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    /// Pins a value on the stack.
    ///
    /// Calls to `async fn` return anonymous [`Future`] values that are `!Unpin`.
    /// These values must be pinned before they can be polled. Calling `.await` will
    /// handle this, but consumes the future. If it is required to call `.await` on
    /// a `&mut _` reference, the caller is responsible for pinning the future.
    ///
    /// Pinning may be done by allocating with [`Box::pin`] or by using the stack
    /// with the `pin!` macro.
    ///
    /// The following will **fail to compile**:
    ///
    /// ```compile_fail
    /// async fn my_async_fn() {
    ///     // async logic here
    /// }
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let mut future = my_async_fn();
    ///     (&mut future).await;
    /// }
    /// ```
    ///
    /// To make this work requires pinning:
    ///
    /// ```
    /// use tokio::pin;
    ///
    /// async fn my_async_fn() {
    ///     // async logic here
    /// }
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let future = my_async_fn();
    ///     pin!(future);
    ///
    ///     (&mut future).await;
    /// }
    /// ```
    ///
    /// Pinning is useful when using `select!` and stream operators that require `T:
    /// Stream + Unpin`.
    ///
    /// [`Future`]: trait@std::future::Future
    /// [`Box::pin`]: #
    ///
    /// # Usage
    ///
    /// The `pin!` macro takes **identifiers** as arguments. It does **not** work
    /// with expressions.
    ///
    /// The following does not compile as an expression is passed to `pin!`.
    ///
    /// ```compile_fail
    /// async fn my_async_fn() {
    ///     // async logic here
    /// }
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let mut future = pin!(my_async_fn());
    ///     (&mut future).await;
    /// }
    /// ```
    ///
    /// # Examples
    ///
    /// Using with select:
    ///
    /// ```
    /// use tokio::{pin, select};
    /// use tokio_stream::{self as stream, StreamExt};
    ///
    /// async fn my_async_fn() {
    ///     // async logic here
    /// }
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let mut stream = stream::iter(vec![1, 2, 3, 4]);
    ///
    ///     let future = my_async_fn();
    ///     pin!(future);
    ///
    ///     loop {
    ///         select! {
    ///             _ = &mut future => {
    ///                 // Stop looping `future` will be polled after completion
    ///                 break;
    ///             }
    ///             Some(val) = stream.next() => {
    ///                 println!("got value = {}", val);
    ///             }
    ///         }
    ///     }
    /// }
    /// ```
    ///
    /// Because assigning to a variable followed by pinning is common, there is also
    /// a variant of the macro that supports doing both in one go.
    ///
    /// ```
    /// use tokio::{pin, select};
    ///
    /// async fn my_async_fn() {
    ///     // async logic here
    /// }
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     pin! {
    ///         let future1 = my_async_fn();
    ///         let future2 = my_async_fn();
    ///     }
    ///
    ///     select! {
    ///         _ = &mut future1 => {}
    ///         _ = &mut future2 => {}
    ///     }
    /// }
    /// ```
    #[macro_export]
    macro_rules! pin {
        ($($x:ident),*) => { $(
            // Move the value to ensure that it is owned
            let mut $x = $x;
            // Shadow the original binding so that it can't be directly accessed
            // ever again.
            #[allow(unused_mut)]
            let mut $x = unsafe {
                $crate::macros::support::Pin::new_unchecked(&mut $x)
            };
        )* };
        ($(
                let $x:ident = $init:expr;
        )*) => {
            $(
                let $x = $init;
                $crate::pin!($x);
            )*
        };
    }