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

@chazkiker2

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.

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)) {
   |                                             ^^^^^^^^^^^^^^^^^^^

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() { }

Rollup merge of rust-lang#87322 - chazkiker2:fix/suggestion-ref-sync-…

Navigation Menu

Repository navigation

suggestion that &T requires Sync to be Send could be clearer #86507

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

suggestion that &T requires Sync to be Send could be clearer #86507

Description

Activity

nikomatsakis commented on Jun 21, 2021

chazkiker2 commented on Jul 20, 2021

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions