Closed
Description
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)