select.rs

/// Wait on multiple concurrent branches, returning when the **first** branch
/// completes, cancelling the remaining branches.
///
/// The `select!` macro must be used inside of async functions, closures, and
/// blocks.
///
/// The `select!` macro accepts one or more branches with the following pattern:
///
/// ```text
/// <pattern> = <async expression> (, if <precondition>)? => <handler>,
/// ```
///
/// Additionally, the `select!` macro may include a single, optional `else`
/// branch, which evaluates if none of the other branches match their patterns:
///
/// ```text
/// else <expression>
/// ```
///
/// The macro aggregates all `<async expression>` expressions and runs them
/// concurrently on the **current** task. Once the **first** expression
/// completes with a value that matches its `<pattern>`, the `select!` macro
/// returns the result of evaluating the completed branch's `<handler>`
/// expression.
///
/// Additionally, each branch may include an optional `if` precondition. This
/// precondition is evaluated **before** the `<async expression>`. If the
/// precondition returns `false`, the branch is entirely disabled. This
/// capability is useful when using `select!` within a loop.
///
/// The complete lifecycle of a `select!` expression is as follows:
///
/// 1. Evaluate all provided `<precondition>` expressions. If the precondition
///    returns `false`, disable the branch for the remainder of the current call
///    to `select!`. Re-entering `select!` due to a loop clears the "disabled"
///    state.
/// 2. Aggregate the `<async expression>`s from each branch, including the
///    disabled ones. If the branch is disabled, `<async expression>` is still
///    evaluated, but the resulting future is not polled.
/// 3. Concurrently await on the results for all remaining `<async expression>`s.
/// 4. Once an `<async expression>` returns a value, attempt to apply the value
///    to the provided `<pattern>`, if the pattern matches, evaluate `<handler>`
///    and return. If the pattern **does not** match, disable the current branch
///    and for the remainder of the current call to `select!`. Continue from step 3.
/// 5. If **all** branches are disabled, evaluate the `else` expression. If none
///    is provided, panic.
///
/// # Notes
///
/// ### Runtime characteristics
///
/// By running all async expressions on the current task, the expressions are
/// able to run **concurrently** but not in **parallel**. This means all
/// expressions are run on the same thread and if one branch blocks the thread,
/// all other expressions will be unable to continue. If parallelism is
/// required, spawn each async expression using [`tokio::spawn`] and pass the
/// join handle to `select!`.
///
/// [`tokio::spawn`]: crate::spawn
///
/// ### Avoid racy `if` preconditions
///
/// Given that `if` preconditions are used to disable `select!` branches, some
/// caution must be used to avoid missing values.
///
/// For example, here is **incorrect** usage of `delay` with `if`. The objective
/// is to repeatedly run an asynchronous task for up to 50 milliseconds.
/// However, there is a potential for the `delay` completion to be missed.
///
/// ```no_run
/// use tokio::time::{self, Duration};
///
/// async fn some_async_work() {
///     // do work
/// }
///
/// #[tokio::main]
/// async fn main() {
///     let mut delay = time::sleep(Duration::from_millis(50));
///
///     while !delay.is_elapsed() {
///         tokio::select! {
///             _ = &mut delay, if !delay.is_elapsed() => {
///                 println!("operation timed out");
///             }
///             _ = some_async_work() => {
///                 println!("operation completed");
///             }
///         }
///     }
/// }
/// ```
///
/// In the above example, `delay.is_elapsed()` may return `true` even if
/// `delay.poll()` never returned `Ready`. This opens up a potential race
/// condition where `delay` expires between the `while !delay.is_elapsed()`
/// check and the call to `select!` resulting in the `some_async_work()` call to
/// run uninterrupted despite the delay having elapsed.
///
/// One way to write the above example without the race would be:
///
/// ```
/// use tokio::time::{self, Duration};
///
/// async fn some_async_work() {
/// # time::sleep(Duration::from_millis(10)).await;
///     // do work
/// }
///
/// #[tokio::main]
/// async fn main() {
///     let mut delay = time::sleep(Duration::from_millis(50));
///
///     loop {
///         tokio::select! {
///             _ = &mut delay => {
///                 println!("operation timed out");
///                 break;
///             }
///             _ = some_async_work() => {
///                 println!("operation completed");
///             }
///         }
///     }
/// }
/// ```
///
/// ### Fairness
///
/// `select!` randomly picks a branch to check first. This provides some level
/// of fairness when calling `select!` in a loop with branches that are always
/// ready.
///
/// # Panics
///
/// `select!` panics if all branches are disabled **and** there is no provided
/// `else` branch. A branch is disabled when the provided `if` precondition
/// returns `false` **or** when the pattern does not match the result of `<async
/// expression>.
///
/// # Examples
///
/// Basic select with two branches.
///
/// ```
/// async fn do_stuff_async() {
///     // async work
/// }
///
/// async fn more_async_work() {
///     // more here
/// }
///
/// #[tokio::main]
/// async fn main() {
///     tokio::select! {
///         _ = do_stuff_async() => {
///             println!("do_stuff_async() completed first")
///         }
///         _ = more_async_work() => {
///             println!("more_async_work() completed first")
///         }
///     };
/// }
/// ```
///
/// Basic stream selecting.
///
/// ```
/// use tokio::stream::{self, StreamExt};
///
/// #[tokio::main]
/// async fn main() {
///     let mut stream1 = stream::iter(vec![1, 2, 3]);
///     let mut stream2 = stream::iter(vec![4, 5, 6]);
///
///     let next = tokio::select! {
///         v = stream1.next() => v.unwrap(),
///         v = stream2.next() => v.unwrap(),
///     };
///
///     assert!(next == 1 || next == 4);
/// }
/// ```
///
/// Collect the contents of two streams. In this example, we rely on pattern
/// matching and the fact that `stream::iter` is "fused", i.e. once the stream
/// is complete, all calls to `next()` return `None`.
///
/// ```
/// use tokio::stream::{self, StreamExt};
///
/// #[tokio::main]
/// async fn main() {
///     let mut stream1 = stream::iter(vec![1, 2, 3]);
///     let mut stream2 = stream::iter(vec![4, 5, 6]);
///
///     let mut values = vec![];
///
///     loop {
///         tokio::select! {
///             Some(v) = stream1.next() => values.push(v),
///             Some(v) = stream2.next() => values.push(v),
///             else => break,
///         }
///     }
///
///     values.sort();
///     assert_eq!(&[1, 2, 3, 4, 5, 6], &values[..]);
/// }
/// ```
///
/// Using the same future in multiple `select!` expressions can be done by passing
/// a reference to the future. Doing so requires the future to be [`Unpin`]. A
/// future can be made [`Unpin`] by either using [`Box::pin`] or stack pinning.
///
/// [`Unpin`]: std::marker::Unpin
/// [`Box::pin`]: std::boxed::Box::pin
///
/// Here, a stream is consumed for at most 1 second.
///
/// ```
/// use tokio::stream::{self, StreamExt};
/// use tokio::time::{self, Duration};
///
/// #[tokio::main]
/// async fn main() {
///     let mut stream = stream::iter(vec![1, 2, 3]);
///     let mut delay = time::sleep(Duration::from_secs(1));
///
///     loop {
///         tokio::select! {
///             maybe_v = stream.next() => {
///                 if let Some(v) = maybe_v {
///                     println!("got = {}", v);
///                 } else {
///                     break;
///                 }
///             }
///             _ = &mut delay => {
///                 println!("timeout");
///                 break;
///             }
///         }
///     }
/// }
/// ```
///
/// Joining two values using `select!`.
///
/// ```
/// use tokio::sync::oneshot;
///
/// #[tokio::main]
/// async fn main() {
///     let (tx1, mut rx1) = oneshot::channel();
///     let (tx2, mut rx2) = oneshot::channel();
///
///     tokio::spawn(async move {
///         tx1.send("first").unwrap();
///     });
///
///     tokio::spawn(async move {
///         tx2.send("second").unwrap();
///     });
///
///     let mut a = None;
///     let mut b = None;
///
///     while a.is_none() || b.is_none() {
///         tokio::select! {
///             v1 = (&mut rx1), if a.is_none() => a = Some(v1.unwrap()),
///             v2 = (&mut rx2), if b.is_none() => b = Some(v2.unwrap()),
///         }
///     }
///
///     let res = (a.unwrap(), b.unwrap());
///
///     assert_eq!(res.0, "first");
///     assert_eq!(res.1, "second");
/// }
/// ```
#[macro_export]
#[cfg_attr(docsrs, doc(cfg(feature = "macros")))]
macro_rules! select {
    // Uses a declarative macro to do **most** of the work. While it is possible
    // to implement fully with a declarative macro, a procedural macro is used
    // to enable improved error messages.
    //
    // The macro is structured as a tt-muncher. All branches are processed and
    // normalized. Once the input is normalized, it is passed to the top-most
    // rule. When entering the macro, `@{ }` is inserted at the front. This is
    // used to collect the normalized input.
    //
    // The macro only recurses once per branch. This allows using `select!`
    // without requiring the user to increase the recursion limit.

    // All input is normalized, now transform.
    (@ {
        // One `_` for each branch in the `select!` macro. Passing this to
        // `count!` converts $skip to an integer.
        ( $($count:tt)* )

        // Normalized select branches. `( $skip )` is a set of `_` characters.
        // There is one `_` for each select branch **before** this one. Given
        // that all input futures are stored in a tuple, $skip is useful for
        // generating a pattern to reference the future for the current branch.
        // $skip is also used as an argument to `count!`, returning the index of
        // the current select branch.
        $( ( $($skip:tt)* ) $bind:pat = $fut:expr, if $c:expr => $handle:expr, )+

        // Fallback expression used when all select branches have been disabled.
        ; $else:expr

    }) => {{
        // Enter a context where stable "function-like" proc macros can be used.
        //
        // This module is defined within a scope and should not leak out of this
        // macro.
        mod util {
            // Generate an enum with one variant per select branch
            $crate::select_priv_declare_output_enum!( ( $($count)* ) );
        }

        // `tokio::macros::support` is a public, but doc(hidden) module
        // including a re-export of all types needed by this macro.
        use $crate::macros::support::Future;
        use $crate::macros::support::Pin;
        use $crate::macros::support::Poll::{Ready, Pending};

        const BRANCHES: u32 = $crate::count!( $($count)* );

        let mut disabled: util::Mask = Default::default();

        // First, invoke all the pre-conditions. For any that return true,
        // set the appropriate bit in `disabled`.
        $(
            if !$c {
                let mask = 1 << $crate::count!( $($skip)* );
                disabled |= mask;
            }
        )*

        // Create a scope to separate polling from handling the output. This
        // adds borrow checker flexibility when using the macro.
        let mut output = {
            // Safety: Nothing must be moved out of `futures`. This is to
            // satisfy the requirement of `Pin::new_unchecked` called below.
            let mut futures = ( $( $fut , )+ );

            $crate::macros::support::poll_fn(|cx| {
                // Track if any branch returns pending. If no branch completes
                // **or** returns pending, this implies that all branches are
                // disabled.
                let mut is_pending = false;

                // Randomly generate a starting point. This makes `select!` a
                // bit more fair and avoids always polling the first future.
                let start = $crate::macros::support::thread_rng_n(BRANCHES);

                for i in 0..BRANCHES {
                    let branch;
                    #[allow(clippy::modulo_one)]
                    {
                        branch = (start + i) % BRANCHES;
                    }
                    match branch {
                        $(
                            #[allow(unreachable_code)]
                            $crate::count!( $($skip)* ) => {
                                // First, if the future has previously been
                                // disabled, do not poll it again. This is done
                                // by checking the associated bit in the
                                // `disabled` bit field.
                                let mask = 1 << branch;

                                if disabled & mask == mask {
                                    // The future has been disabled.
                                    continue;
                                }

                                // Extract the future for this branch from the
                                // tuple
                                let ( $($skip,)* fut, .. ) = &mut futures;

                                // Safety: future is stored on the stack above
                                // and never moved.
                                let mut fut = unsafe { Pin::new_unchecked(fut) };

                                // Try polling it
                                let out = match fut.poll(cx) {
                                    Ready(out) => out,
                                    Pending => {
                                        // Track that at least one future is
                                        // still pending and continue polling.
                                        is_pending = true;
                                        continue;
                                    }
                                };

                                // Disable the future from future polling.
                                disabled |= mask;

                                // The future returned a value, check if matches
                                // the specified pattern.
                                #[allow(unused_variables)]
                                #[allow(unused_mut)]
                                match &out {
                                    $bind => {}
                                    _ => continue,
                                }

                                // The select is complete, return the value
                                return Ready($crate::select_variant!(util::Out, ($($skip)*))(out));
                            }
                        )*
                        _ => unreachable!("reaching this means there probably is an off by one bug"),
                    }
                }

                if is_pending {
                    Pending
                } else {
                    // All branches have been disabled.
                    Ready(util::Out::Disabled)
                }
            }).await
        };

        match output {
            $(
                $crate::select_variant!(util::Out, ($($skip)*) ($bind)) => $handle,
            )*
            util::Out::Disabled => $else,
            _ => unreachable!("failed to match bind"),
        }
    }};

    // ==== Normalize =====

    // These rules match a single `select!` branch and normalize it for
    // processing by the first rule.

    (@ { $($t:tt)* } ) => {
        // No `else` branch
        $crate::select!(@{ $($t)*; unreachable!() })
    };
    (@ { $($t:tt)* } else => $else:expr $(,)?) => {
        $crate::select!(@{ $($t)*; $else })
    };
    (@ { ( $($s:tt)* ) $($t:tt)* } $p:pat = $f:expr, if $c:expr => $h:block, $($r:tt)* ) => {
        $crate::select!(@{ ($($s)* _) $($t)* ($($s)*) $p = $f, if $c => $h, } $($r)*)
    };
    (@ { ( $($s:tt)* ) $($t:tt)* } $p:pat = $f:expr => $h:block, $($r:tt)* ) => {
        $crate::select!(@{ ($($s)* _) $($t)* ($($s)*) $p = $f, if true => $h, } $($r)*)
    };
    (@ { ( $($s:tt)* ) $($t:tt)* } $p:pat = $f:expr, if $c:expr => $h:block $($r:tt)* ) => {
        $crate::select!(@{ ($($s)* _) $($t)* ($($s)*) $p = $f, if $c => $h, } $($r)*)
    };
    (@ { ( $($s:tt)* ) $($t:tt)* } $p:pat = $f:expr => $h:block $($r:tt)* ) => {
        $crate::select!(@{ ($($s)* _) $($t)* ($($s)*) $p = $f, if true => $h, } $($r)*)
    };
    (@ { ( $($s:tt)* ) $($t:tt)* } $p:pat = $f:expr, if $c:expr => $h:expr ) => {
        $crate::select!(@{ ($($s)* _) $($t)* ($($s)*) $p = $f, if $c => $h, })
    };
    (@ { ( $($s:tt)* ) $($t:tt)* } $p:pat = $f:expr => $h:expr ) => {
        $crate::select!(@{ ($($s)* _) $($t)* ($($s)*) $p = $f, if true => $h, })
    };
    (@ { ( $($s:tt)* ) $($t:tt)* } $p:pat = $f:expr, if $c:expr => $h:expr, $($r:tt)* ) => {
        $crate::select!(@{ ($($s)* _) $($t)* ($($s)*) $p = $f, if $c => $h, } $($r)*)
    };
    (@ { ( $($s:tt)* ) $($t:tt)* } $p:pat = $f:expr => $h:expr, $($r:tt)* ) => {
        $crate::select!(@{ ($($s)* _) $($t)* ($($s)*) $p = $f, if true => $h, } $($r)*)
    };

    // ===== Entry point =====

    ( $p:pat = $($t:tt)* ) => {
        $crate::select!(@{ () } $p = $($t)*)
    };
    () => {
        compile_error!("select! requires at least one branch.")
    };
}

// And here... we manually list out matches for up to 64 branches... I'm not
// happy about it either, but this is how we manage to use a declarative macro!

#[macro_export]
#[doc(hidden)]
macro_rules! count {
    () => {
        0
    };
    (_) => {
        1
    };
    (_ _) => {
        2
    };
    (_ _ _) => {
        3
    };
    (_ _ _ _) => {
        4
    };
    (_ _ _ _ _) => {
        5
    };
    (_ _ _ _ _ _) => {
        6
    };
    (_ _ _ _ _ _ _) => {
        7
    };
    (_ _ _ _ _ _ _ _) => {
        8
    };
    (_ _ _ _ _ _ _ _ _) => {
        9
    };
    (_ _ _ _ _ _ _ _ _ _) => {
        10
    };
    (_ _ _ _ _ _ _ _ _ _ _) => {
        11
    };
    (_ _ _ _ _ _ _ _ _ _ _ _) => {
        12
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _) => {
        13
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        14
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        15
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        16
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        17
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        18
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        19
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        20
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        21
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        22
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        23
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        24
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        25
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        26
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        27
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        28
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        29
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        30
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        31
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        32
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        33
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        34
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        35
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        36
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        37
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        38
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        39
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        40
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        41
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        42
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        43
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        44
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        45
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        46
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        47
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        48
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        49
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        50
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        51
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        52
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        53
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        54
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        55
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        56
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        57
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        58
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        59
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        60
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        61
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        62
    };
    (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) => {
        63
    };
}

#[macro_export]
#[doc(hidden)]
macro_rules! select_variant {
    ($($p:ident)::*, () $($t:tt)*) => {
        $($p)::*::_0 $($t)*
    };
    ($($p:ident)::*, (_) $($t:tt)*) => {
        $($p)::*::_1 $($t)*
    };
    ($($p:ident)::*, (_ _) $($t:tt)*) => {
        $($p)::*::_2 $($t)*
    };
    ($($p:ident)::*, (_ _ _) $($t:tt)*) => {
        $($p)::*::_3 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _) $($t:tt)*) => {
        $($p)::*::_4 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_5 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_6 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_7 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_8 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_9 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_10 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_11 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_12 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_13 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_14 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_15 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_16 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_17 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_18 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_19 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_20 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_21 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_22 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_23 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_24 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_25 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_26 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_27 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_28 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_29 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_30 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_31 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_32 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_33 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_34 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_35 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_36 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_37 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_38 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_39 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_40 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_41 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_42 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_43 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_44 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_45 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_46 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_47 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_48 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_49 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_50 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_51 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_52 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_53 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_54 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_55 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_56 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_57 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_58 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_59 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_60 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_61 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_62 $($t)*
    };
    ($($p:ident)::*, (_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _) $($t:tt)*) => {
        $($p)::*::_63 $($t)*
    };
}