Select macro incorrectly binds expression result as shared reference

[tobz]

Version
tokio 1.10.1

Description
When using select! and taking advantage of the built-in pattern matching i.e. Some(foo) = fut_here(), the macro incorrectly binds the result as a shared reference, which turns into confusing errors like this:

192 | /         select! {
193 | |             // TODO: why the hell is this coming out as &S3Request?!?!
194 | |             Some(mut req) = rx.recv() => {
    | |                  -------
    | |                  |
    | |                  data moved here
    | |                  move occurs because `req` has type `S3Request`, which does not implement the `Copy` trait
195 | |                 let seqno = seq_head;
...   |
236 | |             else => break
237 | |         }
    | |_________^

The full code can be seen here, but rx is Receiver<S3Request>, so clearly the returned value is/should be owned. @Darksonn pointed out that the code in the macro responsible for this issue is here.

Note to whoever is interested in fixing it: @Darksonn also pointed out that changing from match &out { to match &mut out { would likely be a suitable fix.

[blasrodri]

This update

diff --git a/tokio/src/macros/select.rs b/tokio/src/macros/select.rs
index a90ee9eb..88c26adb 100644
--- a/tokio/src/macros/select.rs
+++ b/tokio/src/macros/select.rs
@@ -502,7 +502,7 @@ macro_rules! select {
                                 let mut fut = unsafe { Pin::new_unchecked(fut) };
 
                                 // Try polling it
-                                let out = match fut.poll(cx) {
+                                let mut out = match fut.poll(cx) {
                                     Ready(out) => out,
                                     Pending => {
                                         // Track that at least one future is
@@ -519,8 +519,8 @@ macro_rules! select {
                                 // the specified pattern.
                                 #[allow(unused_variables)]
                                 #[allow(unused_mut)]
-                                match &out {
-                                    $bind => {}
+                                match &mut out {
+                                    $bind => {},
                                     _ => continue,
                                 }
 

allows me to do the following:

    Some(ref mut req) = rx.recv() => {...}

However, I still cannot get the following pattern:

    Some(mut req) = rx.recv() => {...}

[Darksonn]

Try using the matches! macro instead.

[blasrodri]

Try using the matches! macro instead.

Do you mean

if !matches!(&mut out, $bind) {continue}

same result

[Darksonn]

Hmm, yeah I tried a few things. It doesn't seem like we can get it to work. We probably have to change the macro to not require this check in the first place.

[blasrodri]

Hmm, yeah I tried a few things. It doesn't seem like we can get it to work. We probably have to change the macro to not require this check in the first place.

Any suggestions? I tried playing around and couldn't find an easy way.

[Darksonn]

I don't think there's an easy way.