Invalid "future is not Send as this value is used across an await"

[nazar-pc]

I tried this code:

use std::future::{pending, Future};
use std::sync::Mutex;

fn foo() -> impl Future<Output = ()> + Send {
    async {
        let sync_mutex = Mutex::new(());

        let m = sync_mutex.lock().unwrap();
        drop(m);
        pending::<()>().await;
    }
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=bdb1d0f24fab8490d5487520cb050c93

I expected to see this happen: Should compile.

Instead, this happened:

error: future cannot be sent between threads safely
  --> src/lib.rs:4:13
   |
4  | fn foo() -> impl Future<Output = ()> + Send {
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ future created by async block is not `Send`
   |
   = help: within `[async block@src/lib.rs:5:5: 11:6]`, the trait `Send` is not implemented for `MutexGuard<'_, ()>`
note: future is not `Send` as this value is used across an await
  --> src/lib.rs:10:25
   |
8  |         let m = sync_mutex.lock().unwrap();
   |             - has type `MutexGuard<'_, ()>` which is not `Send`
9  |         drop(m);
10 |         pending::<()>().await;
   |                         ^^^^^ await occurs here, with `m` maybe used later
11 |     }
   |     - `m` is later dropped here

error: could not compile `playground` (lib) due to previous error

But it makes no sense, m is clearly dropped earlier than that explicitly.

Meta

rustc --version --verbose:

rustc 1.74.0-nightly (2f5df8a94 2023-08-31)
binary: rustc
commit-hash: 2f5df8a94bb3c5fae4e3fcbfc8ef20f1f976cb19
commit-date: 2023-08-31
host: x86_64-unknown-linux-gnu
release: 1.74.0-nightly
LLVM version: 17.0.0

[nazar-pc]

After additional search maybe duplicate of #57478?