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    //! Optional values.
    //!
    //! Type [`Option`] represents an optional value: every [`Option`]
    //! is either [`Some`] and contains a value, or [`None`], and
    //! does not. [`Option`] types are very common in Rust code, as
    //! they have a number of uses:
    //!
    //! * Initial values
    //! * Return values for functions that are not defined
    //!   over their entire input range (partial functions)
    //! * Return value for otherwise reporting simple errors, where `None` is
    //!   returned on error
    //! * Optional struct fields
    //! * Struct fields that can be loaned or "taken"
    //! * Optional function arguments
    //! * Nullable pointers
    //! * Swapping things out of difficult situations
    //!
    //! [`Option`]s are commonly paired with pattern matching to query the presence
    //! of a value and take action, always accounting for the [`None`] case.
    //!
    //! ```
    //! fn divide(numerator: f64, denominator: f64) -> Option<f64> {
    //!     if denominator == 0.0 {
    //!         None
    //!     } else {
    //!         Some(numerator / denominator)
    //!     }
    //! }
    //!
    //! // The return value of the function is an option
    //! let result = divide(2.0, 3.0);
    //!
    //! // Pattern match to retrieve the value
    //! match result {
    //!     // The division was valid
    //!     Some(x) => println!("Result: {}", x),
    //!     // The division was invalid
    //!     None    => println!("Cannot divide by 0"),
    //! }
    //! ```
    //!
    //
    // FIXME: Show how `Option` is used in practice, with lots of methods
    //
    //! # Options and pointers ("nullable" pointers)
    //!
    //! Rust's pointer types must always point to a valid location; there are
    //! no "null" pointers. Instead, Rust has *optional* pointers, like
    //! the optional owned box, [`Option`]`<`[`Box<T>`]`>`.
    //!
    //! The following example uses [`Option`] to create an optional box of
    //! [`i32`]. Notice that in order to use the inner [`i32`] value first, the
    //! `check_optional` function needs to use pattern matching to
    //! determine whether the box has a value (i.e., it is [`Some(...)`][`Some`]) or
    //! not ([`None`]).
    //!
    //! ```
    //! let optional = None;
    //! check_optional(optional);
    //!
    //! let optional = Some(Box::new(9000));
    //! check_optional(optional);
    //!
    //! fn check_optional(optional: Option<Box<i32>>) {
    //!     match optional {
    //!         Some(ref p) => println!("has value {}", p),
    //!         None => println!("has no value"),
    //!     }
    //! }
    //! ```
    //!
    //! This usage of [`Option`] to create safe nullable pointers is so
    //! common that Rust does special optimizations to make the
    //! representation of [`Option`]`<`[`Box<T>`]`>` a single pointer. Optional pointers
    //! in Rust are stored as efficiently as any other pointer type.
    //!
    //! # Examples
    //!
    //! Basic pattern matching on [`Option`]:
    //!
    //! ```
    //! let msg = Some("howdy");
    //!
    //! // Take a reference to the contained string
    //! if let Some(ref m) = msg {
    //!     println!("{}", *m);
    //! }
    //!
    //! // Remove the contained string, destroying the Option
    //! let unwrapped_msg = msg.unwrap_or("default message");
    //! ```
    //!
    //! Initialize a result to [`None`] before a loop:
    //!
    //! ```
    //! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }
    //!
    //! // A list of data to search through.
    //! let all_the_big_things = [
    //!     Kingdom::Plant(250, "redwood"),
    //!     Kingdom::Plant(230, "noble fir"),
    //!     Kingdom::Plant(229, "sugar pine"),
    //!     Kingdom::Animal(25, "blue whale"),
    //!     Kingdom::Animal(19, "fin whale"),
    //!     Kingdom::Animal(15, "north pacific right whale"),
    //! ];
    //!
    //! // We're going to search for the name of the biggest animal,
    //! // but to start with we've just got `None`.
    //! let mut name_of_biggest_animal = None;
    //! let mut size_of_biggest_animal = 0;
    //! for big_thing in &all_the_big_things {
    //!     match *big_thing {
    //!         Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
    //!             // Now we've found the name of some big animal
    //!             size_of_biggest_animal = size;
    //!             name_of_biggest_animal = Some(name);
    //!         }
    //!         Kingdom::Animal(..) | Kingdom::Plant(..) => ()
    //!     }
    //! }
    //!
    //! match name_of_biggest_animal {
    //!     Some(name) => println!("the biggest animal is {}", name),
    //!     None => println!("there are no animals :("),
    //! }
    //! ```
    //!
    //! [`Option`]: enum.Option.html
    //! [`Some`]: enum.Option.html#variant.Some
    //! [`None`]: enum.Option.html#variant.None
    //! [`Box<T>`]: ../../std/boxed/struct.Box.html
    //! [`i32`]: ../../std/primitive.i32.html

    #![stable(feature = "rust1", since = "1.0.0")]

    use iter::{FromIterator, FusedIterator, TrustedLen};
    use {hint, mem, ops::{self, Deref}};
    use pin::Pin;

    // Note that this is not a lang item per se, but it has a hidden dependency on
    // `Iterator`, which is one. The compiler assumes that the `next` method of
    // `Iterator` is an enumeration with one type parameter and two variants,
    // which basically means it must be `Option`.

    /// The `Option` type. See [the module level documentation](index.html) for more.
    #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub enum Option<T> {
        /// No value
        #[stable(feature = "rust1", since = "1.0.0")]
        None,
        /// Some value `T`
        #[stable(feature = "rust1", since = "1.0.0")]
        Some(#[stable(feature = "rust1", since = "1.0.0")] T),
    }

    /////////////////////////////////////////////////////////////////////////////
    // Type implementation
    /////////////////////////////////////////////////////////////////////////////

    impl<T> Option<T> {
        /////////////////////////////////////////////////////////////////////////
        // Querying the contained values
        /////////////////////////////////////////////////////////////////////////

        /// Returns `true` if the option is a [`Some`] value.
        ///
        /// # Examples
        ///
        /// ```
        /// let x: Option<u32> = Some(2);
        /// assert_eq!(x.is_some(), true);
        ///
        /// let x: Option<u32> = None;
        /// assert_eq!(x.is_some(), false);
        /// ```
        ///
        /// [`Some`]: #variant.Some
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn is_some(&self) -> bool {
            match *self {
                Some(_) => true,
                None => false,
            }
        }

        /// Returns `true` if the option is a [`None`] value.
        ///
        /// # Examples
        ///
        /// ```
        /// let x: Option<u32> = Some(2);
        /// assert_eq!(x.is_none(), false);
        ///
        /// let x: Option<u32> = None;
        /// assert_eq!(x.is_none(), true);
        /// ```
        ///
        /// [`None`]: #variant.None
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn is_none(&self) -> bool {
            !self.is_some()
        }

        /////////////////////////////////////////////////////////////////////////
        // Adapter for working with references
        /////////////////////////////////////////////////////////////////////////

        /// Converts from `Option<T>` to `Option<&T>`.
        ///
        /// # Examples
        ///
        /// Convert an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, preserving the original.
        /// The [`map`] method takes the `self` argument by value, consuming the original,
        /// so this technique uses `as_ref` to first take an `Option` to a reference
        /// to the value inside the original.
        ///
        /// [`map`]: enum.Option.html#method.map
        /// [`String`]: ../../std/string/struct.String.html
        /// [`usize`]: ../../std/primitive.usize.html
        ///
        /// ```
        /// let text: Option<String> = Some("Hello, world!".to_string());
        /// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
        /// // then consume *that* with `map`, leaving `text` on the stack.
        /// let text_length: Option<usize> = text.as_ref().map(|s| s.len());
        /// println!("still can print text: {:?}", text);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn as_ref(&self) -> Option<&T> {
            match *self {
                Some(ref x) => Some(x),
                None => None,
            }
        }

        /// Converts from `Option<T>` to `Option<&mut T>`.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut x = Some(2);
        /// match x.as_mut() {
        ///     Some(v) => *v = 42,
        ///     None => {},
        /// }
        /// assert_eq!(x, Some(42));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn as_mut(&mut self) -> Option<&mut T> {
            match *self {
                Some(ref mut x) => Some(x),
                None => None,
            }
        }

        /// Converts from `Pin<&Option<T>>` to `Option<Pin<&T>>`
        #[inline]
        #[stable(feature = "pin", since = "1.33.0")]
        pub fn as_pin_ref<'a>(self: Pin<&'a Option<T>>) -> Option<Pin<&'a T>> {
            unsafe {
                Pin::get_ref(self).as_ref().map(|x| Pin::new_unchecked(x))
            }
        }

        /// Converts from `Pin<&mut Option<T>>` to `Option<Pin<&mut T>>`
        #[inline]
        #[stable(feature = "pin", since = "1.33.0")]
        pub fn as_pin_mut<'a>(self: Pin<&'a mut Option<T>>) -> Option<Pin<&'a mut T>> {
            unsafe {
                Pin::get_unchecked_mut(self).as_mut().map(|x| Pin::new_unchecked(x))
            }
        }

        /////////////////////////////////////////////////////////////////////////
        // Getting to contained values
        /////////////////////////////////////////////////////////////////////////

        /// Unwraps an option, yielding the content of a [`Some`].
        ///
        /// # Panics
        ///
        /// Panics if the value is a [`None`] with a custom panic message provided by
        /// `msg`.
        ///
        /// [`Some`]: #variant.Some
        /// [`None`]: #variant.None
        ///
        /// # Examples
        ///
        /// ```
        /// let x = Some("value");
        /// assert_eq!(x.expect("the world is ending"), "value");
        /// ```
        ///
        /// ```{.should_panic}
        /// let x: Option<&str> = None;
        /// x.expect("the world is ending"); // panics with `the world is ending`
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn expect(self, msg: &str) -> T {
            match self {
                Some(val) => val,
                None => expect_failed(msg),
            }
        }

        /// Moves the value `v` out of the `Option<T>` if it is [`Some(v)`].
        ///
        /// In general, because this function may panic, its use is discouraged.
        /// Instead, prefer to use pattern matching and handle the [`None`]
        /// case explicitly.
        ///
        /// # Panics
        ///
        /// Panics if the self value equals [`None`].
        ///
        /// [`Some(v)`]: #variant.Some
        /// [`None`]: #variant.None
        ///
        /// # Examples
        ///
        /// ```
        /// let x = Some("air");
        /// assert_eq!(x.unwrap(), "air");
        /// ```
        ///
        /// ```{.should_panic}
        /// let x: Option<&str> = None;
        /// assert_eq!(x.unwrap(), "air"); // fails
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn unwrap(self) -> T {
            match self {
                Some(val) => val,
                None => panic!("called `Option::unwrap()` on a `None` value"),
            }
        }

        /// Returns the contained value or a default.
        ///
        /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
        /// the result of a function call, it is recommended to use [`unwrap_or_else`],
        /// which is lazily evaluated.
        ///
        /// [`unwrap_or_else`]: #method.unwrap_or_else
        ///
        /// # Examples
        ///
        /// ```
        /// assert_eq!(Some("car").unwrap_or("bike"), "car");
        /// assert_eq!(None.unwrap_or("bike"), "bike");
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn unwrap_or(self, def: T) -> T {
            match self {
                Some(x) => x,
                None => def,
            }
        }

        /// Returns the contained value or computes it from a closure.
        ///
        /// # Examples
        ///
        /// ```
        /// let k = 10;
        /// assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
        /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T {
            match self {
                Some(x) => x,
                None => f(),
            }
        }

        /////////////////////////////////////////////////////////////////////////
        // Transforming contained values
        /////////////////////////////////////////////////////////////////////////

        /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value.
        ///
        /// # Examples
        ///
        /// Convert an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, consuming the original:
        ///
        /// [`String`]: ../../std/string/struct.String.html
        /// [`usize`]: ../../std/primitive.usize.html
        ///
        /// ```
        /// let maybe_some_string = Some(String::from("Hello, World!"));
        /// // `Option::map` takes self *by value*, consuming `maybe_some_string`
        /// let maybe_some_len = maybe_some_string.map(|s| s.len());
        ///
        /// assert_eq!(maybe_some_len, Some(13));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn map<U, F: FnOnce(T) -> U>(self, f: F) -> Option<U> {
            match self {
                Some(x) => Some(f(x)),
                None => None,
            }
        }

        /// Applies a function to the contained value (if any),
        /// or returns the provided default (if not).
        ///
        /// # Examples
        ///
        /// ```
        /// let x = Some("foo");
        /// assert_eq!(x.map_or(42, |v| v.len()), 3);
        ///
        /// let x: Option<&str> = None;
        /// assert_eq!(x.map_or(42, |v| v.len()), 42);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
            match self {
                Some(t) => f(t),
                None => default,
            }
        }

        /// Applies a function to the contained value (if any),
        /// or computes a default (if not).
        ///
        /// # Examples
        ///
        /// ```
        /// let k = 21;
        ///
        /// let x = Some("foo");
        /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);
        ///
        /// let x: Option<&str> = None;
        /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn map_or_else<U, D: FnOnce() -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
            match self {
                Some(t) => f(t),
                None => default(),
            }
        }

        /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
        /// [`Ok(v)`] and [`None`] to [`Err(err)`].
        ///
        /// Arguments passed to `ok_or` are eagerly evaluated; if you are passing the
        /// result of a function call, it is recommended to use [`ok_or_else`], which is
        /// lazily evaluated.
        ///
        /// [`Result<T, E>`]: ../../std/result/enum.Result.html
        /// [`Ok(v)`]: ../../std/result/enum.Result.html#variant.Ok
        /// [`Err(err)`]: ../../std/result/enum.Result.html#variant.Err
        /// [`None`]: #variant.None
        /// [`Some(v)`]: #variant.Some
        /// [`ok_or_else`]: #method.ok_or_else
        ///
        /// # Examples
        ///
        /// ```
        /// let x = Some("foo");
        /// assert_eq!(x.ok_or(0), Ok("foo"));
        ///
        /// let x: Option<&str> = None;
        /// assert_eq!(x.ok_or(0), Err(0));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn ok_or<E>(self, err: E) -> Result<T, E> {
            match self {
                Some(v) => Ok(v),
                None => Err(err),
            }
        }

        /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
        /// [`Ok(v)`] and [`None`] to [`Err(err())`].
        ///
        /// [`Result<T, E>`]: ../../std/result/enum.Result.html
        /// [`Ok(v)`]: ../../std/result/enum.Result.html#variant.Ok
        /// [`Err(err())`]: ../../std/result/enum.Result.html#variant.Err
        /// [`None`]: #variant.None
        /// [`Some(v)`]: #variant.Some
        ///
        /// # Examples
        ///
        /// ```
        /// let x = Some("foo");
        /// assert_eq!(x.ok_or_else(|| 0), Ok("foo"));
        ///
        /// let x: Option<&str> = None;
        /// assert_eq!(x.ok_or_else(|| 0), Err(0));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn ok_or_else<E, F: FnOnce() -> E>(self, err: F) -> Result<T, E> {
            match self {
                Some(v) => Ok(v),
                None => Err(err()),
            }
        }

        /////////////////////////////////////////////////////////////////////////
        // Iterator constructors
        /////////////////////////////////////////////////////////////////////////

        /// Returns an iterator over the possibly contained value.
        ///
        /// # Examples
        ///
        /// ```
        /// let x = Some(4);
        /// assert_eq!(x.iter().next(), Some(&4));
        ///
        /// let x: Option<u32> = None;
        /// assert_eq!(x.iter().next(), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn iter(&self) -> Iter<T> {
            Iter { inner: Item { opt: self.as_ref() } }
        }

        /// Returns a mutable iterator over the possibly contained value.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut x = Some(4);
        /// match x.iter_mut().next() {
        ///     Some(v) => *v = 42,
        ///     None => {},
        /// }
        /// assert_eq!(x, Some(42));
        ///
        /// let mut x: Option<u32> = None;
        /// assert_eq!(x.iter_mut().next(), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn iter_mut(&mut self) -> IterMut<T> {
            IterMut { inner: Item { opt: self.as_mut() } }
        }

        /////////////////////////////////////////////////////////////////////////
        // Boolean operations on the values, eager and lazy
        /////////////////////////////////////////////////////////////////////////

        /// Returns [`None`] if the option is [`None`], otherwise returns `optb`.
        ///
        /// [`None`]: #variant.None
        ///
        /// # Examples
        ///
        /// ```
        /// let x = Some(2);
        /// let y: Option<&str> = None;
        /// assert_eq!(x.and(y), None);
        ///
        /// let x: Option<u32> = None;
        /// let y = Some("foo");
        /// assert_eq!(x.and(y), None);
        ///
        /// let x = Some(2);
        /// let y = Some("foo");
        /// assert_eq!(x.and(y), Some("foo"));
        ///
        /// let x: Option<u32> = None;
        /// let y: Option<&str> = None;
        /// assert_eq!(x.and(y), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn and<U>(self, optb: Option<U>) -> Option<U> {
            match self {
                Some(_) => optb,
                None => None,
            }
        }

        /// Returns [`None`] if the option is [`None`], otherwise calls `f` with the
        /// wrapped value and returns the result.
        ///
        /// Some languages call this operation flatmap.
        ///
        /// [`None`]: #variant.None
        ///
        /// # Examples
        ///
        /// ```
        /// fn sq(x: u32) -> Option<u32> { Some(x * x) }
        /// fn nope(_: u32) -> Option<u32> { None }
        ///
        /// assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16));
        /// assert_eq!(Some(2).and_then(sq).and_then(nope), None);
        /// assert_eq!(Some(2).and_then(nope).and_then(sq), None);
        /// assert_eq!(None.and_then(sq).and_then(sq), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn and_then<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> Option<U> {
            match self {
                Some(x) => f(x),
                None => None,
            }
        }

        /// Returns `None` if the option is `None`, otherwise calls `predicate`
        /// with the wrapped value and returns:
        ///
        /// - `Some(t)` if `predicate` returns `true` (where `t` is the wrapped
        ///   value), and
        /// - `None` if `predicate` returns `false`.
        ///
        /// This function works similar to `Iterator::filter()`. You can imagine
        /// the `Option<T>` being an iterator over one or zero elements. `filter()`
        /// lets you decide which elements to keep.
        ///
        /// # Examples
        ///
        /// ```rust
        /// fn is_even(n: &i32) -> bool {
        ///     n % 2 == 0
        /// }
        ///
        /// assert_eq!(None.filter(is_even), None);
        /// assert_eq!(Some(3).filter(is_even), None);
        /// assert_eq!(Some(4).filter(is_even), Some(4));
        /// ```
        #[inline]
        #[stable(feature = "option_filter", since = "1.27.0")]
        pub fn filter<P: FnOnce(&T) -> bool>(self, predicate: P) -> Self {
            if let Some(x) = self {
                if predicate(&x) {
                    return Some(x)
                }
            }
            None
        }

        /// Returns the option if it contains a value, otherwise returns `optb`.
        ///
        /// Arguments passed to `or` are eagerly evaluated; if you are passing the
        /// result of a function call, it is recommended to use [`or_else`], which is
        /// lazily evaluated.
        ///
        /// [`or_else`]: #method.or_else
        ///
        /// # Examples
        ///
        /// ```
        /// let x = Some(2);
        /// let y = None;
        /// assert_eq!(x.or(y), Some(2));
        ///
        /// let x = None;
        /// let y = Some(100);
        /// assert_eq!(x.or(y), Some(100));
        ///
        /// let x = Some(2);
        /// let y = Some(100);
        /// assert_eq!(x.or(y), Some(2));
        ///
        /// let x: Option<u32> = None;
        /// let y = None;
        /// assert_eq!(x.or(y), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn or(self, optb: Option<T>) -> Option<T> {
            match self {
                Some(_) => self,
                None => optb,
            }
        }

        /// Returns the option if it contains a value, otherwise calls `f` and
        /// returns the result.
        ///
        /// # Examples
        ///
        /// ```
        /// fn nobody() -> Option<&'static str> { None }
        /// fn vikings() -> Option<&'static str> { Some("vikings") }
        ///
        /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
        /// assert_eq!(None.or_else(vikings), Some("vikings"));
        /// assert_eq!(None.or_else(nobody), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn or_else<F: FnOnce() -> Option<T>>(self, f: F) -> Option<T> {
            match self {
                Some(_) => self,
                None => f(),
            }
        }

        /// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns `None`.
        ///
        /// [`Some`]: #variant.Some
        /// [`None`]: #variant.None
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(option_xor)]
        ///
        /// let x = Some(2);
        /// let y: Option<u32> = None;
        /// assert_eq!(x.xor(y), Some(2));
        ///
        /// let x: Option<u32> = None;
        /// let y = Some(2);
        /// assert_eq!(x.xor(y), Some(2));
        ///
        /// let x = Some(2);
        /// let y = Some(2);
        /// assert_eq!(x.xor(y), None);
        ///
        /// let x: Option<u32> = None;
        /// let y: Option<u32> = None;
        /// assert_eq!(x.xor(y), None);
        /// ```
        #[inline]
        #[unstable(feature = "option_xor", issue = "50512")]
        pub fn xor(self, optb: Option<T>) -> Option<T> {
            match (self, optb) {
                (Some(a), None) => Some(a),
                (None, Some(b)) => Some(b),
                _ => None,
            }
        }

        /////////////////////////////////////////////////////////////////////////
        // Entry-like operations to insert if None and return a reference
        /////////////////////////////////////////////////////////////////////////

        /// Inserts `v` into the option if it is [`None`], then
        /// returns a mutable reference to the contained value.
        ///
        /// [`None`]: #variant.None
        ///
        /// # Examples
        ///
        /// ```
        /// let mut x = None;
        ///
        /// {
        ///     let y: &mut u32 = x.get_or_insert(5);
        ///     assert_eq!(y, &5);
        ///
        ///     *y = 7;
        /// }
        ///
        /// assert_eq!(x, Some(7));
        /// ```
        #[inline]
        #[stable(feature = "option_entry", since = "1.20.0")]
        pub fn get_or_insert(&mut self, v: T) -> &mut T {
            match *self {
                None => *self = Some(v),
                _ => (),
            }

            match *self {
                Some(ref mut v) => v,
                None => unsafe { hint::unreachable_unchecked() },
            }
        }

        /// Inserts a value computed from `f` into the option if it is [`None`], then
        /// returns a mutable reference to the contained value.
        ///
        /// [`None`]: #variant.None
        ///
        /// # Examples
        ///
        /// ```
        /// let mut x = None;
        ///
        /// {
        ///     let y: &mut u32 = x.get_or_insert_with(|| 5);
        ///     assert_eq!(y, &5);
        ///
        ///     *y = 7;
        /// }
        ///
        /// assert_eq!(x, Some(7));
        /// ```
        #[inline]
        #[stable(feature = "option_entry", since = "1.20.0")]
        pub fn get_or_insert_with<F: FnOnce() -> T>(&mut self, f: F) -> &mut T {
            match *self {
                None => *self = Some(f()),
                _ => (),
            }

            match *self {
                Some(ref mut v) => v,
                None => unsafe { hint::unreachable_unchecked() },
            }
        }

        /////////////////////////////////////////////////////////////////////////
        // Misc
        /////////////////////////////////////////////////////////////////////////

        /// Takes the value out of the option, leaving a [`None`] in its place.
        ///
        /// [`None`]: #variant.None
        ///
        /// # Examples
        ///
        /// ```
        /// let mut x = Some(2);
        /// let y = x.take();
        /// assert_eq!(x, None);
        /// assert_eq!(y, Some(2));
        ///
        /// let mut x: Option<u32> = None;
        /// let y = x.take();
        /// assert_eq!(x, None);
        /// assert_eq!(y, None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn take(&mut self) -> Option<T> {
            mem::replace(self, None)
        }

        /// Replaces the actual value in the option by the value given in parameter,
        /// returning the old value if present,
        /// leaving a [`Some`] in its place without deinitializing either one.
        ///
        /// [`Some`]: #variant.Some
        ///
        /// # Examples
        ///
        /// ```
        /// let mut x = Some(2);
        /// let old = x.replace(5);
        /// assert_eq!(x, Some(5));
        /// assert_eq!(old, Some(2));
        ///
        /// let mut x = None;
        /// let old = x.replace(3);
        /// assert_eq!(x, Some(3));
        /// assert_eq!(old, None);
        /// ```
        #[inline]
        #[stable(feature = "option_replace", since = "1.31.0")]
        pub fn replace(&mut self, value: T) -> Option<T> {
            mem::replace(self, Some(value))
        }
    }

    impl<'a, T: Copy> Option<&'a T> {
        /// Maps an `Option<&T>` to an `Option<T>` by copying the contents of the
        /// option.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(copied)]
        ///
        /// let x = 12;
        /// let opt_x = Some(&x);
        /// assert_eq!(opt_x, Some(&12));
        /// let copied = opt_x.copied();
        /// assert_eq!(copied, Some(12));
        /// ```
        #[unstable(feature = "copied", issue = "57126")]
        pub fn copied(self) -> Option<T> {
            self.map(|&t| t)
        }
    }

    impl<'a, T: Copy> Option<&'a mut T> {
        /// Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the
        /// option.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(copied)]
        ///
        /// let mut x = 12;
        /// let opt_x = Some(&mut x);
        /// assert_eq!(opt_x, Some(&mut 12));
        /// let copied = opt_x.copied();
        /// assert_eq!(copied, Some(12));
        /// ```
        #[unstable(feature = "copied", issue = "57126")]
        pub fn copied(self) -> Option<T> {
            self.map(|&mut t| t)
        }
    }

    impl<'a, T: Clone> Option<&'a T> {
        /// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the
        /// option.
        ///
        /// # Examples
        ///
        /// ```
        /// let x = 12;
        /// let opt_x = Some(&x);
        /// assert_eq!(opt_x, Some(&12));
        /// let cloned = opt_x.cloned();
        /// assert_eq!(cloned, Some(12));
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn cloned(self) -> Option<T> {
            self.map(|t| t.clone())
        }
    }

    impl<'a, T: Clone> Option<&'a mut T> {
        /// Maps an `Option<&mut T>` to an `Option<T>` by cloning the contents of the
        /// option.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut x = 12;
        /// let opt_x = Some(&mut x);
        /// assert_eq!(opt_x, Some(&mut 12));
        /// let cloned = opt_x.cloned();
        /// assert_eq!(cloned, Some(12));
        /// ```
        #[stable(since = "1.26.0", feature = "option_ref_mut_cloned")]
        pub fn cloned(self) -> Option<T> {
            self.map(|t| t.clone())
        }
    }

    impl<T: Default> Option<T> {
        /// Returns the contained value or a default
        ///
        /// Consumes the `self` argument then, if [`Some`], returns the contained
        /// value, otherwise if [`None`], returns the [default value] for that
        /// type.
        ///
        /// # Examples
        ///
        /// Convert a string to an integer, turning poorly-formed strings
        /// into 0 (the default value for integers). [`parse`] converts
        /// a string to any other type that implements [`FromStr`], returning
        /// [`None`] on error.
        ///
        /// ```
        /// let good_year_from_input = "1909";
        /// let bad_year_from_input = "190blarg";
        /// let good_year = good_year_from_input.parse().ok().unwrap_or_default();
        /// let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();
        ///
        /// assert_eq!(1909, good_year);
        /// assert_eq!(0, bad_year);
        /// ```
        ///
        /// [`Some`]: #variant.Some
        /// [`None`]: #variant.None
        /// [default value]: ../default/trait.Default.html#tymethod.default
        /// [`parse`]: ../../std/primitive.str.html#method.parse
        /// [`FromStr`]: ../../std/str/trait.FromStr.html
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn unwrap_or_default(self) -> T {
            match self {
                Some(x) => x,
                None => Default::default(),
            }
        }
    }

    #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
    impl<T: Deref> Option<T> {
        /// Converts from `&Option<T>` to `Option<&T::Target>`.
        ///
        /// Leaves the original Option in-place, creating a new one with a reference
        /// to the original one, additionally coercing the contents via `Deref`.
        pub fn deref(&self) -> Option<&T::Target> {
            self.as_ref().map(|t| t.deref())
        }
    }

    impl<T, E> Option<Result<T, E>> {
        /// Transposes an `Option` of a `Result` into a `Result` of an `Option`.
        ///
        /// `None` will be mapped to `Ok(None)`.
        /// `Some(Ok(_))` and `Some(Err(_))` will be mapped to `Ok(Some(_))` and `Err(_)`.
        ///
        /// # Examples
        ///
        /// ```
        /// #[derive(Debug, Eq, PartialEq)]
        /// struct SomeErr;
        ///
        /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
        /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
        /// assert_eq!(x, y.transpose());
        /// ```
        #[inline]
        #[stable(feature = "transpose_result", since = "1.33.0")]
        pub fn transpose(self) -> Result<Option<T>, E> {
            match self {
                Some(Ok(x)) => Ok(Some(x)),
                Some(Err(e)) => Err(e),
                None => Ok(None),
            }
        }
    }

    // This is a separate function to reduce the code size of .expect() itself.
    #[inline(never)]
    #[cold]
    fn expect_failed(msg: &str) -> ! {
        panic!("{}", msg)
    }

    /////////////////////////////////////////////////////////////////////////////
    // Trait implementations
    /////////////////////////////////////////////////////////////////////////////

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> Default for Option<T> {
        /// Returns [`None`][Option::None].
        #[inline]
        fn default() -> Option<T> { None }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> IntoIterator for Option<T> {
        type Item = T;
        type IntoIter = IntoIter<T>;

        /// Returns a consuming iterator over the possibly contained value.
        ///
        /// # Examples
        ///
        /// ```
        /// let x = Some("string");
        /// let v: Vec<&str> = x.into_iter().collect();
        /// assert_eq!(v, ["string"]);
        ///
        /// let x = None;
        /// let v: Vec<&str> = x.into_iter().collect();
        /// assert!(v.is_empty());
        /// ```
        #[inline]
        fn into_iter(self) -> IntoIter<T> {
            IntoIter { inner: Item { opt: self } }
        }
    }

    #[stable(since = "1.4.0", feature = "option_iter")]
    impl<'a, T> IntoIterator for &'a Option<T> {
        type Item = &'a T;
        type IntoIter = Iter<'a, T>;

        fn into_iter(self) -> Iter<'a, T> {
            self.iter()
        }
    }

    #[stable(since = "1.4.0", feature = "option_iter")]
    impl<'a, T> IntoIterator for &'a mut Option<T> {
        type Item = &'a mut T;
        type IntoIter = IterMut<'a, T>;

        fn into_iter(self) -> IterMut<'a, T> {
            self.iter_mut()
        }
    }

    #[stable(since = "1.12.0", feature = "option_from")]
    impl<T> From<T> for Option<T> {
        fn from(val: T) -> Option<T> {
            Some(val)
        }
    }

    #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
    impl<'a, T> From<&'a Option<T>> for Option<&'a T> {
        fn from(o: &'a Option<T>) -> Option<&'a T> {
            o.as_ref()
        }
    }

    #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
    impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T> {
        fn from(o: &'a mut Option<T>) -> Option<&'a mut T> {
            o.as_mut()
        }
    }

    /////////////////////////////////////////////////////////////////////////////
    // The Option Iterators
    /////////////////////////////////////////////////////////////////////////////

    #[derive(Clone, Debug)]
    struct Item<A> {
        opt: Option<A>
    }

    impl<A> Iterator for Item<A> {
        type Item = A;

        #[inline]
        fn next(&mut self) -> Option<A> {
            self.opt.take()
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            match self.opt {
                Some(_) => (1, Some(1)),
                None => (0, Some(0)),
            }
        }
    }

    impl<A> DoubleEndedIterator for Item<A> {
        #[inline]
        fn next_back(&mut self) -> Option<A> {
            self.opt.take()
        }
    }

    impl<A> ExactSizeIterator for Item<A> {}
    impl<A> FusedIterator for Item<A> {}
    unsafe impl<A> TrustedLen for Item<A> {}

    /// An iterator over a reference to the [`Some`] variant of an [`Option`].
    ///
    /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
    ///
    /// This `struct` is created by the [`Option::iter`] function.
    ///
    /// [`Option`]: enum.Option.html
    /// [`Some`]: enum.Option.html#variant.Some
    /// [`Option::iter`]: enum.Option.html#method.iter
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Debug)]
    pub struct Iter<'a, A: 'a> { inner: Item<&'a A> }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, A> Iterator for Iter<'a, A> {
        type Item = &'a A;

        #[inline]
        fn next(&mut self) -> Option<&'a A> { self.inner.next() }
        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
        #[inline]
        fn next_back(&mut self) -> Option<&'a A> { self.inner.next_back() }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A> ExactSizeIterator for Iter<'_, A> {}

    #[stable(feature = "fused", since = "1.26.0")]
    impl<A> FusedIterator for Iter<'_, A> {}

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<A> TrustedLen for Iter<'_, A> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A> Clone for Iter<'_, A> {
        #[inline]
        fn clone(&self) -> Self {
            Iter { inner: self.inner.clone() }
        }
    }

    /// An iterator over a mutable reference to the [`Some`] variant of an [`Option`].
    ///
    /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
    ///
    /// This `struct` is created by the [`Option::iter_mut`] function.
    ///
    /// [`Option`]: enum.Option.html
    /// [`Some`]: enum.Option.html#variant.Some
    /// [`Option::iter_mut`]: enum.Option.html#method.iter_mut
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Debug)]
    pub struct IterMut<'a, A: 'a> { inner: Item<&'a mut A> }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, A> Iterator for IterMut<'a, A> {
        type Item = &'a mut A;

        #[inline]
        fn next(&mut self) -> Option<&'a mut A> { self.inner.next() }
        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
        #[inline]
        fn next_back(&mut self) -> Option<&'a mut A> { self.inner.next_back() }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A> ExactSizeIterator for IterMut<'_, A> {}

    #[stable(feature = "fused", since = "1.26.0")]
    impl<A> FusedIterator for IterMut<'_, A> {}
    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<A> TrustedLen for IterMut<'_, A> {}

    /// An iterator over the value in [`Some`] variant of an [`Option`].
    ///
    /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
    ///
    /// This `struct` is created by the [`Option::into_iter`] function.
    ///
    /// [`Option`]: enum.Option.html
    /// [`Some`]: enum.Option.html#variant.Some
    /// [`Option::into_iter`]: enum.Option.html#method.into_iter
    #[derive(Clone, Debug)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct IntoIter<A> { inner: Item<A> }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A> Iterator for IntoIter<A> {
        type Item = A;

        #[inline]
        fn next(&mut self) -> Option<A> { self.inner.next() }
        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A> DoubleEndedIterator for IntoIter<A> {
        #[inline]
        fn next_back(&mut self) -> Option<A> { self.inner.next_back() }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A> ExactSizeIterator for IntoIter<A> {}

    #[stable(feature = "fused", since = "1.26.0")]
    impl<A> FusedIterator for IntoIter<A> {}

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<A> TrustedLen for IntoIter<A> {}

    /////////////////////////////////////////////////////////////////////////////
    // FromIterator
    /////////////////////////////////////////////////////////////////////////////

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
        /// Takes each element in the [`Iterator`]: if it is [`None`][Option::None],
        /// no further elements are taken, and the [`None`][Option::None] is
        /// returned. Should no [`None`][Option::None] occur, a container with the
        /// values of each [`Option`] is returned.
        ///
        /// # Examples
        ///
        /// Here is an example which increments every integer in a vector.
        /// `We use the checked variant of `add` that returns `None` when the
        /// calculation would result in an overflow.
        ///
        /// ```
        /// let items = vec![0_u16, 1, 2];
        ///
        /// let res: Option<Vec<u16>> = items
        ///     .iter()
        ///     .map(|x| x.checked_add(1))
        ///     .collect();
        ///
        /// assert_eq!(res, Some(vec![1, 2, 3]));
        /// ```
        ///
        /// As you can see, this will return the expected, valid items.
        ///
        /// Here is another example that tries to subtract one from another list
        /// of integers, this time checking for underflow:
        ///
        /// ```
        /// let items = vec![2_u16, 1, 0];
        ///
        /// let res: Option<Vec<u16>> = items
        ///     .iter()
        ///     .map(|x| x.checked_sub(1))
        ///     .collect();
        ///
        /// assert_eq!(res, None);
        /// ```
        ///
        /// Since the last element is zero, it would underflow. Thus, the resulting
        /// value is `None`.
        ///
        /// [`Iterator`]: ../iter/trait.Iterator.html
        #[inline]
        fn from_iter<I: IntoIterator<Item=Option<A>>>(iter: I) -> Option<V> {
            // FIXME(#11084): This could be replaced with Iterator::scan when this
            // performance bug is closed.

            struct Adapter<Iter> {
                iter: Iter,
                found_none: bool,
            }

            impl<T, Iter: Iterator<Item=Option<T>>> Iterator for Adapter<Iter> {
                type Item = T;

                #[inline]
                fn next(&mut self) -> Option<T> {
                    match self.iter.next() {
                        Some(Some(value)) => Some(value),
                        Some(None) => {
                            self.found_none = true;
                            None
                        }
                        None => None,
                    }
                }

                #[inline]
                fn size_hint(&self) -> (usize, Option<usize>) {
                    if self.found_none {
                        (0, Some(0))
                    } else {
                        let (_, upper) = self.iter.size_hint();
                        (0, upper)
                    }
                }
            }

            let mut adapter = Adapter { iter: iter.into_iter(), found_none: false };
            let v: V = FromIterator::from_iter(adapter.by_ref());

            if adapter.found_none {
                None
            } else {
                Some(v)
            }
        }
    }

    /// The error type that results from applying the try operator (`?`) to a `None` value. If you wish
    /// to allow `x?` (where `x` is an `Option<T>`) to be converted into your error type, you can
    /// implement `impl From<NoneError>` for `YourErrorType`. In that case, `x?` within a function that
    /// returns `Result<_, YourErrorType>` will translate a `None` value into an `Err` result.
    #[unstable(feature = "try_trait", issue = "42327")]
    #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
    pub struct NoneError;

    #[unstable(feature = "try_trait", issue = "42327")]
    impl<T> ops::Try for Option<T> {
        type Ok = T;
        type Error = NoneError;

        #[inline]
        fn into_result(self) -> Result<T, NoneError> {
            self.ok_or(NoneError)
        }

        #[inline]
        fn from_ok(v: T) -> Self {
            Some(v)
        }

        #[inline]
        fn from_error(_: NoneError) -> Self {
            None
        }
    }

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