suggestion that &T requires Sync to be Send could be clearer

[nikomatsakis]

Given the following code: playground

use async_trait::async_trait; // 0.1.50
use serde::Serialize;

#[async_trait]
trait Foo {
    async fn bar(x: &(impl Serialize + Send));
}

#[async_trait]
impl Foo for () {
    async fn bar(x: &(impl Serialize + Send)) {
    }
}

fn main() { }

The current output is:

error: future cannot be sent between threads safely
  --> src/main.rs:11:47
   |
11 |       async fn bar(x: &(impl Serialize + Send)) {
   |  _______________________________________________^
12 | |     }
   | |_____^ future created by async block is not `Send`
   |
note: captured value is not `Send`
  --> src/main.rs:11:18
   |
11 |     async fn bar(x: &(impl Serialize + Send)) {
   |                  ^ has type `&impl Serialize + Send` which is not `Send`
   = note: required for the cast to the object type `dyn Future<Output = ()> + Send`
help: consider further restricting this bound
   |
11 |     async fn bar(x: &(impl Serialize + Send + std::marker::Sync)) {
   |                                             ^^^^^^^^^^^^^^^^^^^

The compiler helpfully points out that Sync is required, but nonetheless @chazkiker2 and I both got confused by this error for some time before we noticed it. The suggestion was kind of buried by all the other details.

[nikomatsakis]

I'm not sure the ideal output, but it might be something like this:

error: future cannot be sent between threads safely
  --> src/main.rs:11:47
   |
11 |       async fn bar(x: &(impl Serialize + Send)) {
   |  _______________________________________________^
12 | |     }
   | |_____^ future created by async block is not `Send`
   |
note: `&` references cannot be sent unless their referent is `Sync`
  --> src/main.rs:11:18
   |
11 |     async fn bar(x: &(impl Serialize + Send)) {
   |                  ^ refers to `impl Serialize + Send` which is not `Sync`
   = note: required for the cast to the object type `dyn Future<Output = ()> + Send`
help: consider further restricting this bound
   |
11 |     async fn bar(x: &(impl Serialize + Send + std::marker::Sync)) {
   |                                             ^^^^^^^^^^^^^^^^^^^

[chazkiker2]

This error can be replicated without any external crates (i.e., without #[async_trait]) like so:

Link to Playground

use ::core::pin::Pin;
use ::core::future::Future;
use ::core::marker::Send;

trait Foo {
    fn bar<'me, 'async_trait, T: Send>(x: &'me T)
        -> Pin<Box<dyn Future<Output = ()> + Send + 'async_trait>>
        where 'me: 'async_trait;
}

impl Foo for () {
    fn bar<'me, 'async_trait, T: Send>(x: &'me T)
        -> Pin<Box<dyn Future<Output = ()> + Send + 'async_trait>>
        where 'me: 'async_trait {
            Box::pin(
                async move {
                    let x = x;
                }
            )
         }
}

fn main() { }