Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 2356 insertions, 0 deletions

diff --git a/library/core/src/option.rs b/library/core/src/option.rs
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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" references. Instead, Rust has *optional* pointers, like
+//! the optional owned box, <code>[Option]<[Box\<T>]></code>.
+//!
+//! [Box\<T>]: ../../std/boxed/struct.Box.html
+//!
+//! The following example uses [`Option`] to create an optional box of
+//! [`i32`]. Notice that in order to use the inner [`i32`] value, the
+//! `check_optional` function first 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(p) => println!("has value {p}"),
+//!         None => println!("has no value"),
+//!     }
+//! }
+//! ```
+//!
+//! # Representation
+//!
+//! Rust guarantees to optimize the following types `T` such that
+//! [`Option<T>`] has the same size as `T`:
+//!
+//! * [`Box<U>`]
+//! * `&U`
+//! * `&mut U`
+//! * `fn`, `extern "C" fn`[^extern_fn]
+//! * [`num::NonZero*`]
+//! * [`ptr::NonNull<U>`]
+//! * `#[repr(transparent)]` struct around one of the types in this list.
+//!
+//! [^extern_fn]: this remains true for any other ABI: `extern "abi" fn` (_e.g._, `extern "system" fn`)
+//!
+//! [`Box<U>`]: ../../std/boxed/struct.Box.html
+//! [`num::NonZero*`]: crate::num
+//! [`ptr::NonNull<U>`]: crate::ptr::NonNull
+//!
+//! This is called the "null pointer optimization" or NPO.
+//!
+//! It is further guaranteed that, for the cases above, one can
+//! [`mem::transmute`] from all valid values of `T` to `Option<T>` and
+//! from `Some::<T>(_)` to `T` (but transmuting `None::<T>` to `T`
+//! is undefined behaviour).
+//!
+//! # Method overview
+//!
+//! In addition to working with pattern matching, [`Option`] provides a wide
+//! variety of different methods.
+//!
+//! ## Querying the variant
+//!
+//! The [`is_some`] and [`is_none`] methods return [`true`] if the [`Option`]
+//! is [`Some`] or [`None`], respectively.
+//!
+//! [`is_none`]: Option::is_none
+//! [`is_some`]: Option::is_some
+//!
+//! ## Adapters for working with references
+//!
+//! * [`as_ref`] converts from <code>[&][][Option]\<T></code> to <code>[Option]<[&]T></code>
+//! * [`as_mut`] converts from <code>[&mut] [Option]\<T></code> to <code>[Option]<[&mut] T></code>
+//! * [`as_deref`] converts from <code>[&][][Option]\<T></code> to
+//!   <code>[Option]<[&]T::[Target]></code>
+//! * [`as_deref_mut`] converts from <code>[&mut] [Option]\<T></code> to
+//!   <code>[Option]<[&mut] T::[Target]></code>
+//! * [`as_pin_ref`] converts from <code>[Pin]<[&][][Option]\<T>></code> to
+//!   <code>[Option]<[Pin]<[&]T>></code>
+//! * [`as_pin_mut`] converts from <code>[Pin]<[&mut] [Option]\<T>></code> to
+//!   <code>[Option]<[Pin]<[&mut] T>></code>
+//!
+//! [&]: reference "shared reference"
+//! [&mut]: reference "mutable reference"
+//! [Target]: Deref::Target "ops::Deref::Target"
+//! [`as_deref`]: Option::as_deref
+//! [`as_deref_mut`]: Option::as_deref_mut
+//! [`as_mut`]: Option::as_mut
+//! [`as_pin_mut`]: Option::as_pin_mut
+//! [`as_pin_ref`]: Option::as_pin_ref
+//! [`as_ref`]: Option::as_ref
+//!
+//! ## Extracting the contained value
+//!
+//! These methods extract the contained value in an [`Option<T>`] when it
+//! is the [`Some`] variant. If the [`Option`] is [`None`]:
+//!
+//! * [`expect`] panics with a provided custom message
+//! * [`unwrap`] panics with a generic message
+//! * [`unwrap_or`] returns the provided default value
+//! * [`unwrap_or_default`] returns the default value of the type `T`
+//!   (which must implement the [`Default`] trait)
+//! * [`unwrap_or_else`] returns the result of evaluating the provided
+//!   function
+//!
+//! [`expect`]: Option::expect
+//! [`unwrap`]: Option::unwrap
+//! [`unwrap_or`]: Option::unwrap_or
+//! [`unwrap_or_default`]: Option::unwrap_or_default
+//! [`unwrap_or_else`]: Option::unwrap_or_else
+//!
+//! ## Transforming contained values
+//!
+//! These methods transform [`Option`] to [`Result`]:
+//!
+//! * [`ok_or`] transforms [`Some(v)`] to [`Ok(v)`], and [`None`] to
+//!   [`Err(err)`] using the provided default `err` value
+//! * [`ok_or_else`] transforms [`Some(v)`] to [`Ok(v)`], and [`None`] to
+//!   a value of [`Err`] using the provided function
+//! * [`transpose`] transposes an [`Option`] of a [`Result`] into a
+//!   [`Result`] of an [`Option`]
+//!
+//! [`Err(err)`]: Err
+//! [`Ok(v)`]: Ok
+//! [`Some(v)`]: Some
+//! [`ok_or`]: Option::ok_or
+//! [`ok_or_else`]: Option::ok_or_else
+//! [`transpose`]: Option::transpose
+//!
+//! These methods transform the [`Some`] variant:
+//!
+//! * [`filter`] calls the provided predicate function on the contained
+//!   value `t` if the [`Option`] is [`Some(t)`], and returns [`Some(t)`]
+//!   if the function returns `true`; otherwise, returns [`None`]
+//! * [`flatten`] removes one level of nesting from an
+//!   [`Option<Option<T>>`]
+//! * [`map`] transforms [`Option<T>`] to [`Option<U>`] by applying the
+//!   provided function to the contained value of [`Some`] and leaving
+//!   [`None`] values unchanged
+//!
+//! [`Some(t)`]: Some
+//! [`filter`]: Option::filter
+//! [`flatten`]: Option::flatten
+//! [`map`]: Option::map
+//!
+//! These methods transform [`Option<T>`] to a value of a possibly
+//! different type `U`:
+//!
+//! * [`map_or`] applies the provided function to the contained value of
+//!   [`Some`], or returns the provided default value if the [`Option`] is
+//!   [`None`]
+//! * [`map_or_else`] applies the provided function to the contained value
+//!   of [`Some`], or returns the result of evaluating the provided
+//!   fallback function if the [`Option`] is [`None`]
+//!
+//! [`map_or`]: Option::map_or
+//! [`map_or_else`]: Option::map_or_else
+//!
+//! These methods combine the [`Some`] variants of two [`Option`] values:
+//!
+//! * [`zip`] returns [`Some((s, o))`] if `self` is [`Some(s)`] and the
+//!   provided [`Option`] value is [`Some(o)`]; otherwise, returns [`None`]
+//! * [`zip_with`] calls the provided function `f` and returns
+//!   [`Some(f(s, o))`] if `self` is [`Some(s)`] and the provided
+//!   [`Option`] value is [`Some(o)`]; otherwise, returns [`None`]
+//!
+//! [`Some(f(s, o))`]: Some
+//! [`Some(o)`]: Some
+//! [`Some(s)`]: Some
+//! [`Some((s, o))`]: Some
+//! [`zip`]: Option::zip
+//! [`zip_with`]: Option::zip_with
+//!
+//! ## Boolean operators
+//!
+//! These methods treat the [`Option`] as a boolean value, where [`Some`]
+//! acts like [`true`] and [`None`] acts like [`false`]. There are two
+//! categories of these methods: ones that take an [`Option`] as input, and
+//! ones that take a function as input (to be lazily evaluated).
+//!
+//! The [`and`], [`or`], and [`xor`] methods take another [`Option`] as
+//! input, and produce an [`Option`] as output. Only the [`and`] method can
+//! produce an [`Option<U>`] value having a different inner type `U` than
+//! [`Option<T>`].
+//!
+//! | method  | self      | input     | output    |
+//! |---------|-----------|-----------|-----------|
+//! | [`and`] | `None`    | (ignored) | `None`    |
+//! | [`and`] | `Some(x)` | `None`    | `None`    |
+//! | [`and`] | `Some(x)` | `Some(y)` | `Some(y)` |
+//! | [`or`]  | `None`    | `None`    | `None`    |
+//! | [`or`]  | `None`    | `Some(y)` | `Some(y)` |
+//! | [`or`]  | `Some(x)` | (ignored) | `Some(x)` |
+//! | [`xor`] | `None`    | `None`    | `None`    |
+//! | [`xor`] | `None`    | `Some(y)` | `Some(y)` |
+//! | [`xor`] | `Some(x)` | `None`    | `Some(x)` |
+//! | [`xor`] | `Some(x)` | `Some(y)` | `None`    |
+//!
+//! [`and`]: Option::and
+//! [`or`]: Option::or
+//! [`xor`]: Option::xor
+//!
+//! The [`and_then`] and [`or_else`] methods take a function as input, and
+//! only evaluate the function when they need to produce a new value. Only
+//! the [`and_then`] method can produce an [`Option<U>`] value having a
+//! different inner type `U` than [`Option<T>`].
+//!
+//! | method       | self      | function input | function result | output    |
+//! |--------------|-----------|----------------|-----------------|-----------|
+//! | [`and_then`] | `None`    | (not provided) | (not evaluated) | `None`    |
+//! | [`and_then`] | `Some(x)` | `x`            | `None`          | `None`    |
+//! | [`and_then`] | `Some(x)` | `x`            | `Some(y)`       | `Some(y)` |
+//! | [`or_else`]  | `None`    | (not provided) | `None`          | `None`    |
+//! | [`or_else`]  | `None`    | (not provided) | `Some(y)`       | `Some(y)` |
+//! | [`or_else`]  | `Some(x)` | (not provided) | (not evaluated) | `Some(x)` |
+//!
+//! [`and_then`]: Option::and_then
+//! [`or_else`]: Option::or_else
+//!
+//! This is an example of using methods like [`and_then`] and [`or`] in a
+//! pipeline of method calls. Early stages of the pipeline pass failure
+//! values ([`None`]) through unchanged, and continue processing on
+//! success values ([`Some`]). Toward the end, [`or`] substitutes an error
+//! message if it receives [`None`].
+//!
+//! ```
+//! # use std::collections::BTreeMap;
+//! let mut bt = BTreeMap::new();
+//! bt.insert(20u8, "foo");
+//! bt.insert(42u8, "bar");
+//! let res = [0u8, 1, 11, 200, 22]
+//!     .into_iter()
+//!     .map(|x| {
+//!         // `checked_sub()` returns `None` on error
+//!         x.checked_sub(1)
+//!             // same with `checked_mul()`
+//!             .and_then(|x| x.checked_mul(2))
+//!             // `BTreeMap::get` returns `None` on error
+//!             .and_then(|x| bt.get(&x))
+//!             // Substitute an error message if we have `None` so far
+//!             .or(Some(&"error!"))
+//!             .copied()
+//!             // Won't panic because we unconditionally used `Some` above
+//!             .unwrap()
+//!     })
+//!     .collect::<Vec<_>>();
+//! assert_eq!(res, ["error!", "error!", "foo", "error!", "bar"]);
+//! ```
+//!
+//! ## Comparison operators
+//!
+//! If `T` implements [`PartialOrd`] then [`Option<T>`] will derive its
+//! [`PartialOrd`] implementation.  With this order, [`None`] compares as
+//! less than any [`Some`], and two [`Some`] compare the same way as their
+//! contained values would in `T`.  If `T` also implements
+//! [`Ord`], then so does [`Option<T>`].
+//!
+//! ```
+//! assert!(None < Some(0));
+//! assert!(Some(0) < Some(1));
+//! ```
+//!
+//! ## Iterating over `Option`
+//!
+//! An [`Option`] can be iterated over. This can be helpful if you need an
+//! iterator that is conditionally empty. The iterator will either produce
+//! a single value (when the [`Option`] is [`Some`]), or produce no values
+//! (when the [`Option`] is [`None`]). For example, [`into_iter`] acts like
+//! [`once(v)`] if the [`Option`] is [`Some(v)`], and like [`empty()`] if
+//! the [`Option`] is [`None`].
+//!
+//! [`Some(v)`]: Some
+//! [`empty()`]: crate::iter::empty
+//! [`once(v)`]: crate::iter::once
+//!
+//! Iterators over [`Option<T>`] come in three types:
+//!
+//! * [`into_iter`] consumes the [`Option`] and produces the contained
+//!   value
+//! * [`iter`] produces an immutable reference of type `&T` to the
+//!   contained value
+//! * [`iter_mut`] produces a mutable reference of type `&mut T` to the
+//!   contained value
+//!
+//! [`into_iter`]: Option::into_iter
+//! [`iter`]: Option::iter
+//! [`iter_mut`]: Option::iter_mut
+//!
+//! An iterator over [`Option`] can be useful when chaining iterators, for
+//! example, to conditionally insert items. (It's not always necessary to
+//! explicitly call an iterator constructor: many [`Iterator`] methods that
+//! accept other iterators will also accept iterable types that implement
+//! [`IntoIterator`], which includes [`Option`].)
+//!
+//! ```
+//! let yep = Some(42);
+//! let nope = None;
+//! // chain() already calls into_iter(), so we don't have to do so
+//! let nums: Vec<i32> = (0..4).chain(yep).chain(4..8).collect();
+//! assert_eq!(nums, [0, 1, 2, 3, 42, 4, 5, 6, 7]);
+//! let nums: Vec<i32> = (0..4).chain(nope).chain(4..8).collect();
+//! assert_eq!(nums, [0, 1, 2, 3, 4, 5, 6, 7]);
+//! ```
+//!
+//! One reason to chain iterators in this way is that a function returning
+//! `impl Iterator` must have all possible return values be of the same
+//! concrete type. Chaining an iterated [`Option`] can help with that.
+//!
+//! ```
+//! fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> {
+//!     // Explicit returns to illustrate return types matching
+//!     match do_insert {
+//!         true => return (0..4).chain(Some(42)).chain(4..8),
+//!         false => return (0..4).chain(None).chain(4..8),
+//!     }
+//! }
+//! println!("{:?}", make_iter(true).collect::<Vec<_>>());
+//! println!("{:?}", make_iter(false).collect::<Vec<_>>());
+//! ```
+//!
+//! If we try to do the same thing, but using [`once()`] and [`empty()`],
+//! we can't return `impl Iterator` anymore because the concrete types of
+//! the return values differ.
+//!
+//! [`empty()`]: crate::iter::empty
+//! [`once()`]: crate::iter::once
+//!
+//! ```compile_fail,E0308
+//! # use std::iter::{empty, once};
+//! // This won't compile because all possible returns from the function
+//! // must have the same concrete type.
+//! fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> {
+//!     // Explicit returns to illustrate return types not matching
+//!     match do_insert {
+//!         true => return (0..4).chain(once(42)).chain(4..8),
+//!         false => return (0..4).chain(empty()).chain(4..8),
+//!     }
+//! }
+//! ```
+//!
+//! ## Collecting into `Option`
+//!
+//! [`Option`] implements the [`FromIterator`][impl-FromIterator] trait,
+//! which allows an iterator over [`Option`] values to be collected into an
+//! [`Option`] of a collection of each contained value of the original
+//! [`Option`] values, or [`None`] if any of the elements was [`None`].
+//!
+//! [impl-FromIterator]: Option#impl-FromIterator%3COption%3CA%3E%3E-for-Option%3CV%3E
+//!
+//! ```
+//! let v = [Some(2), Some(4), None, Some(8)];
+//! let res: Option<Vec<_>> = v.into_iter().collect();
+//! assert_eq!(res, None);
+//! let v = [Some(2), Some(4), Some(8)];
+//! let res: Option<Vec<_>> = v.into_iter().collect();
+//! assert_eq!(res, Some(vec![2, 4, 8]));
+//! ```
+//!
+//! [`Option`] also implements the [`Product`][impl-Product] and
+//! [`Sum`][impl-Sum] traits, allowing an iterator over [`Option`] values
+//! to provide the [`product`][Iterator::product] and
+//! [`sum`][Iterator::sum] methods.
+//!
+//! [impl-Product]: Option#impl-Product%3COption%3CU%3E%3E-for-Option%3CT%3E
+//! [impl-Sum]: Option#impl-Sum%3COption%3CU%3E%3E-for-Option%3CT%3E
+//!
+//! ```
+//! let v = [None, Some(1), Some(2), Some(3)];
+//! let res: Option<i32> = v.into_iter().sum();
+//! assert_eq!(res, None);
+//! let v = [Some(1), Some(2), Some(21)];
+//! let res: Option<i32> = v.into_iter().product();
+//! assert_eq!(res, Some(42));
+//! ```
+//!
+//! ## Modifying an [`Option`] in-place
+//!
+//! These methods return a mutable reference to the contained value of an
+//! [`Option<T>`]:
+//!
+//! * [`insert`] inserts a value, dropping any old contents
+//! * [`get_or_insert`] gets the current value, inserting a provided
+//!   default value if it is [`None`]
+//! * [`get_or_insert_default`] gets the current value, inserting the
+//!   default value of type `T` (which must implement [`Default`]) if it is
+//!   [`None`]
+//! * [`get_or_insert_with`] gets the current value, inserting a default
+//!   computed by the provided function if it is [`None`]
+//!
+//! [`get_or_insert`]: Option::get_or_insert
+//! [`get_or_insert_default`]: Option::get_or_insert_default
+//! [`get_or_insert_with`]: Option::get_or_insert_with
+//! [`insert`]: Option::insert
+//!
+//! These methods transfer ownership of the contained value of an
+//! [`Option`]:
+//!
+//! * [`take`] takes ownership of the contained value of an [`Option`], if
+//!   any, replacing the [`Option`] with [`None`]
+//! * [`replace`] takes ownership of the contained value of an [`Option`],
+//!   if any, replacing the [`Option`] with a [`Some`] containing the
+//!   provided value
+//!
+//! [`replace`]: Option::replace
+//! [`take`]: Option::take
+//!
+//! # Examples
+//!
+//! Basic pattern matching on [`Option`]:
+//!
+//! ```
+//! let msg = Some("howdy");
+//!
+//! // Take a reference to the contained string
+//! if let Some(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 :("),
+//! }
+//! ```
+
+#![stable(feature = "rust1", since = "1.0.0")]
+
+use crate::iter::{self, FromIterator, FusedIterator, TrustedLen};
+use crate::marker::Destruct;
+use crate::panicking::{panic, panic_str};
+use crate::pin::Pin;
+use crate::{
+    convert, hint, mem,
+    ops::{self, ControlFlow, Deref, DerefMut},
+};
+
+/// The `Option` type. See [the module level documentation](self) for more.
+#[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
+#[rustc_diagnostic_item = "Option"]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub enum Option<T> {
+    /// No value.
+    #[lang = "None"]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    None,
+    /// Some value of type `T`.
+    #[lang = "Some"]
+    #[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);
+    /// ```
+    #[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"]
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
+    pub const fn is_some(&self) -> bool {
+        matches!(*self, Some(_))
+    }
+
+    /// Returns `true` if the option is a [`Some`] and the value inside of it matches a predicate.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(is_some_with)]
+    ///
+    /// let x: Option<u32> = Some(2);
+    /// assert_eq!(x.is_some_and(|&x| x > 1), true);
+    ///
+    /// let x: Option<u32> = Some(0);
+    /// assert_eq!(x.is_some_and(|&x| x > 1), false);
+    ///
+    /// let x: Option<u32> = None;
+    /// assert_eq!(x.is_some_and(|&x| x > 1), false);
+    /// ```
+    #[must_use]
+    #[inline]
+    #[unstable(feature = "is_some_with", issue = "93050")]
+    pub fn is_some_and(&self, f: impl FnOnce(&T) -> bool) -> bool {
+        matches!(self, Some(x) if f(x))
+    }
+
+    /// 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);
+    /// ```
+    #[must_use = "if you intended to assert that this doesn't have a value, consider \
+                  `.and_then(|_| panic!(\"`Option` had a value when expected `None`\"))` instead"]
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
+    pub const fn is_none(&self) -> bool {
+        !self.is_some()
+    }
+
+    /////////////////////////////////////////////////////////////////////////
+    // Adapter for working with references
+    /////////////////////////////////////////////////////////////////////////
+
+    /// Converts from `&Option<T>` to `Option<&T>`.
+    ///
+    /// # Examples
+    ///
+    /// Converts an <code>Option<[String]></code> into an <code>Option<[usize]></code>, 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`]: Option::map
+    /// [String]: ../../std/string/struct.String.html "String"
+    ///
+    /// ```
+    /// 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]
+    #[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub const fn as_ref(&self) -> Option<&T> {
+        match *self {
+            Some(ref x) => Some(x),
+            None => None,
+        }
+    }
+
+    /// Converts from `&mut 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")]
+    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
+    pub const fn as_mut(&mut self) -> Option<&mut T> {
+        match *self {
+            Some(ref mut x) => Some(x),
+            None => None,
+        }
+    }
+
+    /// Converts from <code>[Pin]<[&]Option\<T>></code> to <code>Option<[Pin]<[&]T>></code>.
+    ///
+    /// [&]: reference "shared reference"
+    #[inline]
+    #[must_use]
+    #[stable(feature = "pin", since = "1.33.0")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> {
+        match Pin::get_ref(self).as_ref() {
+            // SAFETY: `x` is guaranteed to be pinned because it comes from `self`
+            // which is pinned.
+            Some(x) => unsafe { Some(Pin::new_unchecked(x)) },
+            None => None,
+        }
+    }
+
+    /// Converts from <code>[Pin]<[&mut] Option\<T>></code> to <code>Option<[Pin]<[&mut] T>></code>.
+    ///
+    /// [&mut]: reference "mutable reference"
+    #[inline]
+    #[must_use]
+    #[stable(feature = "pin", since = "1.33.0")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> {
+        // SAFETY: `get_unchecked_mut` is never used to move the `Option` inside `self`.
+        // `x` is guaranteed to be pinned because it comes from `self` which is pinned.
+        unsafe {
+            match Pin::get_unchecked_mut(self).as_mut() {
+                Some(x) => Some(Pin::new_unchecked(x)),
+                None => None,
+            }
+        }
+    }
+
+    /////////////////////////////////////////////////////////////////////////
+    // Getting to contained values
+    /////////////////////////////////////////////////////////////////////////
+
+    /// Returns the contained [`Some`] value, consuming the `self` value.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the value is a [`None`] with a custom panic message provided by
+    /// `msg`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let x = Some("value");
+    /// assert_eq!(x.expect("fruits are healthy"), "value");
+    /// ```
+    ///
+    /// ```should_panic
+    /// let x: Option<&str> = None;
+    /// x.expect("fruits are healthy"); // panics with `fruits are healthy`
+    /// ```
+    ///
+    /// # Recommended Message Style
+    ///
+    /// We recommend that `expect` messages are used to describe the reason you
+    /// _expect_ the `Option` should be `Some`.
+    ///
+    /// ```should_panic
+    /// # let slice: &[u8] = &[];
+    /// let item = slice.get(0)
+    ///     .expect("slice should not be empty");
+    /// ```
+    ///
+    /// **Hint**: If you're having trouble remembering how to phrase expect
+    /// error messages remember to focus on the word "should" as in "env
+    /// variable should be set by blah" or "the given binary should be available
+    /// and executable by the current user".
+    ///
+    /// For more detail on expect message styles and the reasoning behind our
+    /// recommendation please refer to the section on ["Common Message
+    /// Styles"](../../std/error/index.html#common-message-styles) in the [`std::error`](../../std/error/index.html) module docs.
+    #[inline]
+    #[track_caller]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
+    pub const fn expect(self, msg: &str) -> T {
+        match self {
+            Some(val) => val,
+            None => expect_failed(msg),
+        }
+    }
+
+    /// Returns the contained [`Some`] value, consuming the `self` value.
+    ///
+    /// Because this function may panic, its use is generally discouraged.
+    /// Instead, prefer to use pattern matching and handle the [`None`]
+    /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
+    /// [`unwrap_or_default`].
+    ///
+    /// [`unwrap_or`]: Option::unwrap_or
+    /// [`unwrap_or_else`]: Option::unwrap_or_else
+    /// [`unwrap_or_default`]: Option::unwrap_or_default
+    ///
+    /// # Panics
+    ///
+    /// Panics if the self value equals [`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]
+    #[track_caller]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
+    pub const fn unwrap(self) -> T {
+        match self {
+            Some(val) => val,
+            None => panic("called `Option::unwrap()` on a `None` value"),
+        }
+    }
+
+    /// Returns the contained [`Some`] value or a provided 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`]: Option::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")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn unwrap_or(self, default: T) -> T
+    where
+        T: ~const Destruct,
+    {
+        match self {
+            Some(x) => x,
+            None => default,
+        }
+    }
+
+    /// Returns the contained [`Some`] 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")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn unwrap_or_else<F>(self, f: F) -> T
+    where
+        F: ~const FnOnce() -> T,
+        F: ~const Destruct,
+    {
+        match self {
+            Some(x) => x,
+            None => f(),
+        }
+    }
+
+    /// Returns the contained [`Some`] 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
+    ///
+    /// Converts 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);
+    /// ```
+    ///
+    /// [default value]: Default::default
+    /// [`parse`]: str::parse
+    /// [`FromStr`]: crate::str::FromStr
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn unwrap_or_default(self) -> T
+    where
+        T: ~const Default,
+    {
+        match self {
+            Some(x) => x,
+            None => Default::default(),
+        }
+    }
+
+    /// Returns the contained [`Some`] value, consuming the `self` value,
+    /// without checking that the value is not [`None`].
+    ///
+    /// # Safety
+    ///
+    /// Calling this method on [`None`] is *[undefined behavior]*.
+    ///
+    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let x = Some("air");
+    /// assert_eq!(unsafe { x.unwrap_unchecked() }, "air");
+    /// ```
+    ///
+    /// ```no_run
+    /// let x: Option<&str> = None;
+    /// assert_eq!(unsafe { x.unwrap_unchecked() }, "air"); // Undefined behavior!
+    /// ```
+    #[inline]
+    #[track_caller]
+    #[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const unsafe fn unwrap_unchecked(self) -> T {
+        debug_assert!(self.is_some());
+        match self {
+            Some(val) => val,
+            // SAFETY: the safety contract must be upheld by the caller.
+            None => unsafe { hint::unreachable_unchecked() },
+        }
+    }
+
+    /////////////////////////////////////////////////////////////////////////
+    // Transforming contained values
+    /////////////////////////////////////////////////////////////////////////
+
+    /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value.
+    ///
+    /// # Examples
+    ///
+    /// Converts an <code>Option<[String]></code> into an <code>Option<[usize]></code>, consuming
+    /// the original:
+    ///
+    /// [String]: ../../std/string/struct.String.html "String"
+    /// ```
+    /// 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")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn map<U, F>(self, f: F) -> Option<U>
+    where
+        F: ~const FnOnce(T) -> U,
+        F: ~const Destruct,
+    {
+        match self {
+            Some(x) => Some(f(x)),
+            None => None,
+        }
+    }
+
+    /// Calls the provided closure with a reference to the contained value (if [`Some`]).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(result_option_inspect)]
+    ///
+    /// let v = vec![1, 2, 3, 4, 5];
+    ///
+    /// // prints "got: 4"
+    /// let x: Option<&usize> = v.get(3).inspect(|x| println!("got: {x}"));
+    ///
+    /// // prints nothing
+    /// let x: Option<&usize> = v.get(5).inspect(|x| println!("got: {x}"));
+    /// ```
+    #[inline]
+    #[unstable(feature = "result_option_inspect", issue = "91345")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn inspect<F>(self, f: F) -> Self
+    where
+        F: ~const FnOnce(&T),
+        F: ~const Destruct,
+    {
+        if let Some(ref x) = self {
+            f(x);
+        }
+
+        self
+    }
+
+    /// Returns the provided default result (if none),
+    /// or applies a function to the contained value (if any).
+    ///
+    /// Arguments passed to `map_or` are eagerly evaluated; if you are passing
+    /// the result of a function call, it is recommended to use [`map_or_else`],
+    /// which is lazily evaluated.
+    ///
+    /// [`map_or_else`]: Option::map_or_else
+    ///
+    /// # 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")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn map_or<U, F>(self, default: U, f: F) -> U
+    where
+        F: ~const FnOnce(T) -> U,
+        F: ~const Destruct,
+        U: ~const Destruct,
+    {
+        match self {
+            Some(t) => f(t),
+            None => default,
+        }
+    }
+
+    /// Computes a default function result (if none), or
+    /// applies a different function to the contained value (if any).
+    ///
+    /// # 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")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn map_or_else<U, D, F>(self, default: D, f: F) -> U
+    where
+        D: ~const FnOnce() -> U,
+        D: ~const Destruct,
+        F: ~const FnOnce(T) -> U,
+        F: ~const Destruct,
+    {
+        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.
+    ///
+    /// [`Ok(v)`]: Ok
+    /// [`Err(err)`]: Err
+    /// [`Some(v)`]: Some
+    /// [`ok_or_else`]: Option::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")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn ok_or<E>(self, err: E) -> Result<T, E>
+    where
+        E: ~const Destruct,
+    {
+        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())`].
+    ///
+    /// [`Ok(v)`]: Ok
+    /// [`Err(err())`]: Err
+    /// [`Some(v)`]: 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")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn ok_or_else<E, F>(self, err: F) -> Result<T, E>
+    where
+        F: ~const FnOnce() -> E,
+        F: ~const Destruct,
+    {
+        match self {
+            Some(v) => Ok(v),
+            None => Err(err()),
+        }
+    }
+
+    /// Converts from `Option<T>` (or `&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`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let x: Option<String> = Some("hey".to_owned());
+    /// assert_eq!(x.as_deref(), Some("hey"));
+    ///
+    /// let x: Option<String> = None;
+    /// assert_eq!(x.as_deref(), None);
+    /// ```
+    #[stable(feature = "option_deref", since = "1.40.0")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn as_deref(&self) -> Option<&T::Target>
+    where
+        T: ~const Deref,
+    {
+        match self.as_ref() {
+            Some(t) => Some(t.deref()),
+            None => None,
+        }
+    }
+
+    /// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`.
+    ///
+    /// Leaves the original `Option` in-place, creating a new one containing a mutable reference to
+    /// the inner type's [`Deref::Target`] type.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut x: Option<String> = Some("hey".to_owned());
+    /// assert_eq!(x.as_deref_mut().map(|x| {
+    ///     x.make_ascii_uppercase();
+    ///     x
+    /// }), Some("HEY".to_owned().as_mut_str()));
+    /// ```
+    #[stable(feature = "option_deref", since = "1.40.0")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn as_deref_mut(&mut self) -> Option<&mut T::Target>
+    where
+        T: ~const DerefMut,
+    {
+        match self.as_mut() {
+            Some(t) => Some(t.deref_mut()),
+            None => None,
+        }
+    }
+
+    /////////////////////////////////////////////////////////////////////////
+    // 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]
+    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub const 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`.
+    ///
+    /// # 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")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn and<U>(self, optb: Option<U>) -> Option<U>
+    where
+        T: ~const Destruct,
+        U: ~const Destruct,
+    {
+        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.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// fn sq_then_to_string(x: u32) -> Option<String> {
+    ///     x.checked_mul(x).map(|sq| sq.to_string())
+    /// }
+    ///
+    /// assert_eq!(Some(2).and_then(sq_then_to_string), Some(4.to_string()));
+    /// assert_eq!(Some(1_000_000).and_then(sq_then_to_string), None); // overflowed!
+    /// assert_eq!(None.and_then(sq_then_to_string), None);
+    /// ```
+    ///
+    /// Often used to chain fallible operations that may return [`None`].
+    ///
+    /// ```
+    /// let arr_2d = [["A0", "A1"], ["B0", "B1"]];
+    ///
+    /// let item_0_1 = arr_2d.get(0).and_then(|row| row.get(1));
+    /// assert_eq!(item_0_1, Some(&"A1"));
+    ///
+    /// let item_2_0 = arr_2d.get(2).and_then(|row| row.get(0));
+    /// assert_eq!(item_2_0, None);
+    /// ```
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn and_then<U, F>(self, f: F) -> Option<U>
+    where
+        F: ~const FnOnce(T) -> Option<U>,
+        F: ~const Destruct,
+    {
+        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));
+    /// ```
+    ///
+    /// [`Some(t)`]: Some
+    #[inline]
+    #[stable(feature = "option_filter", since = "1.27.0")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn filter<P>(self, predicate: P) -> Self
+    where
+        T: ~const Destruct,
+        P: ~const FnOnce(&T) -> bool,
+        P: ~const Destruct,
+    {
+        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`]: Option::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")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn or(self, optb: Option<T>) -> Option<T>
+    where
+        T: ~const Destruct,
+    {
+        match self {
+            Some(x) => Some(x),
+            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")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn or_else<F>(self, f: F) -> Option<T>
+    where
+        F: ~const FnOnce() -> Option<T>,
+        F: ~const Destruct,
+    {
+        match self {
+            Some(x) => Some(x),
+            None => f(),
+        }
+    }
+
+    /// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns [`None`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// 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]
+    #[stable(feature = "option_xor", since = "1.37.0")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn xor(self, optb: Option<T>) -> Option<T>
+    where
+        T: ~const Destruct,
+    {
+        match (self, optb) {
+            (Some(a), None) => Some(a),
+            (None, Some(b)) => Some(b),
+            _ => None,
+        }
+    }
+
+    /////////////////////////////////////////////////////////////////////////
+    // Entry-like operations to insert a value and return a reference
+    /////////////////////////////////////////////////////////////////////////
+
+    /// Inserts `value` into the option, then returns a mutable reference to it.
+    ///
+    /// If the option already contains a value, the old value is dropped.
+    ///
+    /// See also [`Option::get_or_insert`], which doesn't update the value if
+    /// the option already contains [`Some`].
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// let mut opt = None;
+    /// let val = opt.insert(1);
+    /// assert_eq!(*val, 1);
+    /// assert_eq!(opt.unwrap(), 1);
+    /// let val = opt.insert(2);
+    /// assert_eq!(*val, 2);
+    /// *val = 3;
+    /// assert_eq!(opt.unwrap(), 3);
+    /// ```
+    #[must_use = "if you intended to set a value, consider assignment instead"]
+    #[inline]
+    #[stable(feature = "option_insert", since = "1.53.0")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn insert(&mut self, value: T) -> &mut T
+    where
+        T: ~const Destruct,
+    {
+        *self = Some(value);
+
+        // SAFETY: the code above just filled the option
+        unsafe { self.as_mut().unwrap_unchecked() }
+    }
+
+    /// Inserts `value` into the option if it is [`None`], then
+    /// returns a mutable reference to the contained value.
+    ///
+    /// See also [`Option::insert`], which updates the value even if
+    /// the option already contains [`Some`].
+    ///
+    /// # 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")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn get_or_insert(&mut self, value: T) -> &mut T
+    where
+        T: ~const Destruct,
+    {
+        if let None = *self {
+            *self = Some(value);
+        }
+
+        // SAFETY: a `None` variant for `self` would have been replaced by a `Some`
+        // variant in the code above.
+        unsafe { self.as_mut().unwrap_unchecked() }
+    }
+
+    /// Inserts the default value into the option if it is [`None`], then
+    /// returns a mutable reference to the contained value.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(option_get_or_insert_default)]
+    ///
+    /// let mut x = None;
+    ///
+    /// {
+    ///     let y: &mut u32 = x.get_or_insert_default();
+    ///     assert_eq!(y, &0);
+    ///
+    ///     *y = 7;
+    /// }
+    ///
+    /// assert_eq!(x, Some(7));
+    /// ```
+    #[inline]
+    #[unstable(feature = "option_get_or_insert_default", issue = "82901")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn get_or_insert_default(&mut self) -> &mut T
+    where
+        T: ~const Default,
+    {
+        const fn default<T: ~const Default>() -> T {
+            T::default()
+        }
+
+        self.get_or_insert_with(default)
+    }
+
+    /// Inserts a value computed from `f` into the option if it is [`None`],
+    /// then returns a mutable reference to the contained value.
+    ///
+    /// # 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")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn get_or_insert_with<F>(&mut self, f: F) -> &mut T
+    where
+        F: ~const FnOnce() -> T,
+        F: ~const Destruct,
+    {
+        if let None = *self {
+            // the compiler isn't smart enough to know that we are not dropping a `T`
+            // here and wants us to ensure `T` can be dropped at compile time.
+            mem::forget(mem::replace(self, Some(f())))
+        }
+
+        // SAFETY: a `None` variant for `self` would have been replaced by a `Some`
+        // variant in the code above.
+        unsafe { self.as_mut().unwrap_unchecked() }
+    }
+
+    /////////////////////////////////////////////////////////////////////////
+    // Misc
+    /////////////////////////////////////////////////////////////////////////
+
+    /// Takes the value out of the option, leaving a [`None`] in its place.
+    ///
+    /// # 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")]
+    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
+    pub const fn take(&mut self) -> Option<T> {
+        // FIXME replace `mem::replace` by `mem::take` when the latter is const ready
+        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.
+    ///
+    /// # 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]
+    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
+    #[stable(feature = "option_replace", since = "1.31.0")]
+    pub const fn replace(&mut self, value: T) -> Option<T> {
+        mem::replace(self, Some(value))
+    }
+
+    /// Returns `true` if the option is a [`Some`] value containing the given value.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(option_result_contains)]
+    ///
+    /// let x: Option<u32> = Some(2);
+    /// assert_eq!(x.contains(&2), true);
+    ///
+    /// let x: Option<u32> = Some(3);
+    /// assert_eq!(x.contains(&2), false);
+    ///
+    /// let x: Option<u32> = None;
+    /// assert_eq!(x.contains(&2), false);
+    /// ```
+    #[must_use]
+    #[inline]
+    #[unstable(feature = "option_result_contains", issue = "62358")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn contains<U>(&self, x: &U) -> bool
+    where
+        U: ~const PartialEq<T>,
+    {
+        match self {
+            Some(y) => x.eq(y),
+            None => false,
+        }
+    }
+
+    /// Zips `self` with another `Option`.
+    ///
+    /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some((s, o))`.
+    /// Otherwise, `None` is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let x = Some(1);
+    /// let y = Some("hi");
+    /// let z = None::<u8>;
+    ///
+    /// assert_eq!(x.zip(y), Some((1, "hi")));
+    /// assert_eq!(x.zip(z), None);
+    /// ```
+    #[stable(feature = "option_zip_option", since = "1.46.0")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn zip<U>(self, other: Option<U>) -> Option<(T, U)>
+    where
+        T: ~const Destruct,
+        U: ~const Destruct,
+    {
+        match (self, other) {
+            (Some(a), Some(b)) => Some((a, b)),
+            _ => None,
+        }
+    }
+
+    /// Zips `self` and another `Option` with function `f`.
+    ///
+    /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`.
+    /// Otherwise, `None` is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(option_zip)]
+    ///
+    /// #[derive(Debug, PartialEq)]
+    /// struct Point {
+    ///     x: f64,
+    ///     y: f64,
+    /// }
+    ///
+    /// impl Point {
+    ///     fn new(x: f64, y: f64) -> Self {
+    ///         Self { x, y }
+    ///     }
+    /// }
+    ///
+    /// let x = Some(17.5);
+    /// let y = Some(42.7);
+    ///
+    /// assert_eq!(x.zip_with(y, Point::new), Some(Point { x: 17.5, y: 42.7 }));
+    /// assert_eq!(x.zip_with(None, Point::new), None);
+    /// ```
+    #[unstable(feature = "option_zip", issue = "70086")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R>
+    where
+        F: ~const FnOnce(T, U) -> R,
+        F: ~const Destruct,
+        T: ~const Destruct,
+        U: ~const Destruct,
+    {
+        match (self, other) {
+            (Some(a), Some(b)) => Some(f(a, b)),
+            _ => None,
+        }
+    }
+}
+
+impl<T, U> Option<(T, U)> {
+    /// Unzips an option containing a tuple of two options.
+    ///
+    /// If `self` is `Some((a, b))` this method returns `(Some(a), Some(b))`.
+    /// Otherwise, `(None, None)` is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(unzip_option)]
+    ///
+    /// let x = Some((1, "hi"));
+    /// let y = None::<(u8, u32)>;
+    ///
+    /// assert_eq!(x.unzip(), (Some(1), Some("hi")));
+    /// assert_eq!(y.unzip(), (None, None));
+    /// ```
+    #[inline]
+    #[unstable(feature = "unzip_option", issue = "87800", reason = "recently added")]
+    pub const fn unzip(self) -> (Option<T>, Option<U>) {
+        match self {
+            Some((a, b)) => (Some(a), Some(b)),
+            None => (None, None),
+        }
+    }
+}
+
+impl<T> Option<&T> {
+    /// Maps an `Option<&T>` to an `Option<T>` by copying the contents of the
+    /// option.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let x = 12;
+    /// let opt_x = Some(&x);
+    /// assert_eq!(opt_x, Some(&12));
+    /// let copied = opt_x.copied();
+    /// assert_eq!(copied, Some(12));
+    /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
+    #[stable(feature = "copied", since = "1.35.0")]
+    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
+    pub const fn copied(self) -> Option<T>
+    where
+        T: Copy,
+    {
+        // FIXME: this implementation, which sidesteps using `Option::map` since it's not const
+        // ready yet, should be reverted when possible to avoid code repetition
+        match self {
+            Some(&v) => Some(v),
+            None => None,
+        }
+    }
+
+    /// 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));
+    /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_option_cloned", issue = "91582")]
+    pub const fn cloned(self) -> Option<T>
+    where
+        T: ~const Clone,
+    {
+        match self {
+            Some(t) => Some(t.clone()),
+            None => None,
+        }
+    }
+}
+
+impl<T> Option<&mut T> {
+    /// Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the
+    /// option.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// 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));
+    /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
+    #[stable(feature = "copied", since = "1.35.0")]
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn copied(self) -> Option<T>
+    where
+        T: Copy,
+    {
+        match self {
+            Some(&mut t) => Some(t),
+            None => None,
+        }
+    }
+
+    /// 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));
+    /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
+    #[stable(since = "1.26.0", feature = "option_ref_mut_cloned")]
+    #[rustc_const_unstable(feature = "const_option_cloned", issue = "91582")]
+    pub const fn cloned(self) -> Option<T>
+    where
+        T: ~const Clone,
+    {
+        match self {
+            Some(t) => Some(t.clone()),
+            None => None,
+        }
+    }
+}
+
+impl<T, E> Option<Result<T, E>> {
+    /// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`.
+    ///
+    /// [`None`] will be mapped to <code>[Ok]\([None])</code>.
+    /// <code>[Some]\([Ok]\(\_))</code> and <code>[Some]\([Err]\(\_))</code> will be mapped to
+    /// <code>[Ok]\([Some]\(\_))</code> and <code>[Err]\(\_)</code>.
+    ///
+    /// # 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")]
+    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
+    pub const 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.
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
+#[cfg_attr(feature = "panic_immediate_abort", inline)]
+#[cold]
+#[track_caller]
+#[rustc_const_unstable(feature = "const_option", issue = "67441")]
+const fn expect_failed(msg: &str) -> ! {
+    panic_str(msg)
+}
+
+/////////////////////////////////////////////////////////////////////////////
+// Trait implementations
+/////////////////////////////////////////////////////////////////////////////
+
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_const_unstable(feature = "const_clone", issue = "91805")]
+impl<T> const Clone for Option<T>
+where
+    T: ~const Clone + ~const Destruct,
+{
+    #[inline]
+    fn clone(&self) -> Self {
+        match self {
+            Some(x) => Some(x.clone()),
+            None => None,
+        }
+    }
+
+    #[inline]
+    fn clone_from(&mut self, source: &Self) {
+        match (self, source) {
+            (Some(to), Some(from)) => to.clone_from(from),
+            (to, from) => *to = from.clone(),
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
+impl<T> const Default for Option<T> {
+    /// Returns [`None`][Option::None].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let opt: Option<u32> = Option::default();
+    /// assert!(opt.is_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")]
+#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
+impl<T> const From<T> for Option<T> {
+    /// Moves `val` into a new [`Some`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let o: Option<u8> = Option::from(67);
+    ///
+    /// assert_eq!(Some(67), o);
+    /// ```
+    fn from(val: T) -> Option<T> {
+        Some(val)
+    }
+}
+
+#[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
+#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
+impl<'a, T> const From<&'a Option<T>> for Option<&'a T> {
+    /// Converts from `&Option<T>` to `Option<&T>`.
+    ///
+    /// # Examples
+    ///
+    /// Converts an <code>[Option]<[String]></code> into an <code>[Option]<[usize]></code>, preserving
+    /// the original. The [`map`] method takes the `self` argument by value, consuming the original,
+    /// so this technique uses `from` to first take an [`Option`] to a reference
+    /// to the value inside the original.
+    ///
+    /// [`map`]: Option::map
+    /// [String]: ../../std/string/struct.String.html "String"
+    ///
+    /// ```
+    /// let s: Option<String> = Some(String::from("Hello, Rustaceans!"));
+    /// let o: Option<usize> = Option::from(&s).map(|ss: &String| ss.len());
+    ///
+    /// println!("Can still print s: {s:?}");
+    ///
+    /// assert_eq!(o, Some(18));
+    /// ```
+    fn from(o: &'a Option<T>) -> Option<&'a T> {
+        o.as_ref()
+    }
+}
+
+#[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
+#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
+impl<'a, T> const From<&'a mut Option<T>> for Option<&'a mut T> {
+    /// Converts from `&mut Option<T>` to `Option<&mut T>`
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut s = Some(String::from("Hello"));
+    /// let o: Option<&mut String> = Option::from(&mut s);
+    ///
+    /// match o {
+    ///     Some(t) => *t = String::from("Hello, Rustaceans!"),
+    ///     None => (),
+    /// }
+    ///
+    /// assert_eq!(s, Some(String::from("Hello, Rustaceans!")));
+    /// ```
+    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.
+#[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.
+#[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.
+#[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 of type
+    /// `V` containing 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`.
+    ///
+    /// Here is a variation on the previous example, showing that no
+    /// further elements are taken from `iter` after the first `None`.
+    ///
+    /// ```
+    /// let items = vec![3_u16, 2, 1, 10];
+    ///
+    /// let mut shared = 0;
+    ///
+    /// let res: Option<Vec<u16>> = items
+    ///     .iter()
+    ///     .map(|x| { shared += x; x.checked_sub(2) })
+    ///     .collect();
+    ///
+    /// assert_eq!(res, None);
+    /// assert_eq!(shared, 6);
+    /// ```
+    ///
+    /// Since the third element caused an underflow, no further elements were taken,
+    /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
+    #[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.
+
+        iter::try_process(iter.into_iter(), |i| i.collect())
+    }
+}
+
+#[unstable(feature = "try_trait_v2", issue = "84277")]
+#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
+impl<T> const ops::Try for Option<T> {
+    type Output = T;
+    type Residual = Option<convert::Infallible>;
+
+    #[inline]
+    fn from_output(output: Self::Output) -> Self {
+        Some(output)
+    }
+
+    #[inline]
+    fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
+        match self {
+            Some(v) => ControlFlow::Continue(v),
+            None => ControlFlow::Break(None),
+        }
+    }
+}
+
+#[unstable(feature = "try_trait_v2", issue = "84277")]
+#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
+impl<T> const ops::FromResidual for Option<T> {
+    #[inline]
+    fn from_residual(residual: Option<convert::Infallible>) -> Self {
+        match residual {
+            None => None,
+        }
+    }
+}
+
+#[unstable(feature = "try_trait_v2_yeet", issue = "96374")]
+impl<T> ops::FromResidual<ops::Yeet<()>> for Option<T> {
+    #[inline]
+    fn from_residual(ops::Yeet(()): ops::Yeet<()>) -> Self {
+        None
+    }
+}
+
+#[unstable(feature = "try_trait_v2_residual", issue = "91285")]
+impl<T> ops::Residual<T> for Option<convert::Infallible> {
+    type TryType = Option<T>;
+}
+
+impl<T> Option<Option<T>> {
+    /// Converts from `Option<Option<T>>` to `Option<T>`.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// let x: Option<Option<u32>> = Some(Some(6));
+    /// assert_eq!(Some(6), x.flatten());
+    ///
+    /// let x: Option<Option<u32>> = Some(None);
+    /// assert_eq!(None, x.flatten());
+    ///
+    /// let x: Option<Option<u32>> = None;
+    /// assert_eq!(None, x.flatten());
+    /// ```
+    ///
+    /// Flattening only removes one level of nesting at a time:
+    ///
+    /// ```
+    /// let x: Option<Option<Option<u32>>> = Some(Some(Some(6)));
+    /// assert_eq!(Some(Some(6)), x.flatten());
+    /// assert_eq!(Some(6), x.flatten().flatten());
+    /// ```
+    #[inline]
+    #[stable(feature = "option_flattening", since = "1.40.0")]
+    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
+    pub const fn flatten(self) -> Option<T> {
+        match self {
+            Some(inner) => inner,
+            None => None,
+        }
+    }
+}