Future generation can not recognize drop function

[YjyJeff]

drop function does not end the lifetime of a variable in in async block

I tried this code:

async fn hello() {
    println!("hello");
}

tokio::spawn(async move {
    let mut rng = rand::thread_rng();
    let v = rng.gen_range(0..100);
    drop(rng);
    hello().await;
    v
});

The compiler compiles it with an error

error: future cannot be sent between threads safely
   --> src/main.rs:6:26
    |
6   |       let t = tokio::spawn(async move {
    |  __________________________^
7   | |         let mut rng = rand::thread_rng();
8   | |         let v = rng.gen_range(0..100);
9   | |         drop(rng);
10  | |         hello().await;
11  | |         v
12  | |     });
    | |_____^ future created by async block is not `Send`
    |
    = help: within `[async block@src/main.rs:6:26: 12:6]`, the trait `std::marker::Send` is not implemented for `Rc<UnsafeCell<ReseedingRng<rand_chacha::chacha::ChaCha12Core, OsRng>>>`
note: future is not `Send` as this value is used across an await
   --> src/main.rs:10:16
    |
7   |         let mut rng = rand::thread_rng();
    |             ------- has type `ThreadRng` which is not `Send`
...
10  |         hello().await;
    |                ^^^^^^ await occurs here, with `mut rng` maybe used later
11  |         v
12  |     });
    |     - `mut rng` is later dropped here
note: required by a bound in `tokio::spawn`
   --> /Users/jefffffyang/.cargo/registry/src/mirrors.ustc.edu.cn-61ef6e0cd06fb9b8/tokio-1.21.2/src/task/spawn.rs:127:21
    |
127 |         T: Future + Send + 'static,
    |                     ^^^^ required by this bound in `spawn`

I dropped the rng variable in line 9, but the compiler says rng is dropped in line 12. I also write the following code to confirm it is caused by the drop function:

tokio::spawn(async move {
    let v = {
        let mut rng = rand::thread_rng();
        rng.gen_range(0..100)
    };
    hello().await;
    v
});

In the above function, rng is dropped by the compiler implicitly. It works well.

Meta

rustc --version --verbose:

rustc 1.71.0-nightly (ce5919fce 2023-05-15)

[SkiFire13]

This is a known issue, see #69663