Why futures `select!` takes async action as mutable?

1

Why futures select! takes async action as mutable?

I have a following snippet:

let running_server = runtime.spawn(server_future).fuse();
let async_tasks = async { select! {
        res = running_server => match res { ... },

and it doesn't compile with an error:

error[E0596]: cannot borrow `running_server` as mutable, as it is not declared as mutable
    |
    = note: this error originates in the macro `$crate::select_internal` which comes from the expansion of the macro `select` (in Nightly builds, run with -Z macro-backtrace for more info)

In my (nightly) rust version the above mentioned flag is not supported.

2

The select! macro has to be able to poll the future, which requires mutable access to it.

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