src/libcore/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 //! This usage of [`Option`] to create safe nullable pointers is so
  74 //! common that Rust does special optimizations to make the
  75 //! representation of [`Option`]`<`[`Box<T>`]`>` a single pointer. Optional pointers
  76 //! in Rust are stored as efficiently as any other pointer type.
  77 //!
  78 //! # Examples
  79 //!
  80 //! Basic pattern matching on [`Option`]:
  81 //!
  82 //! ```
  83 //! let msg = Some("howdy");
  84 //!
  85 //! // Take a reference to the contained string
  86 //! if let Some(m) = &msg {
  87 //!     println!("{}", *m);
  88 //! }
  89 //!
  90 //! // Remove the contained string, destroying the Option
  91 //! let unwrapped_msg = msg.unwrap_or("default message");
  92 //! ```
  93 //!
  94 //! Initialize a result to [`None`] before a loop:
  95 //!
  96 //! ```
  97 //! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }
  98 //!
  99 //! // A list of data to search through.
 100 //! let all_the_big_things = [
 101 //!     Kingdom::Plant(250, "redwood"),
 102 //!     Kingdom::Plant(230, "noble fir"),
 103 //!     Kingdom::Plant(229, "sugar pine"),
 104 //!     Kingdom::Animal(25, "blue whale"),
 105 //!     Kingdom::Animal(19, "fin whale"),
 106 //!     Kingdom::Animal(15, "north pacific right whale"),
 107 //! ];
 108 //!
 109 //! // We're going to search for the name of the biggest animal,
 110 //! // but to start with we've just got `None`.
 111 //! let mut name_of_biggest_animal = None;
 112 //! let mut size_of_biggest_animal = 0;
 113 //! for big_thing in &all_the_big_things {
 114 //!     match *big_thing {
 115 //!         Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
 116 //!             // Now we've found the name of some big animal
 117 //!             size_of_biggest_animal = size;
 118 //!             name_of_biggest_animal = Some(name);
 119 //!         }
 120 //!         Kingdom::Animal(..) | Kingdom::Plant(..) => ()
 121 //!     }
 122 //! }
 123 //!
 124 //! match name_of_biggest_animal {
 125 //!     Some(name) => println!("the biggest animal is {}", name),
 126 //!     None => println!("there are no animals :("),
 127 //! }
 128 //! ```
 129 //!
 130 //! [`Option`]: enum.Option.html
 131 //! [`Some`]: enum.Option.html#variant.Some
 132 //! [`None`]: enum.Option.html#variant.None
 133 //! [`Box<T>`]: ../../std/boxed/struct.Box.html
 134 //! [`i32`]: ../../std/primitive.i32.html
 135
 136 // ignore-tidy-undocumented-unsafe
 137
 138 #![stable(feature = "rust1", since = "1.0.0")]
 139
 140 use crate::iter::{FromIterator, FusedIterator, TrustedLen};
 141 use crate::pin::Pin;
 142 use crate::{
 143     convert, fmt, hint, mem,
 144     ops::{self, Deref, DerefMut},
 145 };
 146
 147 // Note that this is not a lang item per se, but it has a hidden dependency on
 148 // `Iterator`, which is one. The compiler assumes that the `next` method of
 149 // `Iterator` is an enumeration with one type parameter and two variants,
 150 // which basically means it must be `Option`.
 151
 152 /// The `Option` type. See [the module level documentation](index.html) for more.
 153 #[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
 154 #[rustc_diagnostic_item = "option_type"]
 155 #[stable(feature = "rust1", since = "1.0.0")]
 156 pub enum Option<T> {
 157     /// No value
 158     #[stable(feature = "rust1", since = "1.0.0")]
 159     None,
 160     /// Some value `T`
 161     #[stable(feature = "rust1", since = "1.0.0")]
 162     Some(#[stable(feature = "rust1", since = "1.0.0")] T),
 163 }
 164
 165 /////////////////////////////////////////////////////////////////////////////
 166 // Type implementation
 167 /////////////////////////////////////////////////////////////////////////////
 168
 169 impl<T> Option<T> {
 170     /////////////////////////////////////////////////////////////////////////
 171     // Querying the contained values
 172     /////////////////////////////////////////////////////////////////////////
 173
 174     /// Returns `true` if the option is a [`Some`] value.
 175     ///
 176     /// # Examples
 177     ///
 178     /// ```
 179     /// let x: Option<u32> = Some(2);
 180     /// assert_eq!(x.is_some(), true);
 181     ///
 182     /// let x: Option<u32> = None;
 183     /// assert_eq!(x.is_some(), false);
 184     /// ```
 185     ///
 186     /// [`Some`]: #variant.Some
 187     #[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"]
 188     #[inline]
 189     #[stable(feature = "rust1", since = "1.0.0")]
 190     pub fn is_some(&self) -> bool {
 191         matches!(*self, Some(_))
 192     }
 193
 194     /// Returns `true` if the option is a [`None`] value.
 195     ///
 196     /// # Examples
 197     ///
 198     /// ```
 199     /// let x: Option<u32> = Some(2);
 200     /// assert_eq!(x.is_none(), false);
 201     ///
 202     /// let x: Option<u32> = None;
 203     /// assert_eq!(x.is_none(), true);
 204     /// ```
 205     ///
 206     /// [`None`]: #variant.None
 207     #[must_use = "if you intended to assert that this doesn't have a value, consider \
 208                   `.and_then(|| panic!(\"`Option` had a value when expected `None`\"))` instead"]
 209     #[inline]
 210     #[stable(feature = "rust1", since = "1.0.0")]
 211     pub fn is_none(&self) -> bool {
 212         !self.is_some()
 213     }
 214
 215     /// Returns `true` if the option is a [`Some`] value containing the given value.
 216     ///
 217     /// # Examples
 218     ///
 219     /// ```
 220     /// #![feature(option_result_contains)]
 221     ///
 222     /// let x: Option<u32> = Some(2);
 223     /// assert_eq!(x.contains(&2), true);
 224     ///
 225     /// let x: Option<u32> = Some(3);
 226     /// assert_eq!(x.contains(&2), false);
 227     ///
 228     /// let x: Option<u32> = None;
 229     /// assert_eq!(x.contains(&2), false);
 230     /// ```
 231     #[must_use]
 232     #[inline]
 233     #[unstable(feature = "option_result_contains", issue = "62358")]
 234     pub fn contains<U>(&self, x: &U) -> bool
 235     where
 236         U: PartialEq<T>,
 237     {
 238         match self {
 239             Some(y) => x == y,
 240             None => false,
 241         }
 242     }
 243
 244     /////////////////////////////////////////////////////////////////////////
 245     // Adapter for working with references
 246     /////////////////////////////////////////////////////////////////////////
 247
 248     /// Converts from `&Option<T>` to `Option<&T>`.
 249     ///
 250     /// # Examples
 251     ///
 252     /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, preserving the original.
 253     /// The [`map`] method takes the `self` argument by value, consuming the original,
 254     /// so this technique uses `as_ref` to first take an `Option` to a reference
 255     /// to the value inside the original.
 256     ///
 257     /// [`map`]: enum.Option.html#method.map
 258     /// [`String`]: ../../std/string/struct.String.html
 259     /// [`usize`]: ../../std/primitive.usize.html
 260     ///
 261     /// ```
 262     /// let text: Option<String> = Some("Hello, world!".to_string());
 263     /// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
 264     /// // then consume *that* with `map`, leaving `text` on the stack.
 265     /// let text_length: Option<usize> = text.as_ref().map(|s| s.len());
 266     /// println!("still can print text: {:?}", text);
 267     /// ```
 268     #[inline]
 269     #[stable(feature = "rust1", since = "1.0.0")]
 270     pub fn as_ref(&self) -> Option<&T> {
 271         match *self {
 272             Some(ref x) => Some(x),
 273             None => None,
 274         }
 275     }
 276
 277     /// Converts from `&mut Option<T>` to `Option<&mut T>`.
 278     ///
 279     /// # Examples
 280     ///
 281     /// ```
 282     /// let mut x = Some(2);
 283     /// match x.as_mut() {
 284     ///     Some(v) => *v = 42,
 285     ///     None => {},
 286     /// }
 287     /// assert_eq!(x, Some(42));
 288     /// ```
 289     #[inline]
 290     #[stable(feature = "rust1", since = "1.0.0")]
 291     pub fn as_mut(&mut self) -> Option<&mut T> {
 292         match *self {
 293             Some(ref mut x) => Some(x),
 294             None => None,
 295         }
 296     }
 297
 298     /// Converts from [`Pin`]`<&Option<T>>` to `Option<`[`Pin`]`<&T>>`.
 299     ///
 300     /// [`Pin`]: ../pin/struct.Pin.html
 301     #[inline]
 302     #[stable(feature = "pin", since = "1.33.0")]
 303     pub fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> {
 304         unsafe { Pin::get_ref(self).as_ref().map(|x| Pin::new_unchecked(x)) }
 305     }
 306
 307     /// Converts from [`Pin`]`<&mut Option<T>>` to `Option<`[`Pin`]`<&mut T>>`.
 308     ///
 309     /// [`Pin`]: ../pin/struct.Pin.html
 310     #[inline]
 311     #[stable(feature = "pin", since = "1.33.0")]
 312     pub fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> {
 313         unsafe { Pin::get_unchecked_mut(self).as_mut().map(|x| Pin::new_unchecked(x)) }
 314     }
 315
 316     /////////////////////////////////////////////////////////////////////////
 317     // Getting to contained values
 318     /////////////////////////////////////////////////////////////////////////
 319
 320     /// Returns the contained [`Some`] value, consuming the `self` value.
 321     ///
 322     /// # Panics
 323     ///
 324     /// Panics if the value is a [`None`] with a custom panic message provided by
 325     /// `msg`.
 326     ///
 327     /// [`Some`]: #variant.Some
 328     /// [`None`]: #variant.None
 329     ///
 330     /// # Examples
 331     ///
 332     /// ```
 333     /// let x = Some("value");
 334     /// assert_eq!(x.expect("fruits are healthy"), "value");
 335     /// ```
 336     ///
 337     /// ```{.should_panic}
 338     /// let x: Option<&str> = None;
 339     /// x.expect("fruits are healthy"); // panics with `fruits are healthy`
 340     /// ```
 341     #[inline]
 342     #[track_caller]
 343     #[stable(feature = "rust1", since = "1.0.0")]
 344     pub fn expect(self, msg: &str) -> T {
 345         match self {
 346             Some(val) => val,
 347             None => expect_failed(msg),
 348         }
 349     }
 350
 351     /// Returns the contained [`Some`] value, consuming the `self` value.
 352     ///
 353     /// Because this function may panic, its use is generally discouraged.
 354     /// Instead, prefer to use pattern matching and handle the [`None`]
 355     /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
 356     /// [`unwrap_or_default`].
 357     ///
 358     /// [`unwrap_or`]: #method.unwrap_or
 359     /// [`unwrap_or_else`]: #method.unwrap_or_else
 360     /// [`unwrap_or_default`]: #method.unwrap_or_default
 361     ///
 362     /// # Panics
 363     ///
 364     /// Panics if the self value equals [`None`].
 365     ///
 366     /// [`Some`]: #variant.Some
 367     /// [`None`]: #variant.None
 368     ///
 369     /// # Examples
 370     ///
 371     /// ```
 372     /// let x = Some("air");
 373     /// assert_eq!(x.unwrap(), "air");
 374     /// ```
 375     ///
 376     /// ```{.should_panic}
 377     /// let x: Option<&str> = None;
 378     /// assert_eq!(x.unwrap(), "air"); // fails
 379     /// ```
 380     #[inline]
 381     #[track_caller]
 382     #[stable(feature = "rust1", since = "1.0.0")]
 383     pub 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     /// [`Some`]: #variant.Some
 397     /// [`unwrap_or_else`]: #method.unwrap_or_else
 398     ///
 399     /// # Examples
 400     ///
 401     /// ```
 402     /// assert_eq!(Some("car").unwrap_or("bike"), "car");
 403     /// assert_eq!(None.unwrap_or("bike"), "bike");
 404     /// ```
 405     #[inline]
 406     #[stable(feature = "rust1", since = "1.0.0")]
 407     pub fn unwrap_or(self, default: T) -> T {
 408         match self {
 409             Some(x) => x,
 410             None => default,
 411         }
 412     }
 413
 414     /// Returns the contained [`Some`] value or computes it from a closure.
 415     ///
 416     /// # Examples
 417     ///
 418     /// ```
 419     /// let k = 10;
 420     /// assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
 421     /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20);
 422     /// ```
 423     #[inline]
 424     #[stable(feature = "rust1", since = "1.0.0")]
 425     pub fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T {
 426         match self {
 427             Some(x) => x,
 428             None => f(),
 429         }
 430     }
 431
 432     /////////////////////////////////////////////////////////////////////////
 433     // Transforming contained values
 434     /////////////////////////////////////////////////////////////////////////
 435
 436     /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value.
 437     ///
 438     /// # Examples
 439     ///
 440     /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, consuming the original:
 441     ///
 442     /// [`String`]: ../../std/string/struct.String.html
 443     /// [`usize`]: ../../std/primitive.usize.html
 444     ///
 445     /// ```
 446     /// let maybe_some_string = Some(String::from("Hello, World!"));
 447     /// // `Option::map` takes self *by value*, consuming `maybe_some_string`
 448     /// let maybe_some_len = maybe_some_string.map(|s| s.len());
 449     ///
 450     /// assert_eq!(maybe_some_len, Some(13));
 451     /// ```
 452     #[inline]
 453     #[stable(feature = "rust1", since = "1.0.0")]
 454     pub fn map<U, F: FnOnce(T) -> U>(self, f: F) -> Option<U> {
 455         match self {
 456             Some(x) => Some(f(x)),
 457             None => None,
 458         }
 459     }
 460
 461     /// Applies a function to the contained value (if any),
 462     /// or returns the provided default (if not).
 463     ///
 464     /// Arguments passed to `map_or` are eagerly evaluated; if you are passing
 465     /// the result of a function call, it is recommended to use [`map_or_else`],
 466     /// which is lazily evaluated.
 467     ///
 468     /// [`map_or_else`]: #method.map_or_else
 469     ///
 470     /// # Examples
 471     ///
 472     /// ```
 473     /// let x = Some("foo");
 474     /// assert_eq!(x.map_or(42, |v| v.len()), 3);
 475     ///
 476     /// let x: Option<&str> = None;
 477     /// assert_eq!(x.map_or(42, |v| v.len()), 42);
 478     /// ```
 479     #[inline]
 480     #[stable(feature = "rust1", since = "1.0.0")]
 481     pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
 482         match self {
 483             Some(t) => f(t),
 484             None => default,
 485         }
 486     }
 487
 488     /// Applies a function to the contained value (if any),
 489     /// or computes a default (if not).
 490     ///
 491     /// # Examples
 492     ///
 493     /// ```
 494     /// let k = 21;
 495     ///
 496     /// let x = Some("foo");
 497     /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);
 498     ///
 499     /// let x: Option<&str> = None;
 500     /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);
 501     /// ```
 502     #[inline]
 503     #[stable(feature = "rust1", since = "1.0.0")]
 504     pub fn map_or_else<U, D: FnOnce() -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
 505         match self {
 506             Some(t) => f(t),
 507             None => default(),
 508         }
 509     }
 510
 511     /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
 512     /// [`Ok(v)`] and [`None`] to [`Err(err)`].
 513     ///
 514     /// Arguments passed to `ok_or` are eagerly evaluated; if you are passing the
 515     /// result of a function call, it is recommended to use [`ok_or_else`], which is
 516     /// lazily evaluated.
 517     ///
 518     /// [`Result<T, E>`]: ../../std/result/enum.Result.html
 519     /// [`Ok(v)`]: ../../std/result/enum.Result.html#variant.Ok
 520     /// [`Err(err)`]: ../../std/result/enum.Result.html#variant.Err
 521     /// [`None`]: #variant.None
 522     /// [`Some(v)`]: #variant.Some
 523     /// [`ok_or_else`]: #method.ok_or_else
 524     ///
 525     /// # Examples
 526     ///
 527     /// ```
 528     /// let x = Some("foo");
 529     /// assert_eq!(x.ok_or(0), Ok("foo"));
 530     ///
 531     /// let x: Option<&str> = None;
 532     /// assert_eq!(x.ok_or(0), Err(0));
 533     /// ```
 534     #[inline]
 535     #[stable(feature = "rust1", since = "1.0.0")]
 536     pub fn ok_or<E>(self, err: E) -> Result<T, E> {
 537         match self {
 538             Some(v) => Ok(v),
 539             None => Err(err),
 540         }
 541     }
 542
 543     /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
 544     /// [`Ok(v)`] and [`None`] to [`Err(err())`].
 545     ///
 546     /// [`Result<T, E>`]: ../../std/result/enum.Result.html
 547     /// [`Ok(v)`]: ../../std/result/enum.Result.html#variant.Ok
 548     /// [`Err(err())`]: ../../std/result/enum.Result.html#variant.Err
 549     /// [`None`]: #variant.None
 550     /// [`Some(v)`]: #variant.Some
 551     ///
 552     /// # Examples
 553     ///
 554     /// ```
 555     /// let x = Some("foo");
 556     /// assert_eq!(x.ok_or_else(|| 0), Ok("foo"));
 557     ///
 558     /// let x: Option<&str> = None;
 559     /// assert_eq!(x.ok_or_else(|| 0), Err(0));
 560     /// ```
 561     #[inline]
 562     #[stable(feature = "rust1", since = "1.0.0")]
 563     pub fn ok_or_else<E, F: FnOnce() -> E>(self, err: F) -> Result<T, E> {
 564         match self {
 565             Some(v) => Ok(v),
 566             None => Err(err()),
 567         }
 568     }
 569
 570     /////////////////////////////////////////////////////////////////////////
 571     // Iterator constructors
 572     /////////////////////////////////////////////////////////////////////////
 573
 574     /// Returns an iterator over the possibly contained value.
 575     ///
 576     /// # Examples
 577     ///
 578     /// ```
 579     /// let x = Some(4);
 580     /// assert_eq!(x.iter().next(), Some(&4));
 581     ///
 582     /// let x: Option<u32> = None;
 583     /// assert_eq!(x.iter().next(), None);
 584     /// ```
 585     #[inline]
 586     #[stable(feature = "rust1", since = "1.0.0")]
 587     pub fn iter(&self) -> Iter<'_, T> {
 588         Iter { inner: Item { opt: self.as_ref() } }
 589     }
 590
 591     /// Returns a mutable iterator over the possibly contained value.
 592     ///
 593     /// # Examples
 594     ///
 595     /// ```
 596     /// let mut x = Some(4);
 597     /// match x.iter_mut().next() {
 598     ///     Some(v) => *v = 42,
 599     ///     None => {},
 600     /// }
 601     /// assert_eq!(x, Some(42));
 602     ///
 603     /// let mut x: Option<u32> = None;
 604     /// assert_eq!(x.iter_mut().next(), None);
 605     /// ```
 606     #[inline]
 607     #[stable(feature = "rust1", since = "1.0.0")]
 608     pub fn iter_mut(&mut self) -> IterMut<'_, T> {
 609         IterMut { inner: Item { opt: self.as_mut() } }
 610     }
 611
 612     /////////////////////////////////////////////////////////////////////////
 613     // Boolean operations on the values, eager and lazy
 614     /////////////////////////////////////////////////////////////////////////
 615
 616     /// Returns [`None`] if the option is [`None`], otherwise returns `optb`.
 617     ///
 618     /// [`None`]: #variant.None
 619     ///
 620     /// # Examples
 621     ///
 622     /// ```
 623     /// let x = Some(2);
 624     /// let y: Option<&str> = None;
 625     /// assert_eq!(x.and(y), None);
 626     ///
 627     /// let x: Option<u32> = None;
 628     /// let y = Some("foo");
 629     /// assert_eq!(x.and(y), None);
 630     ///
 631     /// let x = Some(2);
 632     /// let y = Some("foo");
 633     /// assert_eq!(x.and(y), Some("foo"));
 634     ///
 635     /// let x: Option<u32> = None;
 636     /// let y: Option<&str> = None;
 637     /// assert_eq!(x.and(y), None);
 638     /// ```
 639     #[inline]
 640     #[stable(feature = "rust1", since = "1.0.0")]
 641     pub fn and<U>(self, optb: Option<U>) -> Option<U> {
 642         match self {
 643             Some(_) => optb,
 644             None => None,
 645         }
 646     }
 647
 648     /// Returns [`None`] if the option is [`None`], otherwise calls `f` with the
 649     /// wrapped value and returns the result.
 650     ///
 651     /// Some languages call this operation flatmap.
 652     ///
 653     /// [`None`]: #variant.None
 654     ///
 655     /// # Examples
 656     ///
 657     /// ```
 658     /// fn sq(x: u32) -> Option<u32> { Some(x * x) }
 659     /// fn nope(_: u32) -> Option<u32> { None }
 660     ///
 661     /// assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16));
 662     /// assert_eq!(Some(2).and_then(sq).and_then(nope), None);
 663     /// assert_eq!(Some(2).and_then(nope).and_then(sq), None);
 664     /// assert_eq!(None.and_then(sq).and_then(sq), None);
 665     /// ```
 666     #[inline]
 667     #[stable(feature = "rust1", since = "1.0.0")]
 668     pub fn and_then<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> Option<U> {
 669         match self {
 670             Some(x) => f(x),
 671             None => None,
 672         }
 673     }
 674
 675     /// Returns [`None`] if the option is [`None`], otherwise calls `predicate`
 676     /// with the wrapped value and returns:
 677     ///
 678     /// - [`Some(t)`] if `predicate` returns `true` (where `t` is the wrapped
 679     ///   value), and
 680     /// - [`None`] if `predicate` returns `false`.
 681     ///
 682     /// This function works similar to [`Iterator::filter()`]. You can imagine
 683     /// the `Option<T>` being an iterator over one or zero elements. `filter()`
 684     /// lets you decide which elements to keep.
 685     ///
 686     /// # Examples
 687     ///
 688     /// ```rust
 689     /// fn is_even(n: &i32) -> bool {
 690     ///     n % 2 == 0
 691     /// }
 692     ///
 693     /// assert_eq!(None.filter(is_even), None);
 694     /// assert_eq!(Some(3).filter(is_even), None);
 695     /// assert_eq!(Some(4).filter(is_even), Some(4));
 696     /// ```
 697     ///
 698     /// [`None`]: #variant.None
 699     /// [`Some(t)`]: #variant.Some
 700     /// [`Iterator::filter()`]: ../../std/iter/trait.Iterator.html#method.filter
 701     #[inline]
 702     #[stable(feature = "option_filter", since = "1.27.0")]
 703     pub fn filter<P: FnOnce(&T) -> bool>(self, predicate: P) -> Self {
 704         if let Some(x) = self {
 705             if predicate(&x) {
 706                 return Some(x);
 707             }
 708         }
 709         None
 710     }
 711
 712     /// Returns the option if it contains a value, otherwise returns `optb`.
 713     ///
 714     /// Arguments passed to `or` are eagerly evaluated; if you are passing the
 715     /// result of a function call, it is recommended to use [`or_else`], which is
 716     /// lazily evaluated.
 717     ///
 718     /// [`or_else`]: #method.or_else
 719     ///
 720     /// # Examples
 721     ///
 722     /// ```
 723     /// let x = Some(2);
 724     /// let y = None;
 725     /// assert_eq!(x.or(y), Some(2));
 726     ///
 727     /// let x = None;
 728     /// let y = Some(100);
 729     /// assert_eq!(x.or(y), Some(100));
 730     ///
 731     /// let x = Some(2);
 732     /// let y = Some(100);
 733     /// assert_eq!(x.or(y), Some(2));
 734     ///
 735     /// let x: Option<u32> = None;
 736     /// let y = None;
 737     /// assert_eq!(x.or(y), None);
 738     /// ```
 739     #[inline]
 740     #[stable(feature = "rust1", since = "1.0.0")]
 741     pub fn or(self, optb: Option<T>) -> Option<T> {
 742         match self {
 743             Some(_) => self,
 744             None => optb,
 745         }
 746     }
 747
 748     /// Returns the option if it contains a value, otherwise calls `f` and
 749     /// returns the result.
 750     ///
 751     /// # Examples
 752     ///
 753     /// ```
 754     /// fn nobody() -> Option<&'static str> { None }
 755     /// fn vikings() -> Option<&'static str> { Some("vikings") }
 756     ///
 757     /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
 758     /// assert_eq!(None.or_else(vikings), Some("vikings"));
 759     /// assert_eq!(None.or_else(nobody), None);
 760     /// ```
 761     #[inline]
 762     #[stable(feature = "rust1", since = "1.0.0")]
 763     pub fn or_else<F: FnOnce() -> Option<T>>(self, f: F) -> Option<T> {
 764         match self {
 765             Some(_) => self,
 766             None => f(),
 767         }
 768     }
 769
 770     /// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns [`None`].
 771     ///
 772     /// [`Some`]: #variant.Some
 773     /// [`None`]: #variant.None
 774     ///
 775     /// # Examples
 776     ///
 777     /// ```
 778     /// let x = Some(2);
 779     /// let y: Option<u32> = None;
 780     /// assert_eq!(x.xor(y), Some(2));
 781     ///
 782     /// let x: Option<u32> = None;
 783     /// let y = Some(2);
 784     /// assert_eq!(x.xor(y), Some(2));
 785     ///
 786     /// let x = Some(2);
 787     /// let y = Some(2);
 788     /// assert_eq!(x.xor(y), None);
 789     ///
 790     /// let x: Option<u32> = None;
 791     /// let y: Option<u32> = None;
 792     /// assert_eq!(x.xor(y), None);
 793     /// ```
 794     #[inline]
 795     #[stable(feature = "option_xor", since = "1.37.0")]
 796     pub fn xor(self, optb: Option<T>) -> Option<T> {
 797         match (self, optb) {
 798             (Some(a), None) => Some(a),
 799             (None, Some(b)) => Some(b),
 800             _ => None,
 801         }
 802     }
 803
 804     /////////////////////////////////////////////////////////////////////////
 805     // Entry-like operations to insert if None and return a reference
 806     /////////////////////////////////////////////////////////////////////////
 807
 808     /// Inserts `v` into the option if it is [`None`], then
 809     /// returns a mutable reference to the contained value.
 810     ///
 811     /// [`None`]: #variant.None
 812     ///
 813     /// # Examples
 814     ///
 815     /// ```
 816     /// let mut x = None;
 817     ///
 818     /// {
 819     ///     let y: &mut u32 = x.get_or_insert(5);
 820     ///     assert_eq!(y, &5);
 821     ///
 822     ///     *y = 7;
 823     /// }
 824     ///
 825     /// assert_eq!(x, Some(7));
 826     /// ```
 827     #[inline]
 828     #[stable(feature = "option_entry", since = "1.20.0")]
 829     pub fn get_or_insert(&mut self, v: T) -> &mut T {
 830         self.get_or_insert_with(|| v)
 831     }
 832
 833     /// Inserts a value computed from `f` into the option if it is [`None`], then
 834     /// returns a mutable reference to the contained value.
 835     ///
 836     /// [`None`]: #variant.None
 837     ///
 838     /// # Examples
 839     ///
 840     /// ```
 841     /// let mut x = None;
 842     ///
 843     /// {
 844     ///     let y: &mut u32 = x.get_or_insert_with(|| 5);
 845     ///     assert_eq!(y, &5);
 846     ///
 847     ///     *y = 7;
 848     /// }
 849     ///
 850     /// assert_eq!(x, Some(7));
 851     /// ```
 852     #[inline]
 853     #[stable(feature = "option_entry", since = "1.20.0")]
 854     pub fn get_or_insert_with<F: FnOnce() -> T>(&mut self, f: F) -> &mut T {
 855         if let None = *self {
 856             *self = Some(f());
 857         }
 858
 859         match *self {
 860             Some(ref mut v) => v,
 861             None => unsafe { hint::unreachable_unchecked() },
 862         }
 863     }
 864
 865     /////////////////////////////////////////////////////////////////////////
 866     // Misc
 867     /////////////////////////////////////////////////////////////////////////
 868
 869     /// Takes the value out of the option, leaving a [`None`] in its place.
 870     ///
 871     /// [`None`]: #variant.None
 872     ///
 873     /// # Examples
 874     ///
 875     /// ```
 876     /// let mut x = Some(2);
 877     /// let y = x.take();
 878     /// assert_eq!(x, None);
 879     /// assert_eq!(y, Some(2));
 880     ///
 881     /// let mut x: Option<u32> = None;
 882     /// let y = x.take();
 883     /// assert_eq!(x, None);
 884     /// assert_eq!(y, None);
 885     /// ```
 886     #[inline]
 887     #[stable(feature = "rust1", since = "1.0.0")]
 888     pub fn take(&mut self) -> Option<T> {
 889         mem::take(self)
 890     }
 891
 892     /// Replaces the actual value in the option by the value given in parameter,
 893     /// returning the old value if present,
 894     /// leaving a [`Some`] in its place without deinitializing either one.
 895     ///
 896     /// [`Some`]: #variant.Some
 897     ///
 898     /// # Examples
 899     ///
 900     /// ```
 901     /// let mut x = Some(2);
 902     /// let old = x.replace(5);
 903     /// assert_eq!(x, Some(5));
 904     /// assert_eq!(old, Some(2));
 905     ///
 906     /// let mut x = None;
 907     /// let old = x.replace(3);
 908     /// assert_eq!(x, Some(3));
 909     /// assert_eq!(old, None);
 910     /// ```
 911     #[inline]
 912     #[stable(feature = "option_replace", since = "1.31.0")]
 913     pub fn replace(&mut self, value: T) -> Option<T> {
 914         mem::replace(self, Some(value))
 915     }
 916
 917     /// Zips `self` with another `Option`.
 918     ///
 919     /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some((s, o))`.
 920     /// Otherwise, `None` is returned.
 921     ///
 922     /// # Examples
 923     ///
 924     /// ```
 925     /// #![feature(option_zip)]
 926     /// let x = Some(1);
 927     /// let y = Some("hi");
 928     /// let z = None::<u8>;
 929     ///
 930     /// assert_eq!(x.zip(y), Some((1, "hi")));
 931     /// assert_eq!(x.zip(z), None);
 932     /// ```
 933     #[unstable(feature = "option_zip", issue = "70086")]
 934     pub fn zip<U>(self, other: Option<U>) -> Option<(T, U)> {
 935         self.zip_with(other, |a, b| (a, b))
 936     }
 937
 938     /// Zips `self` and another `Option` with function `f`.
 939     ///
 940     /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`.
 941     /// Otherwise, `None` is returned.
 942     ///
 943     /// # Examples
 944     ///
 945     /// ```
 946     /// #![feature(option_zip)]
 947     ///
 948     /// #[derive(Debug, PartialEq)]
 949     /// struct Point {
 950     ///     x: f64,
 951     ///     y: f64,
 952     /// }
 953     ///
 954     /// impl Point {
 955     ///     fn new(x: f64, y: f64) -> Self {
 956     ///         Self { x, y }
 957     ///     }
 958     /// }
 959     ///
 960     /// let x = Some(17.5);
 961     /// let y = Some(42.7);
 962     ///
 963     /// assert_eq!(x.zip_with(y, Point::new), Some(Point { x: 17.5, y: 42.7 }));
 964     /// assert_eq!(x.zip_with(None, Point::new), None);
 965     /// ```
 966     #[unstable(feature = "option_zip", issue = "70086")]
 967     pub fn zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R>
 968     where
 969         F: FnOnce(T, U) -> R,
 970     {
 971         Some(f(self?, other?))
 972     }
 973 }
 974
 975 impl<T: Copy> Option<&T> {
 976     /// Maps an `Option<&T>` to an `Option<T>` by copying the contents of the
 977     /// option.
 978     ///
 979     /// # Examples
 980     ///
 981     /// ```
 982     /// let x = 12;
 983     /// let opt_x = Some(&x);
 984     /// assert_eq!(opt_x, Some(&12));
 985     /// let copied = opt_x.copied();
 986     /// assert_eq!(copied, Some(12));
 987     /// ```
 988     #[stable(feature = "copied", since = "1.35.0")]
 989     pub fn copied(self) -> Option<T> {
 990         self.map(|&t| t)
 991     }
 992 }
 993
 994 impl<T: Copy> Option<&mut T> {
 995     /// Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the
 996     /// option.
 997     ///
 998     /// # Examples
 999     ///
1000     /// ```
1001     /// let mut x = 12;
1002     /// let opt_x = Some(&mut x);
1003     /// assert_eq!(opt_x, Some(&mut 12));
1004     /// let copied = opt_x.copied();
1005     /// assert_eq!(copied, Some(12));
1006     /// ```
1007     #[stable(feature = "copied", since = "1.35.0")]
1008     pub fn copied(self) -> Option<T> {
1009         self.map(|&mut t| t)
1010     }
1011 }
1012
1013 impl<T: Clone> Option<&T> {
1014     /// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the
1015     /// option.
1016     ///
1017     /// # Examples
1018     ///
1019     /// ```
1020     /// let x = 12;
1021     /// let opt_x = Some(&x);
1022     /// assert_eq!(opt_x, Some(&12));
1023     /// let cloned = opt_x.cloned();
1024     /// assert_eq!(cloned, Some(12));
1025     /// ```
1026     #[stable(feature = "rust1", since = "1.0.0")]
1027     pub fn cloned(self) -> Option<T> {
1028         self.map(|t| t.clone())
1029     }
1030 }
1031
1032 impl<T: Clone> Option<&mut T> {
1033     /// Maps an `Option<&mut T>` to an `Option<T>` by cloning the contents of the
1034     /// option.
1035     ///
1036     /// # Examples
1037     ///
1038     /// ```
1039     /// let mut x = 12;
1040     /// let opt_x = Some(&mut x);
1041     /// assert_eq!(opt_x, Some(&mut 12));
1042     /// let cloned = opt_x.cloned();
1043     /// assert_eq!(cloned, Some(12));
1044     /// ```
1045     #[stable(since = "1.26.0", feature = "option_ref_mut_cloned")]
1046     pub fn cloned(self) -> Option<T> {
1047         self.map(|t| t.clone())
1048     }
1049 }
1050
1051 impl<T: fmt::Debug> Option<T> {
1052     /// Consumes `self` while expecting [`None`] and returning nothing.
1053     ///
1054     /// # Panics
1055     ///
1056     /// Panics if the value is a [`Some`], with a panic message including the
1057     /// passed message, and the content of the [`Some`].
1058     ///
1059     /// [`Some`]: #variant.Some
1060     /// [`None`]: #variant.None
1061     ///
1062     /// # Examples
1063     ///
1064     /// ```
1065     /// #![feature(option_expect_none)]
1066     ///
1067     /// use std::collections::HashMap;
1068     /// let mut squares = HashMap::new();
1069     /// for i in -10..=10 {
1070     ///     // This will not panic, since all keys are unique.
1071     ///     squares.insert(i, i * i).expect_none("duplicate key");
1072     /// }
1073     /// ```
1074     ///
1075     /// ```{.should_panic}
1076     /// #![feature(option_expect_none)]
1077     ///
1078     /// use std::collections::HashMap;
1079     /// let mut sqrts = HashMap::new();
1080     /// for i in -10..=10 {
1081     ///     // This will panic, since both negative and positive `i` will
1082     ///     // insert the same `i * i` key, returning the old `Some(i)`.
1083     ///     sqrts.insert(i * i, i).expect_none("duplicate key");
1084     /// }
1085     /// ```
1086     #[inline]
1087     #[track_caller]
1088     #[unstable(feature = "option_expect_none", reason = "newly added", issue = "62633")]
1089     pub fn expect_none(self, msg: &str) {
1090         if let Some(val) = self {
1091             expect_none_failed(msg, &val);
1092         }
1093     }
1094
1095     /// Consumes `self` while expecting [`None`] and returning nothing.
1096     ///
1097     /// # Panics
1098     ///
1099     /// Panics if the value is a [`Some`], with a custom panic message provided
1100     /// by the [`Some`]'s value.
1101     ///
1102     /// [`Some(v)`]: #variant.Some
1103     /// [`None`]: #variant.None
1104     ///
1105     /// # Examples
1106     ///
1107     /// ```
1108     /// #![feature(option_unwrap_none)]
1109     ///
1110     /// use std::collections::HashMap;
1111     /// let mut squares = HashMap::new();
1112     /// for i in -10..=10 {
1113     ///     // This will not panic, since all keys are unique.
1114     ///     squares.insert(i, i * i).unwrap_none();
1115     /// }
1116     /// ```
1117     ///
1118     /// ```{.should_panic}
1119     /// #![feature(option_unwrap_none)]
1120     ///
1121     /// use std::collections::HashMap;
1122     /// let mut sqrts = HashMap::new();
1123     /// for i in -10..=10 {
1124     ///     // This will panic, since both negative and positive `i` will
1125     ///     // insert the same `i * i` key, returning the old `Some(i)`.
1126     ///     sqrts.insert(i * i, i).unwrap_none();
1127     /// }
1128     /// ```
1129     #[inline]
1130     #[track_caller]
1131     #[unstable(feature = "option_unwrap_none", reason = "newly added", issue = "62633")]
1132     pub fn unwrap_none(self) {
1133         if let Some(val) = self {
1134             expect_none_failed("called `Option::unwrap_none()` on a `Some` value", &val);
1135         }
1136     }
1137 }
1138
1139 impl<T: Default> Option<T> {
1140     /// Returns the contained [`Some`] value or a default
1141     ///
1142     /// Consumes the `self` argument then, if [`Some`], returns the contained
1143     /// value, otherwise if [`None`], returns the [default value] for that
1144     /// type.
1145     ///
1146     /// # Examples
1147     ///
1148     /// Converts a string to an integer, turning poorly-formed strings
1149     /// into 0 (the default value for integers). [`parse`] converts
1150     /// a string to any other type that implements [`FromStr`], returning
1151     /// [`None`] on error.
1152     ///
1153     /// ```
1154     /// let good_year_from_input = "1909";
1155     /// let bad_year_from_input = "190blarg";
1156     /// let good_year = good_year_from_input.parse().ok().unwrap_or_default();
1157     /// let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();
1158     ///
1159     /// assert_eq!(1909, good_year);
1160     /// assert_eq!(0, bad_year);
1161     /// ```
1162     ///
1163     /// [`Some`]: #variant.Some
1164     /// [`None`]: #variant.None
1165     /// [default value]: ../default/trait.Default.html#tymethod.default
1166     /// [`parse`]: ../../std/primitive.str.html#method.parse
1167     /// [`FromStr`]: ../../std/str/trait.FromStr.html
1168     #[inline]
1169     #[stable(feature = "rust1", since = "1.0.0")]
1170     pub fn unwrap_or_default(self) -> T {
1171         match self {
1172             Some(x) => x,
1173             None => Default::default(),
1174         }
1175     }
1176 }
1177
1178 impl<T: Deref> Option<T> {
1179     /// Converts from `Option<T>` (or `&Option<T>`) to `Option<&T::Target>`.
1180     ///
1181     /// Leaves the original Option in-place, creating a new one with a reference
1182     /// to the original one, additionally coercing the contents via [`Deref`].
1183     ///
1184     /// [`Deref`]: ../../std/ops/trait.Deref.html
1185     ///
1186     /// # Examples
1187     ///
1188     /// ```
1189     /// let x: Option<String> = Some("hey".to_owned());
1190     /// assert_eq!(x.as_deref(), Some("hey"));
1191     ///
1192     /// let x: Option<String> = None;
1193     /// assert_eq!(x.as_deref(), None);
1194     /// ```
1195     #[stable(feature = "option_deref", since = "1.40.0")]
1196     pub fn as_deref(&self) -> Option<&T::Target> {
1197         self.as_ref().map(|t| t.deref())
1198     }
1199 }
1200
1201 impl<T: DerefMut> Option<T> {
1202     /// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`.
1203     ///
1204     /// Leaves the original `Option` in-place, creating a new one containing a mutable reference to
1205     /// the inner type's `Deref::Target` type.
1206     ///
1207     /// # Examples
1208     ///
1209     /// ```
1210     /// let mut x: Option<String> = Some("hey".to_owned());
1211     /// assert_eq!(x.as_deref_mut().map(|x| {
1212     ///     x.make_ascii_uppercase();
1213     ///     x
1214     /// }), Some("HEY".to_owned().as_mut_str()));
1215     /// ```
1216     #[stable(feature = "option_deref", since = "1.40.0")]
1217     pub fn as_deref_mut(&mut self) -> Option<&mut T::Target> {
1218         self.as_mut().map(|t| t.deref_mut())
1219     }
1220 }
1221
1222 impl<T, E> Option<Result<T, E>> {
1223     /// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`.
1224     ///
1225     /// [`None`] will be mapped to [`Ok`]`(`[`None`]`)`.
1226     /// [`Some`]`(`[`Ok`]`(_))` and [`Some`]`(`[`Err`]`(_))` will be mapped to
1227     /// [`Ok`]`(`[`Some`]`(_))` and [`Err`]`(_)`.
1228     ///
1229     /// [`None`]: #variant.None
1230     /// [`Ok`]: ../../std/result/enum.Result.html#variant.Ok
1231     /// [`Some`]: #variant.Some
1232     /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
1233     ///
1234     /// # Examples
1235     ///
1236     /// ```
1237     /// #[derive(Debug, Eq, PartialEq)]
1238     /// struct SomeErr;
1239     ///
1240     /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1241     /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1242     /// assert_eq!(x, y.transpose());
1243     /// ```
1244     #[inline]
1245     #[stable(feature = "transpose_result", since = "1.33.0")]
1246     pub fn transpose(self) -> Result<Option<T>, E> {
1247         match self {
1248             Some(Ok(x)) => Ok(Some(x)),
1249             Some(Err(e)) => Err(e),
1250             None => Ok(None),
1251         }
1252     }
1253 }
1254
1255 // This is a separate function to reduce the code size of .expect() itself.
1256 #[inline(never)]
1257 #[cold]
1258 #[track_caller]
1259 fn expect_failed(msg: &str) -> ! {
1260     panic!("{}", msg)
1261 }
1262
1263 // This is a separate function to reduce the code size of .expect_none() itself.
1264 #[inline(never)]
1265 #[cold]
1266 #[track_caller]
1267 fn expect_none_failed(msg: &str, value: &dyn fmt::Debug) -> ! {
1268     panic!("{}: {:?}", msg, value)
1269 }
1270
1271 /////////////////////////////////////////////////////////////////////////////
1272 // Trait implementations
1273 /////////////////////////////////////////////////////////////////////////////
1274
1275 #[stable(feature = "rust1", since = "1.0.0")]
1276 impl<T: Clone> Clone for Option<T> {
1277     #[inline]
1278     fn clone(&self) -> Self {
1279         match self {
1280             Some(x) => Some(x.clone()),
1281             None => None,
1282         }
1283     }
1284
1285     #[inline]
1286     fn clone_from(&mut self, source: &Self) {
1287         match (self, source) {
1288             (Some(to), Some(from)) => to.clone_from(from),
1289             (to, from) => *to = from.clone(),
1290         }
1291     }
1292 }
1293
1294 #[stable(feature = "rust1", since = "1.0.0")]
1295 impl<T> Default for Option<T> {
1296     /// Returns [`None`][Option::None].
1297     ///
1298     /// # Examples
1299     ///
1300     /// ```
1301     /// let opt: Option<u32> = Option::default();
1302     /// assert!(opt.is_none());
1303     /// ```
1304     #[inline]
1305     fn default() -> Option<T> {
1306         None
1307     }
1308 }
1309
1310 #[stable(feature = "rust1", since = "1.0.0")]
1311 impl<T> IntoIterator for Option<T> {
1312     type Item = T;
1313     type IntoIter = IntoIter<T>;
1314
1315     /// Returns a consuming iterator over the possibly contained value.
1316     ///
1317     /// # Examples
1318     ///
1319     /// ```
1320     /// let x = Some("string");
1321     /// let v: Vec<&str> = x.into_iter().collect();
1322     /// assert_eq!(v, ["string"]);
1323     ///
1324     /// let x = None;
1325     /// let v: Vec<&str> = x.into_iter().collect();
1326     /// assert!(v.is_empty());
1327     /// ```
1328     #[inline]
1329     fn into_iter(self) -> IntoIter<T> {
1330         IntoIter { inner: Item { opt: self } }
1331     }
1332 }
1333
1334 #[stable(since = "1.4.0", feature = "option_iter")]
1335 impl<'a, T> IntoIterator for &'a Option<T> {
1336     type Item = &'a T;
1337     type IntoIter = Iter<'a, T>;
1338
1339     fn into_iter(self) -> Iter<'a, T> {
1340         self.iter()
1341     }
1342 }
1343
1344 #[stable(since = "1.4.0", feature = "option_iter")]
1345 impl<'a, T> IntoIterator for &'a mut Option<T> {
1346     type Item = &'a mut T;
1347     type IntoIter = IterMut<'a, T>;
1348
1349     fn into_iter(self) -> IterMut<'a, T> {
1350         self.iter_mut()
1351     }
1352 }
1353
1354 #[stable(since = "1.12.0", feature = "option_from")]
1355 impl<T> From<T> for Option<T> {
1356     fn from(val: T) -> Option<T> {
1357         Some(val)
1358     }
1359 }
1360
1361 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1362 impl<'a, T> From<&'a Option<T>> for Option<&'a T> {
1363     fn from(o: &'a Option<T>) -> Option<&'a T> {
1364         o.as_ref()
1365     }
1366 }
1367
1368 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1369 impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T> {
1370     fn from(o: &'a mut Option<T>) -> Option<&'a mut T> {
1371         o.as_mut()
1372     }
1373 }
1374
1375 /////////////////////////////////////////////////////////////////////////////
1376 // The Option Iterators
1377 /////////////////////////////////////////////////////////////////////////////
1378
1379 #[derive(Clone, Debug)]
1380 struct Item<A> {
1381     opt: Option<A>,
1382 }
1383
1384 impl<A> Iterator for Item<A> {
1385     type Item = A;
1386
1387     #[inline]
1388     fn next(&mut self) -> Option<A> {
1389         self.opt.take()
1390     }
1391
1392     #[inline]
1393     fn size_hint(&self) -> (usize, Option<usize>) {
1394         match self.opt {
1395             Some(_) => (1, Some(1)),
1396             None => (0, Some(0)),
1397         }
1398     }
1399 }
1400
1401 impl<A> DoubleEndedIterator for Item<A> {
1402     #[inline]
1403     fn next_back(&mut self) -> Option<A> {
1404         self.opt.take()
1405     }
1406 }
1407
1408 impl<A> ExactSizeIterator for Item<A> {}
1409 impl<A> FusedIterator for Item<A> {}
1410 unsafe impl<A> TrustedLen for Item<A> {}
1411
1412 /// An iterator over a reference to the [`Some`] variant of an [`Option`].
1413 ///
1414 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1415 ///
1416 /// This `struct` is created by the [`Option::iter`] function.
1417 ///
1418 /// [`Option`]: enum.Option.html
1419 /// [`Some`]: enum.Option.html#variant.Some
1420 /// [`Option::iter`]: enum.Option.html#method.iter
1421 #[stable(feature = "rust1", since = "1.0.0")]
1422 #[derive(Debug)]
1423 pub struct Iter<'a, A: 'a> {
1424     inner: Item<&'a A>,
1425 }
1426
1427 #[stable(feature = "rust1", since = "1.0.0")]
1428 impl<'a, A> Iterator for Iter<'a, A> {
1429     type Item = &'a A;
1430
1431     #[inline]
1432     fn next(&mut self) -> Option<&'a A> {
1433         self.inner.next()
1434     }
1435     #[inline]
1436     fn size_hint(&self) -> (usize, Option<usize>) {
1437         self.inner.size_hint()
1438     }
1439 }
1440
1441 #[stable(feature = "rust1", since = "1.0.0")]
1442 impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
1443     #[inline]
1444     fn next_back(&mut self) -> Option<&'a A> {
1445         self.inner.next_back()
1446     }
1447 }
1448
1449 #[stable(feature = "rust1", since = "1.0.0")]
1450 impl<A> ExactSizeIterator for Iter<'_, A> {}
1451
1452 #[stable(feature = "fused", since = "1.26.0")]
1453 impl<A> FusedIterator for Iter<'_, A> {}
1454
1455 #[unstable(feature = "trusted_len", issue = "37572")]
1456 unsafe impl<A> TrustedLen for Iter<'_, A> {}
1457
1458 #[stable(feature = "rust1", since = "1.0.0")]
1459 impl<A> Clone for Iter<'_, A> {
1460     #[inline]
1461     fn clone(&self) -> Self {
1462         Iter { inner: self.inner.clone() }
1463     }
1464 }
1465
1466 /// An iterator over a mutable reference to the [`Some`] variant of an [`Option`].
1467 ///
1468 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1469 ///
1470 /// This `struct` is created by the [`Option::iter_mut`] function.
1471 ///
1472 /// [`Option`]: enum.Option.html
1473 /// [`Some`]: enum.Option.html#variant.Some
1474 /// [`Option::iter_mut`]: enum.Option.html#method.iter_mut
1475 #[stable(feature = "rust1", since = "1.0.0")]
1476 #[derive(Debug)]
1477 pub struct IterMut<'a, A: 'a> {
1478     inner: Item<&'a mut A>,
1479 }
1480
1481 #[stable(feature = "rust1", since = "1.0.0")]
1482 impl<'a, A> Iterator for IterMut<'a, A> {
1483     type Item = &'a mut A;
1484
1485     #[inline]
1486     fn next(&mut self) -> Option<&'a mut A> {
1487         self.inner.next()
1488     }
1489     #[inline]
1490     fn size_hint(&self) -> (usize, Option<usize>) {
1491         self.inner.size_hint()
1492     }
1493 }
1494
1495 #[stable(feature = "rust1", since = "1.0.0")]
1496 impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
1497     #[inline]
1498     fn next_back(&mut self) -> Option<&'a mut A> {
1499         self.inner.next_back()
1500     }
1501 }
1502
1503 #[stable(feature = "rust1", since = "1.0.0")]
1504 impl<A> ExactSizeIterator for IterMut<'_, A> {}
1505
1506 #[stable(feature = "fused", since = "1.26.0")]
1507 impl<A> FusedIterator for IterMut<'_, A> {}
1508 #[unstable(feature = "trusted_len", issue = "37572")]
1509 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
1510
1511 /// An iterator over the value in [`Some`] variant of an [`Option`].
1512 ///
1513 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1514 ///
1515 /// This `struct` is created by the [`Option::into_iter`] function.
1516 ///
1517 /// [`Option`]: enum.Option.html
1518 /// [`Some`]: enum.Option.html#variant.Some
1519 /// [`Option::into_iter`]: enum.Option.html#method.into_iter
1520 #[derive(Clone, Debug)]
1521 #[stable(feature = "rust1", since = "1.0.0")]
1522 pub struct IntoIter<A> {
1523     inner: Item<A>,
1524 }
1525
1526 #[stable(feature = "rust1", since = "1.0.0")]
1527 impl<A> Iterator for IntoIter<A> {
1528     type Item = A;
1529
1530     #[inline]
1531     fn next(&mut self) -> Option<A> {
1532         self.inner.next()
1533     }
1534     #[inline]
1535     fn size_hint(&self) -> (usize, Option<usize>) {
1536         self.inner.size_hint()
1537     }
1538 }
1539
1540 #[stable(feature = "rust1", since = "1.0.0")]
1541 impl<A> DoubleEndedIterator for IntoIter<A> {
1542     #[inline]
1543     fn next_back(&mut self) -> Option<A> {
1544         self.inner.next_back()
1545     }
1546 }
1547
1548 #[stable(feature = "rust1", since = "1.0.0")]
1549 impl<A> ExactSizeIterator for IntoIter<A> {}
1550
1551 #[stable(feature = "fused", since = "1.26.0")]
1552 impl<A> FusedIterator for IntoIter<A> {}
1553
1554 #[unstable(feature = "trusted_len", issue = "37572")]
1555 unsafe impl<A> TrustedLen for IntoIter<A> {}
1556
1557 /////////////////////////////////////////////////////////////////////////////
1558 // FromIterator
1559 /////////////////////////////////////////////////////////////////////////////
1560
1561 #[stable(feature = "rust1", since = "1.0.0")]
1562 impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
1563     /// Takes each element in the [`Iterator`]: if it is [`None`][Option::None],
1564     /// no further elements are taken, and the [`None`][Option::None] is
1565     /// returned. Should no [`None`][Option::None] occur, a container with the
1566     /// values of each [`Option`] is returned.
1567     ///
1568     /// # Examples
1569     ///
1570     /// Here is an example which increments every integer in a vector.
1571     /// We use the checked variant of `add` that returns `None` when the
1572     /// calculation would result in an overflow.
1573     ///
1574     /// ```
1575     /// let items = vec![0_u16, 1, 2];
1576     ///
1577     /// let res: Option<Vec<u16>> = items
1578     ///     .iter()
1579     ///     .map(|x| x.checked_add(1))
1580     ///     .collect();
1581     ///
1582     /// assert_eq!(res, Some(vec![1, 2, 3]));
1583     /// ```
1584     ///
1585     /// As you can see, this will return the expected, valid items.
1586     ///
1587     /// Here is another example that tries to subtract one from another list
1588     /// of integers, this time checking for underflow:
1589     ///
1590     /// ```
1591     /// let items = vec![2_u16, 1, 0];
1592     ///
1593     /// let res: Option<Vec<u16>> = items
1594     ///     .iter()
1595     ///     .map(|x| x.checked_sub(1))
1596     ///     .collect();
1597     ///
1598     /// assert_eq!(res, None);
1599     /// ```
1600     ///
1601     /// Since the last element is zero, it would underflow. Thus, the resulting
1602     /// value is `None`.
1603     ///
1604     /// Here is a variation on the previous example, showing that no
1605     /// further elements are taken from `iter` after the first `None`.
1606     ///
1607     /// ```
1608     /// let items = vec![3_u16, 2, 1, 10];
1609     ///
1610     /// let mut shared = 0;
1611     ///
1612     /// let res: Option<Vec<u16>> = items
1613     ///     .iter()
1614     ///     .map(|x| { shared += x; x.checked_sub(2) })
1615     ///     .collect();
1616     ///
1617     /// assert_eq!(res, None);
1618     /// assert_eq!(shared, 6);
1619     /// ```
1620     ///
1621     /// Since the third element caused an underflow, no further elements were taken,
1622     /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
1623     ///
1624     /// [`Iterator`]: ../iter/trait.Iterator.html
1625     #[inline]
1626     fn from_iter<I: IntoIterator<Item = Option<A>>>(iter: I) -> Option<V> {
1627         // FIXME(#11084): This could be replaced with Iterator::scan when this
1628         // performance bug is closed.
1629
1630         iter.into_iter().map(|x| x.ok_or(())).collect::<Result<_, _>>().ok()
1631     }
1632 }
1633
1634 /// The error type that results from applying the try operator (`?`) to a `None` value. If you wish
1635 /// to allow `x?` (where `x` is an `Option<T>`) to be converted into your error type, you can
1636 /// implement `impl From<NoneError>` for `YourErrorType`. In that case, `x?` within a function that
1637 /// returns `Result<_, YourErrorType>` will translate a `None` value into an `Err` result.
1638 #[unstable(feature = "try_trait", issue = "42327")]
1639 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
1640 pub struct NoneError;
1641
1642 #[unstable(feature = "try_trait", issue = "42327")]
1643 impl<T> ops::Try for Option<T> {
1644     type Ok = T;
1645     type Error = NoneError;
1646
1647     #[inline]
1648     fn into_result(self) -> Result<T, NoneError> {
1649         self.ok_or(NoneError)
1650     }
1651
1652     #[inline]
1653     fn from_ok(v: T) -> Self {
1654         Some(v)
1655     }
1656
1657     #[inline]
1658     fn from_error(_: NoneError) -> Self {
1659         None
1660     }
1661 }
1662
1663 impl<T> Option<Option<T>> {
1664     /// Converts from `Option<Option<T>>` to `Option<T>`
1665     ///
1666     /// # Examples
1667     /// Basic usage:
1668     /// ```
1669     /// let x: Option<Option<u32>> = Some(Some(6));
1670     /// assert_eq!(Some(6), x.flatten());
1671     ///
1672     /// let x: Option<Option<u32>> = Some(None);
1673     /// assert_eq!(None, x.flatten());
1674     ///
1675     /// let x: Option<Option<u32>> = None;
1676     /// assert_eq!(None, x.flatten());
1677     /// ```
1678     /// Flattening once only removes one level of nesting:
1679     /// ```
1680     /// let x: Option<Option<Option<u32>>> = Some(Some(Some(6)));
1681     /// assert_eq!(Some(Some(6)), x.flatten());
1682     /// assert_eq!(Some(6), x.flatten().flatten());
1683     /// ```
1684     #[inline]
1685     #[stable(feature = "option_flattening", since = "1.40.0")]
1686     pub fn flatten(self) -> Option<T> {
1687         self.and_then(convert::identity)
1688     }
1689 }