Failing to match on a mutable pattern in tokio::select!

[bwc-x]

As requested by Alice in the Users Forum.

Version
cargo tree | grep tokio

├── tokio v1.12.0
│   └── tokio-macros v1.4.1 (proc-macro)

Platform
uname -a

Linux DESKTOP-NT88DIE 5.10.16.3-microsoft-standard-WSL2 #1 SMP Fri Apr 2 22:23:49 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

Description

Mutable pattern matching in tokio::select! appears to be disallowed, with streams yielding immutable references by default.

Code Sample

use tokio::sync::mpsc;
use futures::{FutureExt, StreamExt, future::join};

async fn test() {

  let (tx, rx) = mpsc::channel::<i32>(1);
  let stream = async move { rx }.into_stream();
  let mut pinned = Box::pin(stream);

  loop {
    tokio::select! {
      Some(mut rx) = pinned.next() => {
        let (_, n) = join(tx.send(5), rx.recv()).await;
        println!("n: {:?}", n);
        break;
      }
      else => {
        println!("nothing");
      }
    }
  }
}

Which fails for the following reason:

error[E0507]: cannot move out of a shared reference
  --> src/main.rs:28:5
   |
28 | /     tokio::select! {
29 | |       Some(mut rx) = pinned.next() => {
   | |            ------
   | |            |
   | |            data moved here
   | |            move occurs because `rx` has type `tokio::sync::mpsc::Receiver<i32>`, which does not implement the `Copy` trait
30 | |         let (_, n) = join(tx.send(5), rx.recv()).await;
31 | |         println!("n: {:?}", n);
...  |
36 | |       }
37 | |     }
   | |_____^

Workaround

use tokio::sync::mpsc;
use futures::{FutureExt, StreamExt, future::join};

async fn test() {

  let (tx, rx) = mpsc::channel::<i32>(1);
  let stream = async move { rx }.into_stream();
  let mut pinned = Box::pin(stream);

  loop {
    tokio::select! {
      Some(rx) = pinned.next() => {  // no mut
        let mut rx = rx; // an explicit mutable shadowing
        let (_, n) = join(tx.send(5), rx.recv()).await;
        println!("n: {:?}", n);
        break;
      }
      else => {
        println!("nothing");
      }
    }
  }
}

Solution

Unclear, to me at the very least. The problem definitely seems to be related to the way the tokio::select! expands, but I would have little to no clue whatsoever as to where it might make sense to start looking for the source of the issue.