }
    };
}

If this select macro is intended to work the same as select macro in crossbeam_channel_internal, adding the const _IS_BIASED with value false may help. After I added this value, the cargo test succeeded.

[zesterer]

Thanks!