dead value causes `async` future to not be `Send`

[jameysharp]

Here's the smallest example I could come up with, extracted from the larger project I was working on:

use std::future::Future;
use std::sync::RwLock;

pub async fn f(lock: RwLock<()>) {
    let guard = lock.read().unwrap();
    let () = *guard;
    drop(guard);
    wait().await;
}

pub async fn g(lock: RwLock<()>) {
    {
        let guard = lock.read().unwrap();
        let () = *guard;
    }
    wait().await;
}

pub async fn wait() {}

pub fn require_send(_fut: impl Future + Send) {}

pub fn check() {
    require_send(f(RwLock::new(())));
    require_send(g(RwLock::new(())));
}

I expected to see this happen: Both f and g should be equivalent, I think. The only difference between them is whether guard becomes dead at the end of a block or at the call to drop. In both cases, I believe it should be dead before the await.

Instead, this happened: g compiles without error, but attempting to use f reports "error: future cannot be sent between threads safely", because "future is not Send as [guard] is used across an await".

Meta

rustc --version --verbose:

rustc 1.88.0 (6b00bc388 2025-06-23)
binary: rustc
commit-hash: 6b00bc3880198600130e1cf62b8f8a93494488cc
commit-date: 2025-06-23
host: x86_64-unknown-linux-gnu
release: 1.88.0
LLVM version: 20.1.5

Using the Rust playground, I've also checked 1.89.0-beta.6 (2025-07-21 20c571f) and 1.90.0-nightly (2025-07-23 ace6330).