tokio::select! never return pattern

[yuan-y-chang]

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Sometimes we have a never-returning future Future<Output = !>, or Future<Output = std::convert::Infallible> in stable Rust. Like sync_a2b:

tokio::select! {
    () = sync_a2b => unreachable!(),
    () = sync_b2a => unreachable!(),
    () = main_loop => (),
}

BTW, sync_a2b is coercion to () not ! since placed in select!.
But one day, if this future change the return type to (), since unreachable!() is runtime behavior, it won't be detected by the compiler.

The nightly never_pattern is not stable, so we don't have

tokio::select! {
    ! = sync_a2b => unreachable!(),
    ! = sync_b2a => unreachable!(),
    () = main_loop => (),
}

Also, the stable never ! assertion is match $expr {}, which is not a pattern syntax. This verbose syntax also works:

tokio::select! {
    never = sync_a2b => match never {},
    never = sync_b2a => match never {},
    () = main_loop => (),
}

Describe the solution you'd like
A clear and concise description of what you want to happen.

Since the never pattern had a long way to go, an explicit pattern may be helpful to indicate "always pending futures" in the select branch.

tokio::select! {
    ! = sync_a2b;
    ! = sync_b2a;
    () = main_loop => (),
}

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

I don't know much about the macro design of tokio::select!, if => is required:

tokio::select! {
    ! = sync_a2b => unreachable!(),
    ! = sync_b2a => unreachable!(),
    () = main_loop => (),
}

Also compatible with the future syntax.

Additional context
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None

[Darksonn]

I think we will want to wait for the language to figure out what they want for never patterns. Otherwise we risk being inconsistent with upstream Rust on the syntax of select!.