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    619

    //! Futures
    //!
    //! This module contains a number of functions for working with `Future`s,
    //! including the `FutureExt` trait which adds methods to `Future` types.

    #[cfg(feature = "compat")]
    use crate::compat::Compat;
    use core::pin::Pin;
    use futures_core::{
        future::TryFuture,
        stream::TryStream,
        task::{Context, Poll},
    };
    #[cfg(feature = "sink")]
    use futures_sink::Sink;

    use crate::fns::{
        inspect_err_fn, inspect_ok_fn, into_fn, map_err_fn, map_ok_fn, map_ok_or_else_fn,
        unwrap_or_else_fn, InspectErrFn, InspectOkFn, IntoFn, MapErrFn, MapOkFn, MapOkOrElseFn,
        UnwrapOrElseFn,
    };
    use crate::future::{assert_future, Inspect, Map};
    use crate::stream::assert_stream;

    // Combinators
    mod into_future;
    mod try_flatten;
    mod try_flatten_err;

    delegate_all!(
        /// Future for the [`try_flatten`](TryFutureExt::try_flatten) method.
        TryFlatten<Fut1, Fut2>(
            try_flatten::TryFlatten<Fut1, Fut2>
        ): Debug + Future + FusedFuture + New[|x: Fut1| try_flatten::TryFlatten::new(x)]
    );

    delegate_all!(
        /// Future for the [`try_flatten_err`](TryFutureExt::try_flatten_err) method.
        TryFlattenErr<Fut1, Fut2>(
            try_flatten_err::TryFlattenErr<Fut1, Fut2>
        ): Debug + Future + FusedFuture + New[|x: Fut1| try_flatten_err::TryFlattenErr::new(x)]
    );

    delegate_all!(
        /// Future for the [`try_flatten_stream`](TryFutureExt::try_flatten_stream) method.
        TryFlattenStream<Fut>(
            try_flatten::TryFlatten<Fut, Fut::Ok>
        ): Debug + Sink + Stream + FusedStream + New[|x: Fut| try_flatten::TryFlatten::new(x)]
        where Fut: TryFuture
    );

    #[cfg(feature = "sink")]
    delegate_all!(
        /// Sink for the [`flatten_sink`](TryFutureExt::flatten_sink) method.
        #[cfg_attr(docsrs, doc(cfg(feature = "sink")))]
        FlattenSink<Fut, Si>(
            try_flatten::TryFlatten<Fut, Si>
        ): Debug + Sink + Stream + FusedStream + New[|x: Fut| try_flatten::TryFlatten::new(x)]
    );

    delegate_all!(
        /// Future for the [`and_then`](TryFutureExt::and_then) method.
        AndThen<Fut1, Fut2, F>(
            TryFlatten<MapOk<Fut1, F>, Fut2>
        ): Debug + Future + FusedFuture + New[|x: Fut1, f: F| TryFlatten::new(MapOk::new(x, f))]
    );

    delegate_all!(
        /// Future for the [`or_else`](TryFutureExt::or_else) method.
        OrElse<Fut1, Fut2, F>(
            TryFlattenErr<MapErr<Fut1, F>, Fut2>
        ): Debug + Future + FusedFuture + New[|x: Fut1, f: F| TryFlattenErr::new(MapErr::new(x, f))]
    );

    delegate_all!(
        /// Future for the [`err_into`](TryFutureExt::err_into) method.
        ErrInto<Fut, E>(
            MapErr<Fut, IntoFn<E>>
        ): Debug + Future + FusedFuture + New[|x: Fut| MapErr::new(x, into_fn())]
    );

    delegate_all!(
        /// Future for the [`ok_into`](TryFutureExt::ok_into) method.
        OkInto<Fut, E>(
            MapOk<Fut, IntoFn<E>>
        ): Debug + Future + FusedFuture + New[|x: Fut| MapOk::new(x, into_fn())]
    );

    delegate_all!(
        /// Future for the [`inspect_ok`](super::TryFutureExt::inspect_ok) method.
        InspectOk<Fut, F>(
            Inspect<IntoFuture<Fut>, InspectOkFn<F>>
        ): Debug + Future + FusedFuture + New[|x: Fut, f: F| Inspect::new(IntoFuture::new(x), inspect_ok_fn(f))]
    );

    delegate_all!(
        /// Future for the [`inspect_err`](super::TryFutureExt::inspect_err) method.
        InspectErr<Fut, F>(
            Inspect<IntoFuture<Fut>, InspectErrFn<F>>
        ): Debug + Future + FusedFuture + New[|x: Fut, f: F| Inspect::new(IntoFuture::new(x), inspect_err_fn(f))]
    );

    #[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
    pub use self::into_future::IntoFuture;

    delegate_all!(
        /// Future for the [`map_ok`](TryFutureExt::map_ok) method.
        MapOk<Fut, F>(
            Map<IntoFuture<Fut>, MapOkFn<F>>
        ): Debug + Future + FusedFuture + New[|x: Fut, f: F| Map::new(IntoFuture::new(x), map_ok_fn(f))]
    );

    delegate_all!(
        /// Future for the [`map_err`](TryFutureExt::map_err) method.
        MapErr<Fut, F>(
            Map<IntoFuture<Fut>, MapErrFn<F>>
        ): Debug + Future + FusedFuture + New[|x: Fut, f: F| Map::new(IntoFuture::new(x), map_err_fn(f))]
    );

    delegate_all!(
        /// Future for the [`map_ok_or_else`](TryFutureExt::map_ok_or_else) method.
        MapOkOrElse<Fut, F, G>(
            Map<IntoFuture<Fut>, MapOkOrElseFn<F, G>>
        ): Debug + Future + FusedFuture + New[|x: Fut, f: F, g: G| Map::new(IntoFuture::new(x), map_ok_or_else_fn(f, g))]
    );

    delegate_all!(
        /// Future for the [`unwrap_or_else`](TryFutureExt::unwrap_or_else) method.
        UnwrapOrElse<Fut, F>(
            Map<IntoFuture<Fut>, UnwrapOrElseFn<F>>
        ): Debug + Future + FusedFuture + New[|x: Fut, f: F| Map::new(IntoFuture::new(x), unwrap_or_else_fn(f))]
    );

    impl<Fut: ?Sized + TryFuture> TryFutureExt for Fut {}

    /// Adapters specific to [`Result`]-returning futures
    pub trait TryFutureExt: TryFuture {
        /// Flattens the execution of this future when the successful result of this
        /// future is a [`Sink`].
        ///
        /// This can be useful when sink initialization is deferred, and it is
        /// convenient to work with that sink as if the sink was available at the
        /// call site.
        ///
        /// Note that this function consumes this future and returns a wrapped
        /// version of it.
        ///
        /// # Examples
        ///
        /// ```
        /// use futures::future::{Future, TryFutureExt};
        /// use futures::sink::Sink;
        /// # use futures::channel::mpsc::{self, SendError};
        /// # type T = i32;
        /// # type E = SendError;
        ///
        /// fn make_sink_async() -> impl Future<Output = Result<
        ///     impl Sink<T, Error = E>,
        ///     E,
        /// >> { // ... }
        /// # let (tx, _rx) = mpsc::unbounded::<i32>();
        /// # futures::future::ready(Ok(tx))
        /// # }
        /// fn take_sink(sink: impl Sink<T, Error = E>) { /* ... */ }
        ///
        /// let fut = make_sink_async();
        /// take_sink(fut.flatten_sink())
        /// ```
        #[cfg(feature = "sink")]
        #[cfg_attr(docsrs, doc(cfg(feature = "sink")))]
        fn flatten_sink<Item>(self) -> FlattenSink<Self, Self::Ok>
        where
            Self::Ok: Sink<Item, Error = Self::Error>,
            Self: Sized,
        {
            crate::sink::assert_sink::<Item, Self::Error, _>(FlattenSink::new(self))
        }

        /// Maps this future's success value to a different value.
        ///
        /// This method can be used to change the [`Ok`](TryFuture::Ok) type of the
        /// future into a different type. It is similar to the [`Result::map`]
        /// method. You can use this method to chain along a computation once the
        /// future has been resolved.
        ///
        /// The provided closure `f` will only be called if this future is resolved
        /// to an [`Ok`]. If it resolves to an [`Err`], panics, or is dropped, then
        /// the provided closure will never be invoked.
        ///
        /// Note that this method consumes the future it is called on and returns a
        /// wrapped version of it.
        ///
        /// # Examples
        ///
        /// ```
        /// use futures::future::TryFutureExt;
        ///
        /// # futures::executor::block_on(async {
        /// let future = async { Ok::<i32, i32>(1) };
        /// let future = future.map_ok(|x| x + 3);
        /// assert_eq!(future.await, Ok(4));
        /// # });
        /// ```
        ///
        /// Calling [`map_ok`](TryFutureExt::map_ok) on an errored future has no
        /// effect:
        ///
        /// ```
        /// use futures::future::TryFutureExt;
        ///
        /// # futures::executor::block_on(async {
        /// let future = async { Err::<i32, i32>(1) };
        /// let future = future.map_ok(|x| x + 3);
        /// assert_eq!(future.await, Err(1));
        /// # });
        /// ```
        fn map_ok<T, F>(self, f: F) -> MapOk<Self, F>
        where
            F: FnOnce(Self::Ok) -> T,
            Self: Sized,
        {
            assert_future::<Result<T, Self::Error>, _>(MapOk::new(self, f))
        }

        /// Maps this future's success value to a different value, and permits for error handling resulting in the same type.
        ///
        /// This method can be used to coalesce your [`Ok`](TryFuture::Ok) type and [`Error`](TryFuture::Error) into another type,
        /// where that type is the same for both outcomes.
        ///
        /// The provided closure `f` will only be called if this future is resolved
        /// to an [`Ok`]. If it resolves to an [`Err`], panics, or is dropped, then
        /// the provided closure will never be invoked.
        ///
        /// The provided closure `e` will only be called if this future is resolved
        /// to an [`Err`]. If it resolves to an [`Ok`], panics, or is dropped, then
        /// the provided closure will never be invoked.
        ///
        /// Note that this method consumes the future it is called on and returns a
        /// wrapped version of it.
        ///
        /// # Examples
        ///
        /// ```
        /// use futures::future::TryFutureExt;
        ///
        /// # futures::executor::block_on(async {
        /// let future = async { Ok::<i32, i32>(5) };
        /// let future = future.map_ok_or_else(|x| x * 2, |x| x + 3);
        /// assert_eq!(future.await, 8);
        ///
        /// let future = async { Err::<i32, i32>(5) };
        /// let future = future.map_ok_or_else(|x| x * 2, |x| x + 3);
        /// assert_eq!(future.await, 10);
        /// # });
        /// ```
        ///
        fn map_ok_or_else<T, E, F>(self, e: E, f: F) -> MapOkOrElse<Self, F, E>
        where
            F: FnOnce(Self::Ok) -> T,
            E: FnOnce(Self::Error) -> T,
            Self: Sized,
        {
            assert_future::<T, _>(MapOkOrElse::new(self, f, e))
        }

        /// Maps this future's error value to a different value.
        ///
        /// This method can be used to change the [`Error`](TryFuture::Error) type
        /// of the future into a different type. It is similar to the
        /// [`Result::map_err`] method. You can use this method for example to
        /// ensure that futures have the same [`Error`](TryFuture::Error) type when
        /// using [`select!`] or [`join!`].
        ///
        /// The provided closure `f` will only be called if this future is resolved
        /// to an [`Err`]. If it resolves to an [`Ok`], panics, or is dropped, then
        /// the provided closure will never be invoked.
        ///
        /// Note that this method consumes the future it is called on and returns a
        /// wrapped version of it.
        ///
        /// # Examples
        ///
        /// ```
        /// use futures::future::TryFutureExt;
        ///
        /// # futures::executor::block_on(async {
        /// let future = async { Err::<i32, i32>(1) };
        /// let future = future.map_err(|x| x + 3);
        /// assert_eq!(future.await, Err(4));
        /// # });
        /// ```
        ///
        /// Calling [`map_err`](TryFutureExt::map_err) on a successful future has
        /// no effect:
        ///
        /// ```
        /// use futures::future::TryFutureExt;
        ///
        /// # futures::executor::block_on(async {
        /// let future = async { Ok::<i32, i32>(1) };
        /// let future = future.map_err(|x| x + 3);
        /// assert_eq!(future.await, Ok(1));
        /// # });
        /// ```
        fn map_err<E, F>(self, f: F) -> MapErr<Self, F>
        where
            F: FnOnce(Self::Error) -> E,
            Self: Sized,
        {
            assert_future::<Result<Self::Ok, E>, _>(MapErr::new(self, f))
        }

        /// Maps this future's [`Error`](TryFuture::Error) to a new error type
        /// using the [`Into`](std::convert::Into) trait.
        ///
        /// This method does for futures what the `?`-operator does for
        /// [`Result`]: It lets the compiler infer the type of the resulting
        /// error. Just as [`map_err`](TryFutureExt::map_err), this is useful for
        /// example to ensure that futures have the same [`Error`](TryFuture::Error)
        /// type when using [`select!`] or [`join!`].
        ///
        /// Note that this method consumes the future it is called on and returns a
        /// wrapped version of it.
        ///
        /// # Examples
        ///
        /// ```
        /// use futures::future::TryFutureExt;
        ///
        /// # futures::executor::block_on(async {
        /// let future_err_u8 = async { Err::<(), u8>(1) };
        /// let future_err_i32 = future_err_u8.err_into::<i32>();
        /// # });
        /// ```
        fn err_into<E>(self) -> ErrInto<Self, E>
        where
            Self: Sized,
            Self::Error: Into<E>,
        {
            assert_future::<Result<Self::Ok, E>, _>(ErrInto::new(self))
        }

        /// Maps this future's [`Ok`](TryFuture::Ok) to a new type
        /// using the [`Into`](std::convert::Into) trait.
        fn ok_into<U>(self) -> OkInto<Self, U>
        where
            Self: Sized,
            Self::Ok: Into<U>,
        {
            assert_future::<Result<U, Self::Error>, _>(OkInto::new(self))
        }

        /// Executes another future after this one resolves successfully. The
        /// success value is passed to a closure to create this subsequent future.
        ///
        /// The provided closure `f` will only be called if this future is resolved
        /// to an [`Ok`]. If this future resolves to an [`Err`], panics, or is
        /// dropped, then the provided closure will never be invoked. The
        /// [`Error`](TryFuture::Error) type of this future and the future
        /// returned by `f` have to match.
        ///
        /// Note that this method consumes the future it is called on and returns a
        /// wrapped version of it.
        ///
        /// # Examples
        ///
        /// ```
        /// use futures::future::TryFutureExt;
        ///
        /// # futures::executor::block_on(async {
        /// let future = async { Ok::<i32, i32>(1) };
        /// let future = future.and_then(|x| async move { Ok::<i32, i32>(x + 3) });
        /// assert_eq!(future.await, Ok(4));
        /// # });
        /// ```
        ///
        /// Calling [`and_then`](TryFutureExt::and_then) on an errored future has no
        /// effect:
        ///
        /// ```
        /// use futures::future::TryFutureExt;
        ///
        /// # futures::executor::block_on(async {
        /// let future = async { Err::<i32, i32>(1) };
        /// let future = future.and_then(|x| async move { Err::<i32, i32>(x + 3) });
        /// assert_eq!(future.await, Err(1));
        /// # });
        /// ```
        fn and_then<Fut, F>(self, f: F) -> AndThen<Self, Fut, F>
        where
            F: FnOnce(Self::Ok) -> Fut,
            Fut: TryFuture<Error = Self::Error>,
            Self: Sized,
        {
            assert_future::<Result<Fut::Ok, Fut::Error>, _>(AndThen::new(self, f))
        }

        /// Executes another future if this one resolves to an error. The
        /// error value is passed to a closure to create this subsequent future.
        ///
        /// The provided closure `f` will only be called if this future is resolved
        /// to an [`Err`]. If this future resolves to an [`Ok`], panics, or is
        /// dropped, then the provided closure will never be invoked. The
        /// [`Ok`](TryFuture::Ok) type of this future and the future returned by `f`
        /// have to match.
        ///
        /// Note that this method consumes the future it is called on and returns a
        /// wrapped version of it.
        ///
        /// # Examples
        ///
        /// ```
        /// use futures::future::TryFutureExt;
        ///
        /// # futures::executor::block_on(async {
        /// let future = async { Err::<i32, i32>(1) };
        /// let future = future.or_else(|x| async move { Err::<i32, i32>(x + 3) });
        /// assert_eq!(future.await, Err(4));
        /// # });
        /// ```
        ///
        /// Calling [`or_else`](TryFutureExt::or_else) on a successful future has
        /// no effect:
        ///
        /// ```
        /// use futures::future::TryFutureExt;
        ///
        /// # futures::executor::block_on(async {
        /// let future = async { Ok::<i32, i32>(1) };
        /// let future = future.or_else(|x| async move { Ok::<i32, i32>(x + 3) });
        /// assert_eq!(future.await, Ok(1));
        /// # });
        /// ```
        fn or_else<Fut, F>(self, f: F) -> OrElse<Self, Fut, F>
        where
            F: FnOnce(Self::Error) -> Fut,
            Fut: TryFuture<Ok = Self::Ok>,
            Self: Sized,
        {
            assert_future::<Result<Fut::Ok, Fut::Error>, _>(OrElse::new(self, f))
        }

        /// Do something with the success value of a future before passing it on.
        ///
        /// When using futures, you'll often chain several of them together.  While
        /// working on such code, you might want to check out what's happening at
        /// various parts in the pipeline, without consuming the intermediate
        /// value. To do that, insert a call to `inspect_ok`.
        ///
        /// # Examples
        ///
        /// ```
        /// # futures::executor::block_on(async {
        /// use futures::future::TryFutureExt;
        ///
        /// let future = async { Ok::<_, ()>(1) };
        /// let new_future = future.inspect_ok(|&x| println!("about to resolve: {}", x));
        /// assert_eq!(new_future.await, Ok(1));
        /// # });
        /// ```
        fn inspect_ok<F>(self, f: F) -> InspectOk<Self, F>
        where
            F: FnOnce(&Self::Ok),
            Self: Sized,
        {
            assert_future::<Result<Self::Ok, Self::Error>, _>(InspectOk::new(self, f))
        }

        /// Do something with the error value of a future before passing it on.
        ///
        /// When using futures, you'll often chain several of them together.  While
        /// working on such code, you might want to check out what's happening at
        /// various parts in the pipeline, without consuming the intermediate
        /// value. To do that, insert a call to `inspect_err`.
        ///
        /// # Examples
        ///
        /// ```
        /// # futures::executor::block_on(async {
        /// use futures::future::TryFutureExt;
        ///
        /// let future = async { Err::<(), _>(1) };
        /// let new_future = future.inspect_err(|&x| println!("about to error: {}", x));
        /// assert_eq!(new_future.await, Err(1));
        /// # });
        /// ```
        fn inspect_err<F>(self, f: F) -> InspectErr<Self, F>
        where
            F: FnOnce(&Self::Error),
            Self: Sized,
        {
            assert_future::<Result<Self::Ok, Self::Error>, _>(InspectErr::new(self, f))
        }

        /// Flatten the execution of this future when the successful result of this
        /// future is another future.
        ///
        /// This is equivalent to `future.and_then(|x| x)`.
        fn try_flatten(self) -> TryFlatten<Self, Self::Ok>
        where
            Self::Ok: TryFuture<Error = Self::Error>,
            Self: Sized,
        {
            assert_future::<Result<<Self::Ok as TryFuture>::Ok, Self::Error>, _>(TryFlatten::new(self))
        }

        /// Flatten the execution of this future when the successful result of this
        /// future is a stream.
        ///
        /// This can be useful when stream initialization is deferred, and it is
        /// convenient to work with that stream as if stream was available at the
        /// call site.
        ///
        /// Note that this function consumes this future and returns a wrapped
        /// version of it.
        ///
        /// # Examples
        ///
        /// ```
        /// # futures::executor::block_on(async {
        /// use futures::future::TryFutureExt;
        /// use futures::stream::{self, TryStreamExt};
        ///
        /// let stream_items = vec![17, 18, 19].into_iter().map(Ok);
        /// let future_of_a_stream = async { Ok::<_, ()>(stream::iter(stream_items)) };
        ///
        /// let stream = future_of_a_stream.try_flatten_stream();
        /// let list = stream.try_collect::<Vec<_>>().await;
        /// assert_eq!(list, Ok(vec![17, 18, 19]));
        /// # });
        /// ```
        fn try_flatten_stream(self) -> TryFlattenStream<Self>
        where
            Self::Ok: TryStream<Error = Self::Error>,
            Self: Sized,
        {
            assert_stream::<Result<<Self::Ok as TryStream>::Ok, Self::Error>, _>(TryFlattenStream::new(
                self,
            ))
        }

        /// Unwraps this future's output, producing a future with this future's
        /// [`Ok`](TryFuture::Ok) type as its
        /// [`Output`](std::future::Future::Output) type.
        ///
        /// If this future is resolved successfully, the returned future will
        /// contain the original future's success value as output. Otherwise, the
        /// closure `f` is called with the error value to produce an alternate
        /// success value.
        ///
        /// This method is similar to the [`Result::unwrap_or_else`] method.
        ///
        /// # Examples
        ///
        /// ```
        /// use futures::future::TryFutureExt;
        ///
        /// # futures::executor::block_on(async {
        /// let future = async { Err::<(), &str>("Boom!") };
        /// let future = future.unwrap_or_else(|_| ());
        /// assert_eq!(future.await, ());
        /// # });
        /// ```
        fn unwrap_or_else<F>(self, f: F) -> UnwrapOrElse<Self, F>
        where
            Self: Sized,
            F: FnOnce(Self::Error) -> Self::Ok,
        {
            assert_future::<Self::Ok, _>(UnwrapOrElse::new(self, f))
        }

        /// Wraps a [`TryFuture`] into a future compatible with libraries using
        /// futures 0.1 future definitions. Requires the `compat` feature to enable.
        #[cfg(feature = "compat")]
        #[cfg_attr(docsrs, doc(cfg(feature = "compat")))]
        fn compat(self) -> Compat<Self>
        where
            Self: Sized + Unpin,
        {
            Compat::new(self)
        }

        /// Wraps a [`TryFuture`] into a type that implements
        /// [`Future`](std::future::Future).
        ///
        /// [`TryFuture`]s currently do not implement the
        /// [`Future`](std::future::Future) trait due to limitations of the
        /// compiler.
        ///
        /// # Examples
        ///
        /// ```
        /// use futures::future::{Future, TryFuture, TryFutureExt};
        ///
        /// # type T = i32;
        /// # type E = ();
        /// fn make_try_future() -> impl TryFuture<Ok = T, Error = E> { // ... }
        /// # async { Ok::<i32, ()>(1) }
        /// # }
        /// fn take_future(future: impl Future<Output = Result<T, E>>) { /* ... */ }
        ///
        /// take_future(make_try_future().into_future());
        /// ```
        fn into_future(self) -> IntoFuture<Self>
        where
            Self: Sized,
        {
            assert_future::<Result<Self::Ok, Self::Error>, _>(IntoFuture::new(self))
        }

        /// A convenience method for calling [`TryFuture::try_poll`] on [`Unpin`]
        /// future types.
        fn try_poll_unpin(&mut self, cx: &mut Context<'_>) -> Poll<Result<Self::Ok, Self::Error>>
        where
            Self: Unpin,
        {
            Pin::new(self).try_poll(cx)
        }
    }