Async Borrowing Issue

[mingyang91]

I tried this code:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=5203de66797e0725271a464be3604d0c

#![allow(unused)]

struct NotSend(std::marker::PhantomData<*mut ()>);

impl NotSend {
    fn new() -> Self {
        Self(std::marker::PhantomData)
    }
}

unsafe impl Sync for NotSend {}

#[derive(Clone)]
struct WatchRecv;

impl WatchRecv {
    fn borrow(&self) -> Ref<'_> {
        Ref(self, NotSend::new())
    }
}

struct Ref<'a>(&'a WatchRecv, NotSend);

fn assert_send<F: std::future::Future + Send>(f: F) -> F { f }

impl Ref<'_> {
    fn len(&self) -> usize { 0 }
}

async fn another_future() {
    loop {}
}

async fn real_main() {
    let rx = WatchRecv;
    let mut binding = rx.clone();
    assert_send(async move {

        // This doesn't work.
        let value = binding.borrow();
        let len = value.len();
        drop(value);
        println!("len: {len}");

        /*
        // This works.
        let len = {
            let value = binding.borrow();
            value.len()
        };
        */
        another_future().await;
    }).await;
    println!("{:?}", rx.borrow().len());
}

I expected to see this happen: pass compile

Instead, this happened: error

   Compiling playground v0.0.1 (/playground)
error: future cannot be sent between threads safely
  --> src/lib.rs:37:5
   |
37 | /     assert_send(async move {
38 | |
39 | |         // This doesn't work.
40 | |         let value = binding.borrow();
...  |
52 | |         another_future().await;
53 | |     }).await;
   | |______^ future created by async block is not `Send`
   |
   = help: within `{async block@src/lib.rs:37:17: 53:6}`, the trait `Send` is not implemented for `*mut ()`, which is required by `{async block@src/lib.rs:37:17: 53:6}: Send`
note: future is not `Send` as this value is used across an await
  --> src/lib.rs:52:26
   |
40 |         let value = binding.borrow();
   |             ----- has type `Ref<'_>` which is not `Send`
...
52 |         another_future().await;
   |                          ^^^^^ await occurs here, with `value` maybe used later
note: required by a bound in `assert_send`
  --> src/lib.rs:24:41
   |
24 | fn assert_send<F: std::future::Future + Send>(f: F) -> F { f }
   |                                         ^^^^ required by this bound in `assert_send`

error: could not compile `playground` (lib) due to 1 previous error

but if I use the following code:

let len = {
    let value = binding.borrow();
    value.len()
};

it will pass compile.

Meta

rustc --version --verbose:

rustc --version --verbose
rustc 1.79.0 (129f3b996 2024-06-10)
binary: rustc
commit-hash: 129f3b9964af4d4a709d1383930ade12dfe7c081
commit-date: 2024-06-10
host: aarch64-apple-darwin
release: 1.79.0
LLVM version: 18.1.7

And rust online playground.

[mingyang91]

duplicate with #128095