This demo doesn't compile because rustc said that async block is not Send.
#![allow(unused)]
use std::{sync::Mutex, future::Future};
fn main() {
require_send(async {
let mutex = Mutex::new(());
let guard = mutex.lock().unwrap();
drop(guard);
std::future::pending::<()>().await;
});
}
fn require_send(_: impl Future + Send) {}
error: future cannot be sent between threads safely
--> src/main.rs:6:18
|
6 | require_send(async {
| __________________^
7 | | let mutex = Mutex::new(());
8 | | let guard = mutex.lock().unwrap();
9 | | drop(guard);
10 | | std::future::pending::<()>().await;
11 | | });
| |_____^ future created by async block is not `Send`
|
= help: within `[async block@src/main.rs:6:18: 11:6]`, the trait `Send` is not implemented for `MutexGuard<'_, ()>`
note: future is not `Send` as this value is used across an await
--> src/main.rs:10:37
|
8 | let guard = mutex.lock().unwrap();
| ----- has type `MutexGuard<'_, ()>` which is not `Send`
9 | drop(guard);
10 | std::future::pending::<()>().await;
| ^^^^^^ await occurs here, with `guard` maybe used later
11 | });
| - `guard` is later dropped here
note: required by a bound in `require_send`
--> src/main.rs:14:34
|
14 | fn require_send(_: impl Future + Send) {}
| ^^^^ required by this bound in `require_send`
But why? MutexGuard will be dropped at drop(guard);, not at the end of the async block. guard won't be used later.
This async block will be considered implements Send:
async {
let mutex = Mutex::new(());
{
let guard = mutex.lock().unwrap();
}
std::future::pending::<()>().await;
}
What's the difference between them? Thanks.