False positives of local variables crossing await scopes.

[taoseng]

Hello, I have a piece of code that fails to compile, but I believe it is a false positive.

use rand::Rng;
use std::future::Future;
use std::mem;

#[tokio::main]
async fn main() {
    accept_send_future(foo()).await;
}

fn accept_send_future<F>(f: F) -> F
where
    F: Future + Send,
{
    f
}

async fn foo() {
    let mut rng = rand::rng();                 // rng is not Send.
    let idx = rng.random_range(0..100);
    
    mem::drop(rng);  // drop the rng

    bar(idx).await;
}

async fn bar(id: usize) {
    println!("bar: {}", id);
}

i got an error:

error: future cannot be sent between threads safely
  --> src/main.rs:7:24
   |
7  |     accept_send_future(foo()).await;
   |                        ^^^^^ future returned by `foo` is not `Send`
   |
   = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<UnsafeCell<ReseedingRng<rand_chacha::chacha::ChaCha12Core, OsRng>>>`, which is required by `impl Future<Output = ()>: Send`
note: future is not `Send` as this value is used across an await
  --> src/main.rs:29:14
   |
26 |     let mut rng = rand::rng();
   |         ------- has type `ThreadRng` which is not `Send`
...
29 |     bar(idx).await;
   |              ^^^^^ await occurs here, with `mut rng` maybe used later
note: required by a bound in `accept_send_future`
  --> src/main.rs:12:17
   |
10 | fn accept_send_future<F>(f: F) -> F
   |    ------------------ required by a bound in this function
11 | where
12 |     F: Future + Send,
   |                 ^^^^ required by this bound in `accept_send_future`

the 'rng' is expliciyly dropped, and it never cross the await.

[taoseng]

I had to modify the code of foo() as follows for it to compile successfully：

async fn foo() {
    let idx = {
        let mut rng = rand::rng();
        rng.random_range(0..100)
    };
    bar(idx).await;
}

[taoseng]

oh, Let me add something here， the 'rng' is Copy, so drop is useless.

[theemathas]

Duplicate of #63768

[lqd]

the 'rng' is Copy

Is that accurate, https://docs.rs/rand/latest/rand/fn.rng.html returns https://docs.rs/rand/latest/rand/rngs/struct.ThreadRng.html which is not Copy?

[taoseng]

the 'rng' is Copy

Is that accurate, https://docs.rs/rand/latest/rand/fn.rng.html returns https://docs.rs/rand/latest/rand/rngs/struct.ThreadRng.html which is not Copy?

sorry, I got the wrong version.
In rand_v0.7.3 ThreadRng is Copy, but in 0.9.0 ThreadRng is not Copy.

My example is based on version 0.9.0, and this ThreadRng is indeed not Copy.

[Noratrieb]

going to close this as a duplicate as suggested by @theemathas which seems correct to me, but correct me if I'm wrong.