Bad error message using shared borrow of non-sync type across await point

[lily-mara]

I tried this code:

struct NonSync(*const ());
unsafe impl Send for NonSync {}

impl NonSync {
    async fn foo(&self) {}
    async fn bar(&mut self) {}
}

fn require_send(_: impl Send) {}

fn main() {
    require_send(async {
        let mut foo = NonSync(std::ptr::null());
        foo.foo().await;
    });
    
    require_send(async {
        let foo = NonSync(std::ptr::null());
        foo.bar().await;
    });
}

I expected to see this happen: one of three things:

1. Both branches compile
2. Both branches don't compile
3. foo branch doesn't compile, but bar branch does, and error message explains that &T is non-sync while &mut T is sync where T: !Sync

Instead, this happened: explanation

foo branch doesn't compile, but bar branch does, error message implies that the error is a lifetime issue rather than related to shared vs exclusive borrowing.

error: future cannot be sent between threads safely
  --> src/main.rs:12:5
   |
9  | fn require_send(_: impl Send) {}
   |    ------------         ---- required by this bound in `require_send`
...
12 |     require_send(async {
   |     ^^^^^^^^^^^^ future returned by `main` is not `Send`
   |
   = help: within `NonSync`, the trait `std::marker::Sync` is not implemented for `*const ()`
note: future is not `Send` as this value is used across an await
  --> src/main.rs:14:9
   |
14 |         foo.foo().await;
   |         ---^^^^^^^^^^^^- `foo` is later dropped here
   |         |
   |         await occurs here, with `foo` maybe used later
   |         has type `&NonSync`
help: consider moving this into a `let` binding to create a shorter lived borrow
  --> src/main.rs:14:9
   |
14 |         foo.foo().await;
   |         ^^^^^^^^^

Meta

I ran this code using the latest stable (1.42.0) and nightly (1.44.0-nightly (2020-04-09 94d3463)) versions of the compiler. Same error message occurs both times.

[traviscross]

@rustbot labels +AsyncAwait-Triaged

We reviewed this today in WG-async triage.

The fact that it gives an error is correct here, but the diagnostic could definitely be improved.