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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
/// }
///
/// #[monoio::main]
/// async fn main() {
/// let mut future = my_async_fn();
/// (&mut future).await;
/// }
/// ```
///
/// To make this work requires pinning:
///
/// ```
/// use monoio::pin;
///
/// async fn my_async_fn() {
/// // async logic here
/// }
///
/// #[monoio::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`]: std::boxed::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
/// }
///
/// #[monoio::main]
/// async fn main() {
/// let mut future = pin!(my_async_fn());
/// (&mut future).await;
/// }
/// ```
///
/// 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 monoio::{pin, select};
///
/// async fn my_async_fn() {
/// // async logic here
/// }
///
/// #[monoio::main]
/// async fn main() {
/// pin! {
/// let future1 = my_async_fn();
/// let future2 = my_async_fn();
/// }
///
/// select! {
/// _ = &mut future1 => {}
/// _ = &mut future2 => {}
/// }
/// }
/// ```