warn: where_clauses_object_safety when adding default implementation for async_trait

[NishantJoshi00]

Description

Getting warning #[warn(where_clauses_object_safety)] when adding default implementation for async_trait

Sample Case

use tokio::time;

#[async_trait::async_trait]
trait Waiter {
    async fn wait_inner_sec(&self, name: &str);
}

struct Walker {
    inner: u64,
}

impl Walker {
    fn new(inner: u64) -> Self { Self { inner } }
}

#[async_trait::async_trait]
impl Waiter for Walker {
    async fn wait_inner_sec(&self, name: &str) {
        println!("waiting for: {}", name);
        time::sleep(time::Duration::new(self.inner, 0)).await
    }
}

#[tokio::main]
async fn main() {
    let walker = Walker::new(1);
    let dyn_walker: Box<&(dyn Waiter)> = Box::new(&walker);
    dyn_walker.wait_inner_sec("plane").await;

}

This example works properly and there is no issue with object-safety here!

Now, if I add a default implementation for the trait

#[async_trait::async_trait]
trait Waiter {
  async fn wait_inner_sec(&self, name: &str) {
    println!("waiting for: {}", name);
    time::sleep(time::Duration::new(5, 0)).await
  }
}

After this change, I suddenly get 1 warning and 1 error.

The Error

error[E0277]: `dyn Waiter` cannot be shared between threads safely
  --> src/main.rs:31:5
   |
31 |     dyn_walker.wait_inner_sec("plane").await;
   |     ^^^^^^^^^^ -------------- required by a bound introduced by this call
   |     |
   |     `dyn Waiter` cannot be shared between threads safely
   |
   = help: the trait `Sync` is not implemented for `dyn Waiter`
note: required by a bound in `Waiter::wait_inner_sec`
  --> src/main.rs:5:5
   |
5  |     async fn wait_inner_sec(&self, name: &str) {
   |     ^^^^^ required by this bound in `Waiter::wait_inner_sec`

This can be solved with

- let dyn_walker: Box<&(dyn Waiter)> = Box::new(&walker);
+ let dyn_walker: Box<&(dyn Waiter + Sync)> = Box::new(&walker);

The Warning

warning: the trait `Waiter` cannot be made into an object
 --> src/main.rs:5:14
  |
5 |     async fn wait_inner_sec(&self, name: &str) {
  |              ^^^^^^^^^^^^^^
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #51443 <https://github.com/rust-lang/rust/issues/51443>
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
 --> src/main.rs:5:14
  |
4 | trait Waiter {
  |       ------ this trait cannot be made into an object...
5 |     async fn wait_inner_sec(&self, name: &str) {
  |              ^^^^^^^^^^^^^^ ...because method `wait_inner_sec` references the `Self` type in its `where` clause
  = help: consider moving `wait_inner_sec` to another trait
  = note: `#[warn(where_clauses_object_safety)]` on by default

• The warning doesn't provide the whole context
• In my implementation I haven't used any where clause.
• After expanding the macro turns out the function has a where clause, where self is referenced Self: Sync
• This problem can be solved by trait Waiter: Sync, but should this warning even occur?

[dtolnay]

It's a false positive rust-lang/rust#51443 (comment).
You can use #![allow(where_clauses_object_safety)] in your crate for now.

[dtolnay]

This is fixed in Rust 1.69.0 by rust-lang/rust#107082.

[NishantJoshi00]

Awesome! Thanks a lot