Add disconnected flavor which immediately fails

[ryoqun]

Yet another small niceties, i'd like to add to crossbeam_channel.

This doesn't look meaningful by itself at first glance. However, this is useful for conditionally inserting an artificial select arm at predetermined priority in the select_biased!() #1040, combined with never() like this:

select_biased! {
    recv(important_channel) -> urgent_message => {
        ...
    },
    recv(if is_local_task_available { disconnected() } else { never() }) -> _dummy_signal => {
        ...
    },
    recv(normal_channel) -> normal_message => {
        ...
    },
}

Note that using tick() like below introduces needless calls to Instant::now() (which is a vdso basically) per select invocation on top of the needless stack memory consumption. (On the other hand, the zst & const-code-heavy disconnected() will be optimized out to great degree after the macro expansion)..:

fn always() -> Receiver<Instant> {
    tick(Duration::default())
}

Also, unrolling the select is possible, but creates code duplication. Lastly, checking is_local_task_available after select_biased!() alone would introduce a priority inversion.

Thus, i think there's no acceptable alternative solution currently.

[ryoqun]

I'm happy to expand this doccomment with an example, which will be based on the one in the pr description with polishes, once it's confirmed that this pr has some hope to be merged.

[ryoqun]

oops forgot to update..

[taiki-e]

Thanks for the PR. I guess this could be replaced by never + disconnect once the API for explicit disconnection is exposed?

[ryoqun]

Thanks for the PR. I guess this could be replaced by never + disconnect once the API for explicit disconnection is exposed?

hey, really thanks for replying!

yeah, assuming the disconnected never is implemented in a hard-coded way without any state much like the never itself for maximum optimization opportunity by rustc.

note that the tick approach in the pr description or just let disconnected = unbounded().1 would work much like disconnected(). however the two approaches introduce some overhead, which i'd like to avoid entirely.

[ryoqun]

i think disconnected dummy sender might also be useful. ref: #750

how about this api? let (sender, receiver) = disconnected()? (i.e. align with unbounded() and bounded(...))