tokio::select! with only an "else branch"

[GilShoshan94]

Hi, I wrote a procedural macro and in it I use a tokio::select! where the amount of branches are dependent on the proc macro's input.
I always have the falback else branch.

In my use case, my proc macro uses the user function signature as input and the amount of arguments will determine the amount of branches in the tokio::select! call.

And it is totally valid to have a no argment function that will translate in a select! with 0 branch and only the else fallback. as such:

...
let foo = tokio::select! {
   else => 1,
};
...

But it doesn't compile.

Describe the solution you'd like

A clear and concise description of what you want to happen.
I would like this edge case to be valid. While I understand that this is not very useful by itself, we are talking here in the context of proc macro code.
Also, the else branch exists as a fallback for when all branches are disabled. And no branches at all is kinda equivalent to all branches disabled.

Describe alternatives you've considered

• Alternative 1: I write my proc macro a bit more complicate and take into account the special case when I have 0 branches and skip the tokio::select! altogether.

• Alternative 2: I added a permanent dummy branch always disabled (with a never type ! return type) in the expension of my proc macro. As such:

...
let foo = tokio::select! {
   _ = async {} , if false => unreachable!(),
   else => 1,
};
...

• Alternative 3: a PR to tokio to enhance the select! macro, the simplest is basically to implement "Alternative 2" directly in the macro_rules!.

• Alternative 4: a PR to tokio to enhance the select! macro but done more elegantly than "Alternative 3" but will required more complex changes in the macro and suporting files such tokio\tokio-macros\src\select.rs (because here it doesn't work I think when variants is an empty Vec).

Additional notes
I already implemented Alternative 3 and add 2 tests to cover it in tokio\tokio\tests\macros_select.rs.
I am ready to submit the PR if it is desired by the Tokio team.

Thank you.

[GilShoshan94]

Alternative 5, as suggested by @Darksonn : PR that add a simple rule that expand to to the $else exp immediately.