library/core/src/option.rs

   1 //! Optional values.
   2 //!
   3 //! Type [`Option`] represents an optional value: every [`Option`]
   4 //! is either [`Some`] and contains a value, or [`None`], and
   5 //! does not. [`Option`] types are very common in Rust code, as
   6 //! they have a number of uses:
   7 //!
   8 //! * Initial values
   9 //! * Return values for functions that are not defined
  10 //!   over their entire input range (partial functions)
  11 //! * Return value for otherwise reporting simple errors, where [`None`] is
  12 //!   returned on error
  13 //! * Optional struct fields
  14 //! * Struct fields that can be loaned or "taken"
  15 //! * Optional function arguments
  16 //! * Nullable pointers
  17 //! * Swapping things out of difficult situations
  18 //!
  19 //! [`Option`]s are commonly paired with pattern matching to query the presence
  20 //! of a value and take action, always accounting for the [`None`] case.
  21 //!
  22 //! ```
  23 //! fn divide(numerator: f64, denominator: f64) -> Option<f64> {
  24 //!     if denominator == 0.0 {
  25 //!         None
  26 //!     } else {
  27 //!         Some(numerator / denominator)
  28 //!     }
  29 //! }
  30 //!
  31 //! // The return value of the function is an option
  32 //! let result = divide(2.0, 3.0);
  33 //!
  34 //! // Pattern match to retrieve the value
  35 //! match result {
  36 //!     // The division was valid
  37 //!     Some(x) => println!("Result: {}", x),
  38 //!     // The division was invalid
  39 //!     None    => println!("Cannot divide by 0"),
  40 //! }
  41 //! ```
  42 //!
  43 //
  44 // FIXME: Show how `Option` is used in practice, with lots of methods
  45 //
  46 //! # Options and pointers ("nullable" pointers)
  47 //!
  48 //! Rust's pointer types must always point to a valid location; there are
  49 //! no "null" references. Instead, Rust has *optional* pointers, like
  50 //! the optional owned box, [`Option`]`<`[`Box<T>`]`>`.
  51 //!
  52 //! The following example uses [`Option`] to create an optional box of
  53 //! [`i32`]. Notice that in order to use the inner [`i32`] value first, the
  54 //! `check_optional` function needs to use pattern matching to
  55 //! determine whether the box has a value (i.e., it is [`Some(...)`][`Some`]) or
  56 //! not ([`None`]).
  57 //!
  58 //! ```
  59 //! let optional = None;
  60 //! check_optional(optional);
  61 //!
  62 //! let optional = Some(Box::new(9000));
  63 //! check_optional(optional);
  64 //!
  65 //! fn check_optional(optional: Option<Box<i32>>) {
  66 //!     match optional {
  67 //!         Some(p) => println!("has value {}", p),
  68 //!         None => println!("has no value"),
  69 //!     }
  70 //! }
  71 //! ```
  72 //!
  73 //! # Representation
  74 //!
  75 //! Rust guarantees to optimize the following types `T` such that
  76 //! [`Option<T>`] has the same size as `T`:
  77 //!
  78 //! * [`Box<U>`]
  79 //! * `&U`
  80 //! * `&mut U`
  81 //! * `fn`, `extern "C" fn`
  82 //! * [`num::NonZero*`]
  83 //! * [`ptr::NonNull<U>`]
  84 //! * `#[repr(transparent)]` struct around one of the types in this list.
  85 //!
  86 //! It is further guaranteed that, for the cases above, one can
  87 //! [`mem::transmute`] from all valid values of `T` to `Option<T>` and
  88 //! from `Some::<T>(_)` to `T` (but transmuting `None::<T>` to `T`
  89 //! is undefined behaviour).
  90 //!
  91 //! # Examples
  92 //!
  93 //! Basic pattern matching on [`Option`]:
  94 //!
  95 //! ```
  96 //! let msg = Some("howdy");
  97 //!
  98 //! // Take a reference to the contained string
  99 //! if let Some(m) = &msg {
 100 //!     println!("{}", *m);
 101 //! }
 102 //!
 103 //! // Remove the contained string, destroying the Option
 104 //! let unwrapped_msg = msg.unwrap_or("default message");
 105 //! ```
 106 //!
 107 //! Initialize a result to [`None`] before a loop:
 108 //!
 109 //! ```
 110 //! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }
 111 //!
 112 //! // A list of data to search through.
 113 //! let all_the_big_things = [
 114 //!     Kingdom::Plant(250, "redwood"),
 115 //!     Kingdom::Plant(230, "noble fir"),
 116 //!     Kingdom::Plant(229, "sugar pine"),
 117 //!     Kingdom::Animal(25, "blue whale"),
 118 //!     Kingdom::Animal(19, "fin whale"),
 119 //!     Kingdom::Animal(15, "north pacific right whale"),
 120 //! ];
 121 //!
 122 //! // We're going to search for the name of the biggest animal,
 123 //! // but to start with we've just got `None`.
 124 //! let mut name_of_biggest_animal = None;
 125 //! let mut size_of_biggest_animal = 0;
 126 //! for big_thing in &all_the_big_things {
 127 //!     match *big_thing {
 128 //!         Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
 129 //!             // Now we've found the name of some big animal
 130 //!             size_of_biggest_animal = size;
 131 //!             name_of_biggest_animal = Some(name);
 132 //!         }
 133 //!         Kingdom::Animal(..) | Kingdom::Plant(..) => ()
 134 //!     }
 135 //! }
 136 //!
 137 //! match name_of_biggest_animal {
 138 //!     Some(name) => println!("the biggest animal is {}", name),
 139 //!     None => println!("there are no animals :("),
 140 //! }
 141 //! ```
 142 //!
 143 //! [`Box<T>`]: ../../std/boxed/struct.Box.html
 144 //! [`Box<U>`]: ../../std/boxed/struct.Box.html
 145 //! [`num::NonZero*`]: crate::num
 146 //! [`ptr::NonNull<U>`]: crate::ptr::NonNull
 147
 148 #![stable(feature = "rust1", since = "1.0.0")]
 149
 150 use crate::iter::{FromIterator, FusedIterator, TrustedLen};
 151 use crate::pin::Pin;
 152 use crate::{
 153     convert, hint, mem,
 154     ops::{self, ControlFlow, Deref, DerefMut},
 155 };
 156
 157 /// The `Option` type. See [the module level documentation](self) for more.
 158 #[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
 159 #[rustc_diagnostic_item = "option_type"]
 160 #[stable(feature = "rust1", since = "1.0.0")]
 161 pub enum Option<T> {
 162     /// No value
 163     #[lang = "None"]
 164     #[stable(feature = "rust1", since = "1.0.0")]
 165     None,
 166     /// Some value `T`
 167     #[lang = "Some"]
 168     #[stable(feature = "rust1", since = "1.0.0")]
 169     Some(#[stable(feature = "rust1", since = "1.0.0")] T),
 170 }
 171
 172 /////////////////////////////////////////////////////////////////////////////
 173 // Type implementation
 174 /////////////////////////////////////////////////////////////////////////////
 175
 176 impl<T> Option<T> {
 177     /////////////////////////////////////////////////////////////////////////
 178     // Querying the contained values
 179     /////////////////////////////////////////////////////////////////////////
 180
 181     /// Returns `true` if the option is a [`Some`] value.
 182     ///
 183     /// # Examples
 184     ///
 185     /// ```
 186     /// let x: Option<u32> = Some(2);
 187     /// assert_eq!(x.is_some(), true);
 188     ///
 189     /// let x: Option<u32> = None;
 190     /// assert_eq!(x.is_some(), false);
 191     /// ```
 192     #[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"]
 193     #[inline]
 194     #[rustc_const_stable(feature = "const_option", since = "1.48.0")]
 195     #[stable(feature = "rust1", since = "1.0.0")]
 196     pub const fn is_some(&self) -> bool {
 197         matches!(*self, Some(_))
 198     }
 199
 200     /// Returns `true` if the option is a [`None`] value.
 201     ///
 202     /// # Examples
 203     ///
 204     /// ```
 205     /// let x: Option<u32> = Some(2);
 206     /// assert_eq!(x.is_none(), false);
 207     ///
 208     /// let x: Option<u32> = None;
 209     /// assert_eq!(x.is_none(), true);
 210     /// ```
 211     #[must_use = "if you intended to assert that this doesn't have a value, consider \
 212                   `.and_then(|| panic!(\"`Option` had a value when expected `None`\"))` instead"]
 213     #[inline]
 214     #[rustc_const_stable(feature = "const_option", since = "1.48.0")]
 215     #[stable(feature = "rust1", since = "1.0.0")]
 216     pub const fn is_none(&self) -> bool {
 217         !self.is_some()
 218     }
 219
 220     /// Returns `true` if the option is a [`Some`] value containing the given value.
 221     ///
 222     /// # Examples
 223     ///
 224     /// ```
 225     /// #![feature(option_result_contains)]
 226     ///
 227     /// let x: Option<u32> = Some(2);
 228     /// assert_eq!(x.contains(&2), true);
 229     ///
 230     /// let x: Option<u32> = Some(3);
 231     /// assert_eq!(x.contains(&2), false);
 232     ///
 233     /// let x: Option<u32> = None;
 234     /// assert_eq!(x.contains(&2), false);
 235     /// ```
 236     #[must_use]
 237     #[inline]
 238     #[unstable(feature = "option_result_contains", issue = "62358")]
 239     pub fn contains<U>(&self, x: &U) -> bool
 240     where
 241         U: PartialEq<T>,
 242     {
 243         match self {
 244             Some(y) => x == y,
 245             None => false,
 246         }
 247     }
 248
 249     /////////////////////////////////////////////////////////////////////////
 250     // Adapter for working with references
 251     /////////////////////////////////////////////////////////////////////////
 252
 253     /// Converts from `&Option<T>` to `Option<&T>`.
 254     ///
 255     /// # Examples
 256     ///
 257     /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, preserving the original.
 258     /// The [`map`] method takes the `self` argument by value, consuming the original,
 259     /// so this technique uses `as_ref` to first take an `Option` to a reference
 260     /// to the value inside the original.
 261     ///
 262     /// [`map`]: Option::map
 263     /// [`String`]: ../../std/string/struct.String.html
 264     ///
 265     /// ```
 266     /// let text: Option<String> = Some("Hello, world!".to_string());
 267     /// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
 268     /// // then consume *that* with `map`, leaving `text` on the stack.
 269     /// let text_length: Option<usize> = text.as_ref().map(|s| s.len());
 270     /// println!("still can print text: {:?}", text);
 271     /// ```
 272     #[inline]
 273     #[rustc_const_stable(feature = "const_option", since = "1.48.0")]
 274     #[stable(feature = "rust1", since = "1.0.0")]
 275     pub const fn as_ref(&self) -> Option<&T> {
 276         match *self {
 277             Some(ref x) => Some(x),
 278             None => None,
 279         }
 280     }
 281
 282     /// Converts from `&mut Option<T>` to `Option<&mut T>`.
 283     ///
 284     /// # Examples
 285     ///
 286     /// ```
 287     /// let mut x = Some(2);
 288     /// match x.as_mut() {
 289     ///     Some(v) => *v = 42,
 290     ///     None => {},
 291     /// }
 292     /// assert_eq!(x, Some(42));
 293     /// ```
 294     #[inline]
 295     #[stable(feature = "rust1", since = "1.0.0")]
 296     pub fn as_mut(&mut self) -> Option<&mut T> {
 297         match *self {
 298             Some(ref mut x) => Some(x),
 299             None => None,
 300         }
 301     }
 302
 303     /// Converts from [`Pin`]`<&Option<T>>` to `Option<`[`Pin`]`<&T>>`.
 304     #[inline]
 305     #[stable(feature = "pin", since = "1.33.0")]
 306     pub fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> {
 307         // SAFETY: `x` is guaranteed to be pinned because it comes from `self`
 308         // which is pinned.
 309         unsafe { Pin::get_ref(self).as_ref().map(|x| Pin::new_unchecked(x)) }
 310     }
 311
 312     /// Converts from [`Pin`]`<&mut Option<T>>` to `Option<`[`Pin`]`<&mut T>>`.
 313     #[inline]
 314     #[stable(feature = "pin", since = "1.33.0")]
 315     pub fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> {
 316         // SAFETY: `get_unchecked_mut` is never used to move the `Option` inside `self`.
 317         // `x` is guaranteed to be pinned because it comes from `self` which is pinned.
 318         unsafe { Pin::get_unchecked_mut(self).as_mut().map(|x| Pin::new_unchecked(x)) }
 319     }
 320
 321     /////////////////////////////////////////////////////////////////////////
 322     // Getting to contained values
 323     /////////////////////////////////////////////////////////////////////////
 324
 325     /// Returns the contained [`Some`] value, consuming the `self` value.
 326     ///
 327     /// # Panics
 328     ///
 329     /// Panics if the value is a [`None`] with a custom panic message provided by
 330     /// `msg`.
 331     ///
 332     /// # Examples
 333     ///
 334     /// ```
 335     /// let x = Some("value");
 336     /// assert_eq!(x.expect("fruits are healthy"), "value");
 337     /// ```
 338     ///
 339     /// ```should_panic
 340     /// let x: Option<&str> = None;
 341     /// x.expect("fruits are healthy"); // panics with `fruits are healthy`
 342     /// ```
 343     #[inline]
 344     #[track_caller]
 345     #[stable(feature = "rust1", since = "1.0.0")]
 346     pub fn expect(self, msg: &str) -> T {
 347         match self {
 348             Some(val) => val,
 349             None => expect_failed(msg),
 350         }
 351     }
 352
 353     /// Returns the contained [`Some`] value, consuming the `self` value.
 354     ///
 355     /// Because this function may panic, its use is generally discouraged.
 356     /// Instead, prefer to use pattern matching and handle the [`None`]
 357     /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
 358     /// [`unwrap_or_default`].
 359     ///
 360     /// [`unwrap_or`]: Option::unwrap_or
 361     /// [`unwrap_or_else`]: Option::unwrap_or_else
 362     /// [`unwrap_or_default`]: Option::unwrap_or_default
 363     ///
 364     /// # Panics
 365     ///
 366     /// Panics if the self value equals [`None`].
 367     ///
 368     /// # Examples
 369     ///
 370     /// ```
 371     /// let x = Some("air");
 372     /// assert_eq!(x.unwrap(), "air");
 373     /// ```
 374     ///
 375     /// ```should_panic
 376     /// let x: Option<&str> = None;
 377     /// assert_eq!(x.unwrap(), "air"); // fails
 378     /// ```
 379     #[inline]
 380     #[track_caller]
 381     #[stable(feature = "rust1", since = "1.0.0")]
 382     #[rustc_const_unstable(feature = "const_option", issue = "67441")]
 383     pub const fn unwrap(self) -> T {
 384         match self {
 385             Some(val) => val,
 386             None => panic!("called `Option::unwrap()` on a `None` value"),
 387         }
 388     }
 389
 390     /// Returns the contained [`Some`] value or a provided default.
 391     ///
 392     /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
 393     /// the result of a function call, it is recommended to use [`unwrap_or_else`],
 394     /// which is lazily evaluated.
 395     ///
 396     /// [`unwrap_or_else`]: Option::unwrap_or_else
 397     ///
 398     /// # Examples
 399     ///
 400     /// ```
 401     /// assert_eq!(Some("car").unwrap_or("bike"), "car");
 402     /// assert_eq!(None.unwrap_or("bike"), "bike");
 403     /// ```
 404     #[inline]
 405     #[stable(feature = "rust1", since = "1.0.0")]
 406     pub fn unwrap_or(self, default: T) -> T {
 407         match self {
 408             Some(x) => x,
 409             None => default,
 410         }
 411     }
 412
 413     /// Returns the contained [`Some`] value or computes it from a closure.
 414     ///
 415     /// # Examples
 416     ///
 417     /// ```
 418     /// let k = 10;
 419     /// assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
 420     /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20);
 421     /// ```
 422     #[inline]
 423     #[stable(feature = "rust1", since = "1.0.0")]
 424     pub fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T {
 425         match self {
 426             Some(x) => x,
 427             None => f(),
 428         }
 429     }
 430
 431     /// Returns the contained [`Some`] value, consuming the `self` value,
 432     /// without checking that the value is not [`None`].
 433     ///
 434     /// # Safety
 435     ///
 436     /// Calling this method on [`None`] is *[undefined behavior]*.
 437     ///
 438     /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
 439     ///
 440     /// # Examples
 441     ///
 442     /// ```
 443     /// #![feature(option_result_unwrap_unchecked)]
 444     /// let x = Some("air");
 445     /// assert_eq!(unsafe { x.unwrap_unchecked() }, "air");
 446     /// ```
 447     ///
 448     /// ```no_run
 449     /// #![feature(option_result_unwrap_unchecked)]
 450     /// let x: Option<&str> = None;
 451     /// assert_eq!(unsafe { x.unwrap_unchecked() }, "air"); // Undefined behavior!
 452     /// ```
 453     #[inline]
 454     #[track_caller]
 455     #[unstable(feature = "option_result_unwrap_unchecked", reason = "newly added", issue = "81383")]
 456     pub unsafe fn unwrap_unchecked(self) -> T {
 457         debug_assert!(self.is_some());
 458         match self {
 459             Some(val) => val,
 460             // SAFETY: the safety contract must be upheld by the caller.
 461             None => unsafe { hint::unreachable_unchecked() },
 462         }
 463     }
 464
 465     /////////////////////////////////////////////////////////////////////////
 466     // Transforming contained values
 467     /////////////////////////////////////////////////////////////////////////
 468
 469     /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value.
 470     ///
 471     /// # Examples
 472     ///
 473     /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, consuming the original:
 474     ///
 475     /// [`String`]: ../../std/string/struct.String.html
 476     /// ```
 477     /// let maybe_some_string = Some(String::from("Hello, World!"));
 478     /// // `Option::map` takes self *by value*, consuming `maybe_some_string`
 479     /// let maybe_some_len = maybe_some_string.map(|s| s.len());
 480     ///
 481     /// assert_eq!(maybe_some_len, Some(13));
 482     /// ```
 483     #[inline]
 484     #[stable(feature = "rust1", since = "1.0.0")]
 485     pub fn map<U, F: FnOnce(T) -> U>(self, f: F) -> Option<U> {
 486         match self {
 487             Some(x) => Some(f(x)),
 488             None => None,
 489         }
 490     }
 491
 492     /// Returns the provided default result (if none),
 493     /// or applies a function to the contained value (if any).
 494     ///
 495     /// Arguments passed to `map_or` are eagerly evaluated; if you are passing
 496     /// the result of a function call, it is recommended to use [`map_or_else`],
 497     /// which is lazily evaluated.
 498     ///
 499     /// [`map_or_else`]: Option::map_or_else
 500     ///
 501     /// # Examples
 502     ///
 503     /// ```
 504     /// let x = Some("foo");
 505     /// assert_eq!(x.map_or(42, |v| v.len()), 3);
 506     ///
 507     /// let x: Option<&str> = None;
 508     /// assert_eq!(x.map_or(42, |v| v.len()), 42);
 509     /// ```
 510     #[inline]
 511     #[stable(feature = "rust1", since = "1.0.0")]
 512     pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
 513         match self {
 514             Some(t) => f(t),
 515             None => default,
 516         }
 517     }
 518
 519     /// Computes a default function result (if none), or
 520     /// applies a different function to the contained value (if any).
 521     ///
 522     /// # Examples
 523     ///
 524     /// ```
 525     /// let k = 21;
 526     ///
 527     /// let x = Some("foo");
 528     /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);
 529     ///
 530     /// let x: Option<&str> = None;
 531     /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);
 532     /// ```
 533     #[inline]
 534     #[stable(feature = "rust1", since = "1.0.0")]
 535     pub fn map_or_else<U, D: FnOnce() -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
 536         match self {
 537             Some(t) => f(t),
 538             None => default(),
 539         }
 540     }
 541
 542     /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
 543     /// [`Ok(v)`] and [`None`] to [`Err(err)`].
 544     ///
 545     /// Arguments passed to `ok_or` are eagerly evaluated; if you are passing the
 546     /// result of a function call, it is recommended to use [`ok_or_else`], which is
 547     /// lazily evaluated.
 548     ///
 549     /// [`Ok(v)`]: Ok
 550     /// [`Err(err)`]: Err
 551     /// [`Some(v)`]: Some
 552     /// [`ok_or_else`]: Option::ok_or_else
 553     ///
 554     /// # Examples
 555     ///
 556     /// ```
 557     /// let x = Some("foo");
 558     /// assert_eq!(x.ok_or(0), Ok("foo"));
 559     ///
 560     /// let x: Option<&str> = None;
 561     /// assert_eq!(x.ok_or(0), Err(0));
 562     /// ```
 563     #[inline]
 564     #[stable(feature = "rust1", since = "1.0.0")]
 565     pub fn ok_or<E>(self, err: E) -> Result<T, E> {
 566         match self {
 567             Some(v) => Ok(v),
 568             None => Err(err),
 569         }
 570     }
 571
 572     /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
 573     /// [`Ok(v)`] and [`None`] to [`Err(err())`].
 574     ///
 575     /// [`Ok(v)`]: Ok
 576     /// [`Err(err())`]: Err
 577     /// [`Some(v)`]: Some
 578     ///
 579     /// # Examples
 580     ///
 581     /// ```
 582     /// let x = Some("foo");
 583     /// assert_eq!(x.ok_or_else(|| 0), Ok("foo"));
 584     ///
 585     /// let x: Option<&str> = None;
 586     /// assert_eq!(x.ok_or_else(|| 0), Err(0));
 587     /// ```
 588     #[inline]
 589     #[stable(feature = "rust1", since = "1.0.0")]
 590     pub fn ok_or_else<E, F: FnOnce() -> E>(self, err: F) -> Result<T, E> {
 591         match self {
 592             Some(v) => Ok(v),
 593             None => Err(err()),
 594         }
 595     }
 596
 597     /////////////////////////////////////////////////////////////////////////
 598     // Iterator constructors
 599     /////////////////////////////////////////////////////////////////////////
 600
 601     /// Returns an iterator over the possibly contained value.
 602     ///
 603     /// # Examples
 604     ///
 605     /// ```
 606     /// let x = Some(4);
 607     /// assert_eq!(x.iter().next(), Some(&4));
 608     ///
 609     /// let x: Option<u32> = None;
 610     /// assert_eq!(x.iter().next(), None);
 611     /// ```
 612     #[inline]
 613     #[rustc_const_unstable(feature = "const_option", issue = "67441")]
 614     #[stable(feature = "rust1", since = "1.0.0")]
 615     pub const fn iter(&self) -> Iter<'_, T> {
 616         Iter { inner: Item { opt: self.as_ref() } }
 617     }
 618
 619     /// Returns a mutable iterator over the possibly contained value.
 620     ///
 621     /// # Examples
 622     ///
 623     /// ```
 624     /// let mut x = Some(4);
 625     /// match x.iter_mut().next() {
 626     ///     Some(v) => *v = 42,
 627     ///     None => {},
 628     /// }
 629     /// assert_eq!(x, Some(42));
 630     ///
 631     /// let mut x: Option<u32> = None;
 632     /// assert_eq!(x.iter_mut().next(), None);
 633     /// ```
 634     #[inline]
 635     #[stable(feature = "rust1", since = "1.0.0")]
 636     pub fn iter_mut(&mut self) -> IterMut<'_, T> {
 637         IterMut { inner: Item { opt: self.as_mut() } }
 638     }
 639
 640     /////////////////////////////////////////////////////////////////////////
 641     // Boolean operations on the values, eager and lazy
 642     /////////////////////////////////////////////////////////////////////////
 643
 644     /// Returns [`None`] if the option is [`None`], otherwise returns `optb`.
 645     ///
 646     /// # Examples
 647     ///
 648     /// ```
 649     /// let x = Some(2);
 650     /// let y: Option<&str> = None;
 651     /// assert_eq!(x.and(y), None);
 652     ///
 653     /// let x: Option<u32> = None;
 654     /// let y = Some("foo");
 655     /// assert_eq!(x.and(y), None);
 656     ///
 657     /// let x = Some(2);
 658     /// let y = Some("foo");
 659     /// assert_eq!(x.and(y), Some("foo"));
 660     ///
 661     /// let x: Option<u32> = None;
 662     /// let y: Option<&str> = None;
 663     /// assert_eq!(x.and(y), None);
 664     /// ```
 665     #[inline]
 666     #[stable(feature = "rust1", since = "1.0.0")]
 667     pub fn and<U>(self, optb: Option<U>) -> Option<U> {
 668         match self {
 669             Some(_) => optb,
 670             None => None,
 671         }
 672     }
 673
 674     /// Returns [`None`] if the option is [`None`], otherwise calls `f` with the
 675     /// wrapped value and returns the result.
 676     ///
 677     /// Some languages call this operation flatmap.
 678     ///
 679     /// # Examples
 680     ///
 681     /// ```
 682     /// fn sq(x: u32) -> Option<u32> { Some(x * x) }
 683     /// fn nope(_: u32) -> Option<u32> { None }
 684     ///
 685     /// assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16));
 686     /// assert_eq!(Some(2).and_then(sq).and_then(nope), None);
 687     /// assert_eq!(Some(2).and_then(nope).and_then(sq), None);
 688     /// assert_eq!(None.and_then(sq).and_then(sq), None);
 689     /// ```
 690     #[inline]
 691     #[stable(feature = "rust1", since = "1.0.0")]
 692     pub fn and_then<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> Option<U> {
 693         match self {
 694             Some(x) => f(x),
 695             None => None,
 696         }
 697     }
 698
 699     /// Returns [`None`] if the option is [`None`], otherwise calls `predicate`
 700     /// with the wrapped value and returns:
 701     ///
 702     /// - [`Some(t)`] if `predicate` returns `true` (where `t` is the wrapped
 703     ///   value), and
 704     /// - [`None`] if `predicate` returns `false`.
 705     ///
 706     /// This function works similar to [`Iterator::filter()`]. You can imagine
 707     /// the `Option<T>` being an iterator over one or zero elements. `filter()`
 708     /// lets you decide which elements to keep.
 709     ///
 710     /// # Examples
 711     ///
 712     /// ```rust
 713     /// fn is_even(n: &i32) -> bool {
 714     ///     n % 2 == 0
 715     /// }
 716     ///
 717     /// assert_eq!(None.filter(is_even), None);
 718     /// assert_eq!(Some(3).filter(is_even), None);
 719     /// assert_eq!(Some(4).filter(is_even), Some(4));
 720     /// ```
 721     ///
 722     /// [`Some(t)`]: Some
 723     #[inline]
 724     #[stable(feature = "option_filter", since = "1.27.0")]
 725     pub fn filter<P: FnOnce(&T) -> bool>(self, predicate: P) -> Self {
 726         if let Some(x) = self {
 727             if predicate(&x) {
 728                 return Some(x);
 729             }
 730         }
 731         None
 732     }
 733
 734     /// Returns the option if it contains a value, otherwise returns `optb`.
 735     ///
 736     /// Arguments passed to `or` are eagerly evaluated; if you are passing the
 737     /// result of a function call, it is recommended to use [`or_else`], which is
 738     /// lazily evaluated.
 739     ///
 740     /// [`or_else`]: Option::or_else
 741     ///
 742     /// # Examples
 743     ///
 744     /// ```
 745     /// let x = Some(2);
 746     /// let y = None;
 747     /// assert_eq!(x.or(y), Some(2));
 748     ///
 749     /// let x = None;
 750     /// let y = Some(100);
 751     /// assert_eq!(x.or(y), Some(100));
 752     ///
 753     /// let x = Some(2);
 754     /// let y = Some(100);
 755     /// assert_eq!(x.or(y), Some(2));
 756     ///
 757     /// let x: Option<u32> = None;
 758     /// let y = None;
 759     /// assert_eq!(x.or(y), None);
 760     /// ```
 761     #[inline]
 762     #[stable(feature = "rust1", since = "1.0.0")]
 763     pub fn or(self, optb: Option<T>) -> Option<T> {
 764         match self {
 765             Some(_) => self,
 766             None => optb,
 767         }
 768     }
 769
 770     /// Returns the option if it contains a value, otherwise calls `f` and
 771     /// returns the result.
 772     ///
 773     /// # Examples
 774     ///
 775     /// ```
 776     /// fn nobody() -> Option<&'static str> { None }
 777     /// fn vikings() -> Option<&'static str> { Some("vikings") }
 778     ///
 779     /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
 780     /// assert_eq!(None.or_else(vikings), Some("vikings"));
 781     /// assert_eq!(None.or_else(nobody), None);
 782     /// ```
 783     #[inline]
 784     #[stable(feature = "rust1", since = "1.0.0")]
 785     pub fn or_else<F: FnOnce() -> Option<T>>(self, f: F) -> Option<T> {
 786         match self {
 787             Some(_) => self,
 788             None => f(),
 789         }
 790     }
 791
 792     /// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns [`None`].
 793     ///
 794     /// # Examples
 795     ///
 796     /// ```
 797     /// let x = Some(2);
 798     /// let y: Option<u32> = None;
 799     /// assert_eq!(x.xor(y), Some(2));
 800     ///
 801     /// let x: Option<u32> = None;
 802     /// let y = Some(2);
 803     /// assert_eq!(x.xor(y), Some(2));
 804     ///
 805     /// let x = Some(2);
 806     /// let y = Some(2);
 807     /// assert_eq!(x.xor(y), None);
 808     ///
 809     /// let x: Option<u32> = None;
 810     /// let y: Option<u32> = None;
 811     /// assert_eq!(x.xor(y), None);
 812     /// ```
 813     #[inline]
 814     #[stable(feature = "option_xor", since = "1.37.0")]
 815     pub fn xor(self, optb: Option<T>) -> Option<T> {
 816         match (self, optb) {
 817             (Some(a), None) => Some(a),
 818             (None, Some(b)) => Some(b),
 819             _ => None,
 820         }
 821     }
 822
 823     /////////////////////////////////////////////////////////////////////////
 824     // Entry-like operations to insert a value and return a reference
 825     /////////////////////////////////////////////////////////////////////////
 826
 827     /// Inserts `value` into the option then returns a mutable reference to it.
 828     ///
 829     /// If the option already contains a value, the old value is dropped.
 830     ///
 831     /// See also [`Option::get_or_insert`], which doesn't update the value if
 832     /// the option already contains [`Some`].
 833     ///
 834     /// # Example
 835     ///
 836     /// ```
 837     /// let mut opt = None;
 838     /// let val = opt.insert(1);
 839     /// assert_eq!(*val, 1);
 840     /// assert_eq!(opt.unwrap(), 1);
 841     /// let val = opt.insert(2);
 842     /// assert_eq!(*val, 2);
 843     /// *val = 3;
 844     /// assert_eq!(opt.unwrap(), 3);
 845     /// ```
 846     #[inline]
 847     #[stable(feature = "option_insert", since = "1.53.0")]
 848     pub fn insert(&mut self, value: T) -> &mut T {
 849         *self = Some(value);
 850
 851         match self {
 852             Some(v) => v,
 853             // SAFETY: the code above just filled the option
 854             None => unsafe { hint::unreachable_unchecked() },
 855         }
 856     }
 857
 858     /// Inserts `value` into the option if it is [`None`], then
 859     /// returns a mutable reference to the contained value.
 860     ///
 861     /// See also [`Option::insert`], which updates the value even if
 862     /// the option already contains [`Some`].
 863     ///
 864     /// # Examples
 865     ///
 866     /// ```
 867     /// let mut x = None;
 868     ///
 869     /// {
 870     ///     let y: &mut u32 = x.get_or_insert(5);
 871     ///     assert_eq!(y, &5);
 872     ///
 873     ///     *y = 7;
 874     /// }
 875     ///
 876     /// assert_eq!(x, Some(7));
 877     /// ```
 878     #[inline]
 879     #[stable(feature = "option_entry", since = "1.20.0")]
 880     pub fn get_or_insert(&mut self, value: T) -> &mut T {
 881         self.get_or_insert_with(|| value)
 882     }
 883
 884     /// Inserts the default value into the option if it is [`None`], then
 885     /// returns a mutable reference to the contained value.
 886     ///
 887     /// # Examples
 888     ///
 889     /// ```
 890     /// #![feature(option_get_or_insert_default)]
 891     ///
 892     /// let mut x = None;
 893     ///
 894     /// {
 895     ///     let y: &mut u32 = x.get_or_insert_default();
 896     ///     assert_eq!(y, &0);
 897     ///
 898     ///     *y = 7;
 899     /// }
 900     ///
 901     /// assert_eq!(x, Some(7));
 902     /// ```
 903     #[inline]
 904     #[unstable(feature = "option_get_or_insert_default", issue = "82901")]
 905     pub fn get_or_insert_default(&mut self) -> &mut T
 906     where
 907         T: Default,
 908     {
 909         self.get_or_insert_with(Default::default)
 910     }
 911
 912     /// Inserts a value computed from `f` into the option if it is [`None`],
 913     /// then returns a mutable reference to the contained value.
 914     ///
 915     /// # Examples
 916     ///
 917     /// ```
 918     /// let mut x = None;
 919     ///
 920     /// {
 921     ///     let y: &mut u32 = x.get_or_insert_with(|| 5);
 922     ///     assert_eq!(y, &5);
 923     ///
 924     ///     *y = 7;
 925     /// }
 926     ///
 927     /// assert_eq!(x, Some(7));
 928     /// ```
 929     #[inline]
 930     #[stable(feature = "option_entry", since = "1.20.0")]
 931     pub fn get_or_insert_with<F: FnOnce() -> T>(&mut self, f: F) -> &mut T {
 932         if let None = *self {
 933             *self = Some(f());
 934         }
 935
 936         match self {
 937             Some(v) => v,
 938             // SAFETY: a `None` variant for `self` would have been replaced by a `Some`
 939             // variant in the code above.
 940             None => unsafe { hint::unreachable_unchecked() },
 941         }
 942     }
 943
 944     /////////////////////////////////////////////////////////////////////////
 945     // Misc
 946     /////////////////////////////////////////////////////////////////////////
 947
 948     /// Takes the value out of the option, leaving a [`None`] in its place.
 949     ///
 950     /// # Examples
 951     ///
 952     /// ```
 953     /// let mut x = Some(2);
 954     /// let y = x.take();
 955     /// assert_eq!(x, None);
 956     /// assert_eq!(y, Some(2));
 957     ///
 958     /// let mut x: Option<u32> = None;
 959     /// let y = x.take();
 960     /// assert_eq!(x, None);
 961     /// assert_eq!(y, None);
 962     /// ```
 963     #[inline]
 964     #[stable(feature = "rust1", since = "1.0.0")]
 965     pub fn take(&mut self) -> Option<T> {
 966         mem::take(self)
 967     }
 968
 969     /// Replaces the actual value in the option by the value given in parameter,
 970     /// returning the old value if present,
 971     /// leaving a [`Some`] in its place without deinitializing either one.
 972     ///
 973     /// # Examples
 974     ///
 975     /// ```
 976     /// let mut x = Some(2);
 977     /// let old = x.replace(5);
 978     /// assert_eq!(x, Some(5));
 979     /// assert_eq!(old, Some(2));
 980     ///
 981     /// let mut x = None;
 982     /// let old = x.replace(3);
 983     /// assert_eq!(x, Some(3));
 984     /// assert_eq!(old, None);
 985     /// ```
 986     #[inline]
 987     #[stable(feature = "option_replace", since = "1.31.0")]
 988     pub fn replace(&mut self, value: T) -> Option<T> {
 989         mem::replace(self, Some(value))
 990     }
 991
 992     /// Zips `self` with another `Option`.
 993     ///
 994     /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some((s, o))`.
 995     /// Otherwise, `None` is returned.
 996     ///
 997     /// # Examples
 998     ///
 999     /// ```
1000     /// let x = Some(1);
1001     /// let y = Some("hi");
1002     /// let z = None::<u8>;
1003     ///
1004     /// assert_eq!(x.zip(y), Some((1, "hi")));
1005     /// assert_eq!(x.zip(z), None);
1006     /// ```
1007     #[stable(feature = "option_zip_option", since = "1.46.0")]
1008     pub fn zip<U>(self, other: Option<U>) -> Option<(T, U)> {
1009         match (self, other) {
1010             (Some(a), Some(b)) => Some((a, b)),
1011             _ => None,
1012         }
1013     }
1014
1015     /// Zips `self` and another `Option` with function `f`.
1016     ///
1017     /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`.
1018     /// Otherwise, `None` is returned.
1019     ///
1020     /// # Examples
1021     ///
1022     /// ```
1023     /// #![feature(option_zip)]
1024     ///
1025     /// #[derive(Debug, PartialEq)]
1026     /// struct Point {
1027     ///     x: f64,
1028     ///     y: f64,
1029     /// }
1030     ///
1031     /// impl Point {
1032     ///     fn new(x: f64, y: f64) -> Self {
1033     ///         Self { x, y }
1034     ///     }
1035     /// }
1036     ///
1037     /// let x = Some(17.5);
1038     /// let y = Some(42.7);
1039     ///
1040     /// assert_eq!(x.zip_with(y, Point::new), Some(Point { x: 17.5, y: 42.7 }));
1041     /// assert_eq!(x.zip_with(None, Point::new), None);
1042     /// ```
1043     #[unstable(feature = "option_zip", issue = "70086")]
1044     pub fn zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R>
1045     where
1046         F: FnOnce(T, U) -> R,
1047     {
1048         Some(f(self?, other?))
1049     }
1050 }
1051
1052 impl<T: Copy> Option<&T> {
1053     /// Maps an `Option<&T>` to an `Option<T>` by copying the contents of the
1054     /// option.
1055     ///
1056     /// # Examples
1057     ///
1058     /// ```
1059     /// let x = 12;
1060     /// let opt_x = Some(&x);
1061     /// assert_eq!(opt_x, Some(&12));
1062     /// let copied = opt_x.copied();
1063     /// assert_eq!(copied, Some(12));
1064     /// ```
1065     #[stable(feature = "copied", since = "1.35.0")]
1066     pub fn copied(self) -> Option<T> {
1067         self.map(|&t| t)
1068     }
1069 }
1070
1071 impl<T: Copy> Option<&mut T> {
1072     /// Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the
1073     /// option.
1074     ///
1075     /// # Examples
1076     ///
1077     /// ```
1078     /// let mut x = 12;
1079     /// let opt_x = Some(&mut x);
1080     /// assert_eq!(opt_x, Some(&mut 12));
1081     /// let copied = opt_x.copied();
1082     /// assert_eq!(copied, Some(12));
1083     /// ```
1084     #[stable(feature = "copied", since = "1.35.0")]
1085     pub fn copied(self) -> Option<T> {
1086         self.map(|&mut t| t)
1087     }
1088 }
1089
1090 impl<T: Clone> Option<&T> {
1091     /// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the
1092     /// option.
1093     ///
1094     /// # Examples
1095     ///
1096     /// ```
1097     /// let x = 12;
1098     /// let opt_x = Some(&x);
1099     /// assert_eq!(opt_x, Some(&12));
1100     /// let cloned = opt_x.cloned();
1101     /// assert_eq!(cloned, Some(12));
1102     /// ```
1103     #[stable(feature = "rust1", since = "1.0.0")]
1104     pub fn cloned(self) -> Option<T> {
1105         self.map(|t| t.clone())
1106     }
1107 }
1108
1109 impl<T: Clone> Option<&mut T> {
1110     /// Maps an `Option<&mut T>` to an `Option<T>` by cloning the contents of the
1111     /// option.
1112     ///
1113     /// # Examples
1114     ///
1115     /// ```
1116     /// let mut x = 12;
1117     /// let opt_x = Some(&mut x);
1118     /// assert_eq!(opt_x, Some(&mut 12));
1119     /// let cloned = opt_x.cloned();
1120     /// assert_eq!(cloned, Some(12));
1121     /// ```
1122     #[stable(since = "1.26.0", feature = "option_ref_mut_cloned")]
1123     pub fn cloned(self) -> Option<T> {
1124         self.map(|t| t.clone())
1125     }
1126 }
1127
1128 impl<T: Default> Option<T> {
1129     /// Returns the contained [`Some`] value or a default
1130     ///
1131     /// Consumes the `self` argument then, if [`Some`], returns the contained
1132     /// value, otherwise if [`None`], returns the [default value] for that
1133     /// type.
1134     ///
1135     /// # Examples
1136     ///
1137     /// Converts a string to an integer, turning poorly-formed strings
1138     /// into 0 (the default value for integers). [`parse`] converts
1139     /// a string to any other type that implements [`FromStr`], returning
1140     /// [`None`] on error.
1141     ///
1142     /// ```
1143     /// let good_year_from_input = "1909";
1144     /// let bad_year_from_input = "190blarg";
1145     /// let good_year = good_year_from_input.parse().ok().unwrap_or_default();
1146     /// let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();
1147     ///
1148     /// assert_eq!(1909, good_year);
1149     /// assert_eq!(0, bad_year);
1150     /// ```
1151     ///
1152     /// [default value]: Default::default
1153     /// [`parse`]: str::parse
1154     /// [`FromStr`]: crate::str::FromStr
1155     #[inline]
1156     #[stable(feature = "rust1", since = "1.0.0")]
1157     pub fn unwrap_or_default(self) -> T {
1158         match self {
1159             Some(x) => x,
1160             None => Default::default(),
1161         }
1162     }
1163 }
1164
1165 impl<T: Deref> Option<T> {
1166     /// Converts from `Option<T>` (or `&Option<T>`) to `Option<&T::Target>`.
1167     ///
1168     /// Leaves the original Option in-place, creating a new one with a reference
1169     /// to the original one, additionally coercing the contents via [`Deref`].
1170     ///
1171     /// # Examples
1172     ///
1173     /// ```
1174     /// let x: Option<String> = Some("hey".to_owned());
1175     /// assert_eq!(x.as_deref(), Some("hey"));
1176     ///
1177     /// let x: Option<String> = None;
1178     /// assert_eq!(x.as_deref(), None);
1179     /// ```
1180     #[stable(feature = "option_deref", since = "1.40.0")]
1181     pub fn as_deref(&self) -> Option<&T::Target> {
1182         self.as_ref().map(|t| t.deref())
1183     }
1184 }
1185
1186 impl<T: DerefMut> Option<T> {
1187     /// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`.
1188     ///
1189     /// Leaves the original `Option` in-place, creating a new one containing a mutable reference to
1190     /// the inner type's `Deref::Target` type.
1191     ///
1192     /// # Examples
1193     ///
1194     /// ```
1195     /// let mut x: Option<String> = Some("hey".to_owned());
1196     /// assert_eq!(x.as_deref_mut().map(|x| {
1197     ///     x.make_ascii_uppercase();
1198     ///     x
1199     /// }), Some("HEY".to_owned().as_mut_str()));
1200     /// ```
1201     #[stable(feature = "option_deref", since = "1.40.0")]
1202     pub fn as_deref_mut(&mut self) -> Option<&mut T::Target> {
1203         self.as_mut().map(|t| t.deref_mut())
1204     }
1205 }
1206
1207 impl<T, E> Option<Result<T, E>> {
1208     /// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`.
1209     ///
1210     /// [`None`] will be mapped to [`Ok`]`(`[`None`]`)`.
1211     /// [`Some`]`(`[`Ok`]`(_))` and [`Some`]`(`[`Err`]`(_))` will be mapped to
1212     /// [`Ok`]`(`[`Some`]`(_))` and [`Err`]`(_)`.
1213     ///
1214     /// # Examples
1215     ///
1216     /// ```
1217     /// #[derive(Debug, Eq, PartialEq)]
1218     /// struct SomeErr;
1219     ///
1220     /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1221     /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1222     /// assert_eq!(x, y.transpose());
1223     /// ```
1224     #[inline]
1225     #[stable(feature = "transpose_result", since = "1.33.0")]
1226     #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1227     pub const fn transpose(self) -> Result<Option<T>, E> {
1228         match self {
1229             Some(Ok(x)) => Ok(Some(x)),
1230             Some(Err(e)) => Err(e),
1231             None => Ok(None),
1232         }
1233     }
1234 }
1235
1236 // This is a separate function to reduce the code size of .expect() itself.
1237 #[inline(never)]
1238 #[cold]
1239 #[track_caller]
1240 fn expect_failed(msg: &str) -> ! {
1241     panic!("{}", msg)
1242 }
1243
1244 /////////////////////////////////////////////////////////////////////////////
1245 // Trait implementations
1246 /////////////////////////////////////////////////////////////////////////////
1247
1248 #[stable(feature = "rust1", since = "1.0.0")]
1249 impl<T: Clone> Clone for Option<T> {
1250     #[inline]
1251     fn clone(&self) -> Self {
1252         match self {
1253             Some(x) => Some(x.clone()),
1254             None => None,
1255         }
1256     }
1257
1258     #[inline]
1259     fn clone_from(&mut self, source: &Self) {
1260         match (self, source) {
1261             (Some(to), Some(from)) => to.clone_from(from),
1262             (to, from) => *to = from.clone(),
1263         }
1264     }
1265 }
1266
1267 #[stable(feature = "rust1", since = "1.0.0")]
1268 impl<T> Default for Option<T> {
1269     /// Returns [`None`][Option::None].
1270     ///
1271     /// # Examples
1272     ///
1273     /// ```
1274     /// let opt: Option<u32> = Option::default();
1275     /// assert!(opt.is_none());
1276     /// ```
1277     #[inline]
1278     fn default() -> Option<T> {
1279         None
1280     }
1281 }
1282
1283 #[stable(feature = "rust1", since = "1.0.0")]
1284 impl<T> IntoIterator for Option<T> {
1285     type Item = T;
1286     type IntoIter = IntoIter<T>;
1287
1288     /// Returns a consuming iterator over the possibly contained value.
1289     ///
1290     /// # Examples
1291     ///
1292     /// ```
1293     /// let x = Some("string");
1294     /// let v: Vec<&str> = x.into_iter().collect();
1295     /// assert_eq!(v, ["string"]);
1296     ///
1297     /// let x = None;
1298     /// let v: Vec<&str> = x.into_iter().collect();
1299     /// assert!(v.is_empty());
1300     /// ```
1301     #[inline]
1302     fn into_iter(self) -> IntoIter<T> {
1303         IntoIter { inner: Item { opt: self } }
1304     }
1305 }
1306
1307 #[stable(since = "1.4.0", feature = "option_iter")]
1308 impl<'a, T> IntoIterator for &'a Option<T> {
1309     type Item = &'a T;
1310     type IntoIter = Iter<'a, T>;
1311
1312     fn into_iter(self) -> Iter<'a, T> {
1313         self.iter()
1314     }
1315 }
1316
1317 #[stable(since = "1.4.0", feature = "option_iter")]
1318 impl<'a, T> IntoIterator for &'a mut Option<T> {
1319     type Item = &'a mut T;
1320     type IntoIter = IterMut<'a, T>;
1321
1322     fn into_iter(self) -> IterMut<'a, T> {
1323         self.iter_mut()
1324     }
1325 }
1326
1327 #[stable(since = "1.12.0", feature = "option_from")]
1328 impl<T> From<T> for Option<T> {
1329     /// Copies `val` into a new `Some`.
1330     ///
1331     /// # Examples
1332     ///
1333     /// ```
1334     /// let o: Option<u8> = Option::from(67);
1335     ///
1336     /// assert_eq!(Some(67), o);
1337     /// ```
1338     fn from(val: T) -> Option<T> {
1339         Some(val)
1340     }
1341 }
1342
1343 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1344 impl<'a, T> From<&'a Option<T>> for Option<&'a T> {
1345     /// Converts from `&Option<T>` to `Option<&T>`.
1346     ///
1347     /// # Examples
1348     ///
1349     /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, preserving the original.
1350     /// The [`map`] method takes the `self` argument by value, consuming the original,
1351     /// so this technique uses `as_ref` to first take an `Option` to a reference
1352     /// to the value inside the original.
1353     ///
1354     /// [`map`]: Option::map
1355     /// [`String`]: ../../std/string/struct.String.html
1356     ///
1357     /// ```
1358     /// let s: Option<String> = Some(String::from("Hello, Rustaceans!"));
1359     /// let o: Option<usize> = Option::from(&s).map(|ss: &String| ss.len());
1360     ///
1361     /// println!("Can still print s: {:?}", s);
1362     ///
1363     /// assert_eq!(o, Some(18));
1364     /// ```
1365     fn from(o: &'a Option<T>) -> Option<&'a T> {
1366         o.as_ref()
1367     }
1368 }
1369
1370 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1371 impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T> {
1372     /// Converts from `&mut Option<T>` to `Option<&mut T>`
1373     ///
1374     /// # Examples
1375     ///
1376     /// ```
1377     /// let mut s = Some(String::from("Hello"));
1378     /// let o: Option<&mut String> = Option::from(&mut s);
1379     ///
1380     /// match o {
1381     ///     Some(t) => *t = String::from("Hello, Rustaceans!"),
1382     ///     None => (),
1383     /// }
1384     ///
1385     /// assert_eq!(s, Some(String::from("Hello, Rustaceans!")));
1386     /// ```
1387     fn from(o: &'a mut Option<T>) -> Option<&'a mut T> {
1388         o.as_mut()
1389     }
1390 }
1391
1392 /////////////////////////////////////////////////////////////////////////////
1393 // The Option Iterators
1394 /////////////////////////////////////////////////////////////////////////////
1395
1396 #[derive(Clone, Debug)]
1397 struct Item<A> {
1398     opt: Option<A>,
1399 }
1400
1401 impl<A> Iterator for Item<A> {
1402     type Item = A;
1403
1404     #[inline]
1405     fn next(&mut self) -> Option<A> {
1406         self.opt.take()
1407     }
1408
1409     #[inline]
1410     fn size_hint(&self) -> (usize, Option<usize>) {
1411         match self.opt {
1412             Some(_) => (1, Some(1)),
1413             None => (0, Some(0)),
1414         }
1415     }
1416 }
1417
1418 impl<A> DoubleEndedIterator for Item<A> {
1419     #[inline]
1420     fn next_back(&mut self) -> Option<A> {
1421         self.opt.take()
1422     }
1423 }
1424
1425 impl<A> ExactSizeIterator for Item<A> {}
1426 impl<A> FusedIterator for Item<A> {}
1427 unsafe impl<A> TrustedLen for Item<A> {}
1428
1429 /// An iterator over a reference to the [`Some`] variant of an [`Option`].
1430 ///
1431 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1432 ///
1433 /// This `struct` is created by the [`Option::iter`] function.
1434 #[stable(feature = "rust1", since = "1.0.0")]
1435 #[derive(Debug)]
1436 pub struct Iter<'a, A: 'a> {
1437     inner: Item<&'a A>,
1438 }
1439
1440 #[stable(feature = "rust1", since = "1.0.0")]
1441 impl<'a, A> Iterator for Iter<'a, A> {
1442     type Item = &'a A;
1443
1444     #[inline]
1445     fn next(&mut self) -> Option<&'a A> {
1446         self.inner.next()
1447     }
1448     #[inline]
1449     fn size_hint(&self) -> (usize, Option<usize>) {
1450         self.inner.size_hint()
1451     }
1452 }
1453
1454 #[stable(feature = "rust1", since = "1.0.0")]
1455 impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
1456     #[inline]
1457     fn next_back(&mut self) -> Option<&'a A> {
1458         self.inner.next_back()
1459     }
1460 }
1461
1462 #[stable(feature = "rust1", since = "1.0.0")]
1463 impl<A> ExactSizeIterator for Iter<'_, A> {}
1464
1465 #[stable(feature = "fused", since = "1.26.0")]
1466 impl<A> FusedIterator for Iter<'_, A> {}
1467
1468 #[unstable(feature = "trusted_len", issue = "37572")]
1469 unsafe impl<A> TrustedLen for Iter<'_, A> {}
1470
1471 #[stable(feature = "rust1", since = "1.0.0")]
1472 impl<A> Clone for Iter<'_, A> {
1473     #[inline]
1474     fn clone(&self) -> Self {
1475         Iter { inner: self.inner.clone() }
1476     }
1477 }
1478
1479 /// An iterator over a mutable reference to the [`Some`] variant of an [`Option`].
1480 ///
1481 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1482 ///
1483 /// This `struct` is created by the [`Option::iter_mut`] function.
1484 #[stable(feature = "rust1", since = "1.0.0")]
1485 #[derive(Debug)]
1486 pub struct IterMut<'a, A: 'a> {
1487     inner: Item<&'a mut A>,
1488 }
1489
1490 #[stable(feature = "rust1", since = "1.0.0")]
1491 impl<'a, A> Iterator for IterMut<'a, A> {
1492     type Item = &'a mut A;
1493
1494     #[inline]
1495     fn next(&mut self) -> Option<&'a mut A> {
1496         self.inner.next()
1497     }
1498     #[inline]
1499     fn size_hint(&self) -> (usize, Option<usize>) {
1500         self.inner.size_hint()
1501     }
1502 }
1503
1504 #[stable(feature = "rust1", since = "1.0.0")]
1505 impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
1506     #[inline]
1507     fn next_back(&mut self) -> Option<&'a mut A> {
1508         self.inner.next_back()
1509     }
1510 }
1511
1512 #[stable(feature = "rust1", since = "1.0.0")]
1513 impl<A> ExactSizeIterator for IterMut<'_, A> {}
1514
1515 #[stable(feature = "fused", since = "1.26.0")]
1516 impl<A> FusedIterator for IterMut<'_, A> {}
1517 #[unstable(feature = "trusted_len", issue = "37572")]
1518 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
1519
1520 /// An iterator over the value in [`Some`] variant of an [`Option`].
1521 ///
1522 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1523 ///
1524 /// This `struct` is created by the [`Option::into_iter`] function.
1525 #[derive(Clone, Debug)]
1526 #[stable(feature = "rust1", since = "1.0.0")]
1527 pub struct IntoIter<A> {
1528     inner: Item<A>,
1529 }
1530
1531 #[stable(feature = "rust1", since = "1.0.0")]
1532 impl<A> Iterator for IntoIter<A> {
1533     type Item = A;
1534
1535     #[inline]
1536     fn next(&mut self) -> Option<A> {
1537         self.inner.next()
1538     }
1539     #[inline]
1540     fn size_hint(&self) -> (usize, Option<usize>) {
1541         self.inner.size_hint()
1542     }
1543 }
1544
1545 #[stable(feature = "rust1", since = "1.0.0")]
1546 impl<A> DoubleEndedIterator for IntoIter<A> {
1547     #[inline]
1548     fn next_back(&mut self) -> Option<A> {
1549         self.inner.next_back()
1550     }
1551 }
1552
1553 #[stable(feature = "rust1", since = "1.0.0")]
1554 impl<A> ExactSizeIterator for IntoIter<A> {}
1555
1556 #[stable(feature = "fused", since = "1.26.0")]
1557 impl<A> FusedIterator for IntoIter<A> {}
1558
1559 #[unstable(feature = "trusted_len", issue = "37572")]
1560 unsafe impl<A> TrustedLen for IntoIter<A> {}
1561
1562 /////////////////////////////////////////////////////////////////////////////
1563 // FromIterator
1564 /////////////////////////////////////////////////////////////////////////////
1565
1566 #[stable(feature = "rust1", since = "1.0.0")]
1567 impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
1568     /// Takes each element in the [`Iterator`]: if it is [`None`][Option::None],
1569     /// no further elements are taken, and the [`None`][Option::None] is
1570     /// returned. Should no [`None`][Option::None] occur, a container with the
1571     /// values of each [`Option`] is returned.
1572     ///
1573     /// # Examples
1574     ///
1575     /// Here is an example which increments every integer in a vector.
1576     /// We use the checked variant of `add` that returns `None` when the
1577     /// calculation would result in an overflow.
1578     ///
1579     /// ```
1580     /// let items = vec![0_u16, 1, 2];
1581     ///
1582     /// let res: Option<Vec<u16>> = items
1583     ///     .iter()
1584     ///     .map(|x| x.checked_add(1))
1585     ///     .collect();
1586     ///
1587     /// assert_eq!(res, Some(vec![1, 2, 3]));
1588     /// ```
1589     ///
1590     /// As you can see, this will return the expected, valid items.
1591     ///
1592     /// Here is another example that tries to subtract one from another list
1593     /// of integers, this time checking for underflow:
1594     ///
1595     /// ```
1596     /// let items = vec![2_u16, 1, 0];
1597     ///
1598     /// let res: Option<Vec<u16>> = items
1599     ///     .iter()
1600     ///     .map(|x| x.checked_sub(1))
1601     ///     .collect();
1602     ///
1603     /// assert_eq!(res, None);
1604     /// ```
1605     ///
1606     /// Since the last element is zero, it would underflow. Thus, the resulting
1607     /// value is `None`.
1608     ///
1609     /// Here is a variation on the previous example, showing that no
1610     /// further elements are taken from `iter` after the first `None`.
1611     ///
1612     /// ```
1613     /// let items = vec![3_u16, 2, 1, 10];
1614     ///
1615     /// let mut shared = 0;
1616     ///
1617     /// let res: Option<Vec<u16>> = items
1618     ///     .iter()
1619     ///     .map(|x| { shared += x; x.checked_sub(2) })
1620     ///     .collect();
1621     ///
1622     /// assert_eq!(res, None);
1623     /// assert_eq!(shared, 6);
1624     /// ```
1625     ///
1626     /// Since the third element caused an underflow, no further elements were taken,
1627     /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
1628     #[inline]
1629     fn from_iter<I: IntoIterator<Item = Option<A>>>(iter: I) -> Option<V> {
1630         // FIXME(#11084): This could be replaced with Iterator::scan when this
1631         // performance bug is closed.
1632
1633         iter.into_iter().map(|x| x.ok_or(())).collect::<Result<_, _>>().ok()
1634     }
1635 }
1636
1637 /// The error type that results from applying the try operator (`?`) to a `None` value. If you wish
1638 /// to allow `x?` (where `x` is an `Option<T>`) to be converted into your error type, you can
1639 /// implement `impl From<NoneError>` for `YourErrorType`. In that case, `x?` within a function that
1640 /// returns `Result<_, YourErrorType>` will translate a `None` value into an `Err` result.
1641 #[rustc_diagnostic_item = "none_error"]
1642 #[unstable(feature = "try_trait", issue = "42327")]
1643 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
1644 pub struct NoneError;
1645
1646 #[unstable(feature = "try_trait", issue = "42327")]
1647 impl<T> ops::Try for Option<T> {
1648     type Ok = T;
1649     type Error = NoneError;
1650
1651     #[inline]
1652     fn into_result(self) -> Result<T, NoneError> {
1653         self.ok_or(NoneError)
1654     }
1655
1656     #[inline]
1657     fn from_ok(v: T) -> Self {
1658         Some(v)
1659     }
1660
1661     #[inline]
1662     fn from_error(_: NoneError) -> Self {
1663         None
1664     }
1665 }
1666
1667 #[unstable(feature = "try_trait_v2", issue = "84277")]
1668 impl<T> ops::TryV2 for Option<T> {
1669     type Output = T;
1670     type Residual = Option<convert::Infallible>;
1671
1672     #[inline]
1673     fn from_output(output: Self::Output) -> Self {
1674         Some(output)
1675     }
1676
1677     #[inline]
1678     fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
1679         match self {
1680             Some(v) => ControlFlow::Continue(v),
1681             None => ControlFlow::Break(None),
1682         }
1683     }
1684 }
1685
1686 #[unstable(feature = "try_trait_v2", issue = "84277")]
1687 impl<T> ops::FromResidual for Option<T> {
1688     #[inline]
1689     fn from_residual(residual: Option<convert::Infallible>) -> Self {
1690         match residual {
1691             None => None,
1692         }
1693     }
1694 }
1695
1696 impl<T> Option<Option<T>> {
1697     /// Converts from `Option<Option<T>>` to `Option<T>`
1698     ///
1699     /// # Examples
1700     ///
1701     /// Basic usage:
1702     ///
1703     /// ```
1704     /// let x: Option<Option<u32>> = Some(Some(6));
1705     /// assert_eq!(Some(6), x.flatten());
1706     ///
1707     /// let x: Option<Option<u32>> = Some(None);
1708     /// assert_eq!(None, x.flatten());
1709     ///
1710     /// let x: Option<Option<u32>> = None;
1711     /// assert_eq!(None, x.flatten());
1712     /// ```
1713     ///
1714     /// Flattening only removes one level of nesting at a time:
1715     ///
1716     /// ```
1717     /// let x: Option<Option<Option<u32>>> = Some(Some(Some(6)));
1718     /// assert_eq!(Some(Some(6)), x.flatten());
1719     /// assert_eq!(Some(6), x.flatten().flatten());
1720     /// ```
1721     #[inline]
1722     #[stable(feature = "option_flattening", since = "1.40.0")]
1723     #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1724     pub const fn flatten(self) -> Option<T> {
1725         match self {
1726             Some(inner) => inner,
1727             None => None,
1728         }
1729     }
1730 }