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    /// The version of the call operator that takes an immutable receiver.
    ///
    /// Instances of `Fn` can be called repeatedly without mutating state.
    ///
    /// *This trait (`Fn`) is not to be confused with [function pointers][]
    /// (`fn`).*
    ///
    /// `Fn` is implemented automatically by closures which only take immutable
    /// references to captured variables or don't capture anything at all, as well
    /// as (safe) [function pointers][] (with some caveats, see their documentation
    /// for more details). Additionally, for any type `F` that implements `Fn`, `&F`
    /// implements `Fn`, too.
    ///
    /// Since both [`FnMut`] and [`FnOnce`] are supertraits of `Fn`, any
    /// instance of `Fn` can be used as a parameter where a [`FnMut`] or [`FnOnce`]
    /// is expected.
    ///
    /// Use `Fn` as a bound when you want to accept a parameter of function-like
    /// type and need to call it repeatedly and without mutating state (e.g., when
    /// calling it concurrently). If you do not need such strict requirements, use
    /// [`FnMut`] or [`FnOnce`] as bounds.
    ///
    /// See the [chapter on closures in *The Rust Programming Language*][book] for
    /// some more information on this topic.
    ///
    /// Also of note is the special syntax for `Fn` traits (e.g.
    /// `Fn(usize, bool) -> usize`). Those interested in the technical details of
    /// this can refer to [the relevant section in the *Rustonomicon*][nomicon].
    ///
    /// [book]: ../../book/ch13-01-closures.html
    /// [`FnMut`]: trait.FnMut.html
    /// [`FnOnce`]: trait.FnOnce.html
    /// [function pointers]: ../../std/primitive.fn.html
    /// [nomicon]: ../../nomicon/hrtb.html
    ///
    /// # Examples
    ///
    /// ## Calling a closure
    ///
    /// ```
    /// let square = |x| x * x;
    /// assert_eq!(square(5), 25);
    /// ```
    ///
    /// ## Using a `Fn` parameter
    ///
    /// ```
    /// fn call_with_one<F>(func: F) -> usize
    ///     where F: Fn(usize) -> usize {
    ///     func(1)
    /// }
    ///
    /// let double = |x| x * 2;
    /// assert_eq!(call_with_one(double), 2);
    /// ```
    #[lang = "fn"]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_paren_sugar]
    #[rustc_on_unimplemented(
        on(Args="()", note="wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}"),
        message="expected a `{Fn}<{Args}>` closure, found `{Self}`",
        label="expected an `Fn<{Args}>` closure, found `{Self}`",
    )]
    #[fundamental] // so that regex can rely that `&str: !FnMut`
    #[must_use]
    pub trait Fn<Args> : FnMut<Args> {
        /// Performs the call operation.
        #[unstable(feature = "fn_traits", issue = "29625")]
        extern "rust-call" fn call(&self, args: Args) -> Self::Output;
    }

    /// The version of the call operator that takes a mutable receiver.
    ///
    /// Instances of `FnMut` can be called repeatedly and may mutate state.
    ///
    /// `FnMut` is implemented automatically by closures which take mutable
    /// references to captured variables, as well as all types that implement
    /// [`Fn`], e.g., (safe) [function pointers][] (since `FnMut` is a supertrait of
    /// [`Fn`]). Additionally, for any type `F` that implements `FnMut`, `&mut F`
    /// implements `FnMut`, too.
    ///
    /// Since [`FnOnce`] is a supertrait of `FnMut`, any instance of `FnMut` can be
    /// used where a [`FnOnce`] is expected, and since [`Fn`] is a subtrait of
    /// `FnMut`, any instance of [`Fn`] can be used where `FnMut` is expected.
    ///
    /// Use `FnMut` as a bound when you want to accept a parameter of function-like
    /// type and need to call it repeatedly, while allowing it to mutate state.
    /// If you don't want the parameter to mutate state, use [`Fn`] as a
    /// bound; if you don't need to call it repeatedly, use [`FnOnce`].
    ///
    /// See the [chapter on closures in *The Rust Programming Language*][book] for
    /// some more information on this topic.
    ///
    /// Also of note is the special syntax for `Fn` traits (e.g.
    /// `Fn(usize, bool) -> usize`). Those interested in the technical details of
    /// this can refer to [the relevant section in the *Rustonomicon*][nomicon].
    ///
    /// [book]: ../../book/ch13-01-closures.html
    /// [`Fn`]: trait.Fn.html
    /// [`FnOnce`]: trait.FnOnce.html
    /// [function pointers]: ../../std/primitive.fn.html
    /// [nomicon]: ../../nomicon/hrtb.html
    ///
    /// # Examples
    ///
    /// ## Calling a mutably capturing closure
    ///
    /// ```
    /// let mut x = 5;
    /// {
    ///     let mut square_x = || x *= x;
    ///     square_x();
    /// }
    /// assert_eq!(x, 25);
    /// ```
    ///
    /// ## Using a `FnMut` parameter
    ///
    /// ```
    /// fn do_twice<F>(mut func: F)
    ///     where F: FnMut()
    /// {
    ///     func();
    ///     func();
    /// }
    ///
    /// let mut x: usize = 1;
    /// {
    ///     let add_two_to_x = || x += 2;
    ///     do_twice(add_two_to_x);
    /// }
    ///
    /// assert_eq!(x, 5);
    /// ```
    #[lang = "fn_mut"]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_paren_sugar]
    #[rustc_on_unimplemented(
        on(Args="()", note="wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}"),
        message="expected a `{FnMut}<{Args}>` closure, found `{Self}`",
        label="expected an `FnMut<{Args}>` closure, found `{Self}`",
    )]
    #[fundamental] // so that regex can rely that `&str: !FnMut`
    #[must_use]
    pub trait FnMut<Args> : FnOnce<Args> {
        /// Performs the call operation.
        #[unstable(feature = "fn_traits", issue = "29625")]
        extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output;
    }

    /// The version of the call operator that takes a by-value receiver.
    ///
    /// Instances of `FnOnce` can be called, but might not be callable multiple
    /// times. Because of this, if the only thing known about a type is that it
    /// implements `FnOnce`, it can only be called once.
    ///
    /// `FnOnce` is implemented automatically by closure that might consume captured
    /// variables, as well as all types that implement [`FnMut`], e.g., (safe)
    /// [function pointers][] (since `FnOnce` is a supertrait of [`FnMut`]).
    ///
    /// Since both [`Fn`] and [`FnMut`] are subtraits of `FnOnce`, any instance of
    /// [`Fn`] or [`FnMut`] can be used where a `FnOnce` is expected.
    ///
    /// Use `FnOnce` as a bound when you want to accept a parameter of function-like
    /// type and only need to call it once. If you need to call the parameter
    /// repeatedly, use [`FnMut`] as a bound; if you also need it to not mutate
    /// state, use [`Fn`].
    ///
    /// See the [chapter on closures in *The Rust Programming Language*][book] for
    /// some more information on this topic.
    ///
    /// Also of note is the special syntax for `Fn` traits (e.g.
    /// `Fn(usize, bool) -> usize`). Those interested in the technical details of
    /// this can refer to [the relevant section in the *Rustonomicon*][nomicon].
    ///
    /// [book]: ../../book/ch13-01-closures.html
    /// [`Fn`]: trait.Fn.html
    /// [`FnMut`]: trait.FnMut.html
    /// [function pointers]: ../../std/primitive.fn.html
    /// [nomicon]: ../../nomicon/hrtb.html
    ///
    /// # Examples
    ///
    /// ## Calling a by-value closure
    ///
    /// ```
    /// let x = 5;
    /// let square_x = move || x * x;
    /// assert_eq!(square_x(), 25);
    /// ```
    ///
    /// ## Using a `FnOnce` parameter
    ///
    /// ```
    /// fn consume_with_relish<F>(func: F)
    ///     where F: FnOnce() -> String
    /// {
    ///     // `func` consumes its captured variables, so it cannot be run more
    ///     // than once.
    ///     println!("Consumed: {}", func());
    ///
    ///     println!("Delicious!");
    ///
    ///     // Attempting to invoke `func()` again will throw a `use of moved
    ///     // value` error for `func`.
    /// }
    ///
    /// let x = String::from("x");
    /// let consume_and_return_x = move || x;
    /// consume_with_relish(consume_and_return_x);
    ///
    /// // `consume_and_return_x` can no longer be invoked at this point
    /// ```
    #[lang = "fn_once"]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_paren_sugar]
    #[rustc_on_unimplemented(
        on(Args="()", note="wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}"),
        message="expected a `{FnOnce}<{Args}>` closure, found `{Self}`",
        label="expected an `FnOnce<{Args}>` closure, found `{Self}`",
    )]
    #[fundamental] // so that regex can rely that `&str: !FnMut`
    #[must_use]
    pub trait FnOnce<Args> {
        /// The returned type after the call operator is used.
        #[stable(feature = "fn_once_output", since = "1.12.0")]
        type Output;

        /// Performs the call operation.
        #[unstable(feature = "fn_traits", issue = "29625")]
        extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
    }

    mod impls {
        #[stable(feature = "rust1", since = "1.0.0")]
        impl<A,F:?Sized> Fn<A> for &F
            where F : Fn<A>
        {
            extern "rust-call" fn call(&self, args: A) -> F::Output {
                (**self).call(args)
            }
        }

        #[stable(feature = "rust1", since = "1.0.0")]
        impl<A,F:?Sized> FnMut<A> for &F
            where F : Fn<A>
        {
            extern "rust-call" fn call_mut(&mut self, args: A) -> F::Output {
                (**self).call(args)
            }
        }

        #[stable(feature = "rust1", since = "1.0.0")]
        impl<A,F:?Sized> FnOnce<A> for &F
            where F : Fn<A>
        {
            type Output = F::Output;

            extern "rust-call" fn call_once(self, args: A) -> F::Output {
                (*self).call(args)
            }
        }

        #[stable(feature = "rust1", since = "1.0.0")]
        impl<A,F:?Sized> FnMut<A> for &mut F
            where F : FnMut<A>
        {
            extern "rust-call" fn call_mut(&mut self, args: A) -> F::Output {
                (*self).call_mut(args)
            }
        }

        #[stable(feature = "rust1", since = "1.0.0")]
        impl<A,F:?Sized> FnOnce<A> for &mut F
            where F : FnMut<A>
        {
            type Output = F::Output;
            extern "rust-call" fn call_once(self, args: A) -> F::Output {
                (*self).call_mut(args)
            }
        }
    }

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