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     fmt, hint, mem,
 154     ops::{self, 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     /// Applies a function to the contained value (if any),
 493     /// or returns the provided default (if not).
 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     /// Applies a function to the contained value (if any),
 520     /// or computes a default (if not).
 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     /// Inserts `value` into the option then returns a mutable reference to it.
 598     ///
 599     /// If the option already contains a value, the old value is dropped.
 600     ///
 601     /// # Example
 602     ///
 603     /// ```
 604     /// #![feature(option_insert)]
 605     ///
 606     /// let mut opt = None;
 607     /// let val = opt.insert(1);
 608     /// assert_eq!(*val, 1);
 609     /// assert_eq!(opt.unwrap(), 1);
 610     /// let val = opt.insert(2);
 611     /// assert_eq!(*val, 2);
 612     /// *val = 3;
 613     /// assert_eq!(opt.unwrap(), 3);
 614     /// ```
 615     #[inline]
 616     #[unstable(feature = "option_insert", reason = "newly added", issue = "78271")]
 617     pub fn insert(&mut self, value: T) -> &mut T {
 618         *self = Some(value);
 619
 620         match self {
 621             Some(v) => v,
 622             // SAFETY: the code above just filled the option
 623             None => unsafe { hint::unreachable_unchecked() },
 624         }
 625     }
 626
 627     /////////////////////////////////////////////////////////////////////////
 628     // Iterator constructors
 629     /////////////////////////////////////////////////////////////////////////
 630
 631     /// Returns an iterator over the possibly contained value.
 632     ///
 633     /// # Examples
 634     ///
 635     /// ```
 636     /// let x = Some(4);
 637     /// assert_eq!(x.iter().next(), Some(&4));
 638     ///
 639     /// let x: Option<u32> = None;
 640     /// assert_eq!(x.iter().next(), None);
 641     /// ```
 642     #[inline]
 643     #[rustc_const_unstable(feature = "const_option", issue = "67441")]
 644     #[stable(feature = "rust1", since = "1.0.0")]
 645     pub const fn iter(&self) -> Iter<'_, T> {
 646         Iter { inner: Item { opt: self.as_ref() } }
 647     }
 648
 649     /// Returns a mutable iterator over the possibly contained value.
 650     ///
 651     /// # Examples
 652     ///
 653     /// ```
 654     /// let mut x = Some(4);
 655     /// match x.iter_mut().next() {
 656     ///     Some(v) => *v = 42,
 657     ///     None => {},
 658     /// }
 659     /// assert_eq!(x, Some(42));
 660     ///
 661     /// let mut x: Option<u32> = None;
 662     /// assert_eq!(x.iter_mut().next(), None);
 663     /// ```
 664     #[inline]
 665     #[stable(feature = "rust1", since = "1.0.0")]
 666     pub fn iter_mut(&mut self) -> IterMut<'_, T> {
 667         IterMut { inner: Item { opt: self.as_mut() } }
 668     }
 669
 670     /////////////////////////////////////////////////////////////////////////
 671     // Boolean operations on the values, eager and lazy
 672     /////////////////////////////////////////////////////////////////////////
 673
 674     /// Returns [`None`] if the option is [`None`], otherwise returns `optb`.
 675     ///
 676     /// # Examples
 677     ///
 678     /// ```
 679     /// let x = Some(2);
 680     /// let y: Option<&str> = None;
 681     /// assert_eq!(x.and(y), None);
 682     ///
 683     /// let x: Option<u32> = None;
 684     /// let y = Some("foo");
 685     /// assert_eq!(x.and(y), None);
 686     ///
 687     /// let x = Some(2);
 688     /// let y = Some("foo");
 689     /// assert_eq!(x.and(y), Some("foo"));
 690     ///
 691     /// let x: Option<u32> = None;
 692     /// let y: Option<&str> = None;
 693     /// assert_eq!(x.and(y), None);
 694     /// ```
 695     #[inline]
 696     #[stable(feature = "rust1", since = "1.0.0")]
 697     pub fn and<U>(self, optb: Option<U>) -> Option<U> {
 698         match self {
 699             Some(_) => optb,
 700             None => None,
 701         }
 702     }
 703
 704     /// Returns [`None`] if the option is [`None`], otherwise calls `f` with the
 705     /// wrapped value and returns the result.
 706     ///
 707     /// Some languages call this operation flatmap.
 708     ///
 709     /// # Examples
 710     ///
 711     /// ```
 712     /// fn sq(x: u32) -> Option<u32> { Some(x * x) }
 713     /// fn nope(_: u32) -> Option<u32> { None }
 714     ///
 715     /// assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16));
 716     /// assert_eq!(Some(2).and_then(sq).and_then(nope), None);
 717     /// assert_eq!(Some(2).and_then(nope).and_then(sq), None);
 718     /// assert_eq!(None.and_then(sq).and_then(sq), None);
 719     /// ```
 720     #[inline]
 721     #[stable(feature = "rust1", since = "1.0.0")]
 722     pub fn and_then<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> Option<U> {
 723         match self {
 724             Some(x) => f(x),
 725             None => None,
 726         }
 727     }
 728
 729     /// Returns [`None`] if the option is [`None`], otherwise calls `predicate`
 730     /// with the wrapped value and returns:
 731     ///
 732     /// - [`Some(t)`] if `predicate` returns `true` (where `t` is the wrapped
 733     ///   value), and
 734     /// - [`None`] if `predicate` returns `false`.
 735     ///
 736     /// This function works similar to [`Iterator::filter()`]. You can imagine
 737     /// the `Option<T>` being an iterator over one or zero elements. `filter()`
 738     /// lets you decide which elements to keep.
 739     ///
 740     /// # Examples
 741     ///
 742     /// ```rust
 743     /// fn is_even(n: &i32) -> bool {
 744     ///     n % 2 == 0
 745     /// }
 746     ///
 747     /// assert_eq!(None.filter(is_even), None);
 748     /// assert_eq!(Some(3).filter(is_even), None);
 749     /// assert_eq!(Some(4).filter(is_even), Some(4));
 750     /// ```
 751     ///
 752     /// [`Some(t)`]: Some
 753     #[inline]
 754     #[stable(feature = "option_filter", since = "1.27.0")]
 755     pub fn filter<P: FnOnce(&T) -> bool>(self, predicate: P) -> Self {
 756         if let Some(x) = self {
 757             if predicate(&x) {
 758                 return Some(x);
 759             }
 760         }
 761         None
 762     }
 763
 764     /// Returns the option if it contains a value, otherwise returns `optb`.
 765     ///
 766     /// Arguments passed to `or` are eagerly evaluated; if you are passing the
 767     /// result of a function call, it is recommended to use [`or_else`], which is
 768     /// lazily evaluated.
 769     ///
 770     /// [`or_else`]: Option::or_else
 771     ///
 772     /// # Examples
 773     ///
 774     /// ```
 775     /// let x = Some(2);
 776     /// let y = None;
 777     /// assert_eq!(x.or(y), Some(2));
 778     ///
 779     /// let x = None;
 780     /// let y = Some(100);
 781     /// assert_eq!(x.or(y), Some(100));
 782     ///
 783     /// let x = Some(2);
 784     /// let y = Some(100);
 785     /// assert_eq!(x.or(y), Some(2));
 786     ///
 787     /// let x: Option<u32> = None;
 788     /// let y = None;
 789     /// assert_eq!(x.or(y), None);
 790     /// ```
 791     #[inline]
 792     #[stable(feature = "rust1", since = "1.0.0")]
 793     pub fn or(self, optb: Option<T>) -> Option<T> {
 794         match self {
 795             Some(_) => self,
 796             None => optb,
 797         }
 798     }
 799
 800     /// Returns the option if it contains a value, otherwise calls `f` and
 801     /// returns the result.
 802     ///
 803     /// # Examples
 804     ///
 805     /// ```
 806     /// fn nobody() -> Option<&'static str> { None }
 807     /// fn vikings() -> Option<&'static str> { Some("vikings") }
 808     ///
 809     /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
 810     /// assert_eq!(None.or_else(vikings), Some("vikings"));
 811     /// assert_eq!(None.or_else(nobody), None);
 812     /// ```
 813     #[inline]
 814     #[stable(feature = "rust1", since = "1.0.0")]
 815     pub fn or_else<F: FnOnce() -> Option<T>>(self, f: F) -> Option<T> {
 816         match self {
 817             Some(_) => self,
 818             None => f(),
 819         }
 820     }
 821
 822     /// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns [`None`].
 823     ///
 824     /// # Examples
 825     ///
 826     /// ```
 827     /// let x = Some(2);
 828     /// let y: Option<u32> = None;
 829     /// assert_eq!(x.xor(y), Some(2));
 830     ///
 831     /// let x: Option<u32> = None;
 832     /// let y = Some(2);
 833     /// assert_eq!(x.xor(y), Some(2));
 834     ///
 835     /// let x = Some(2);
 836     /// let y = Some(2);
 837     /// assert_eq!(x.xor(y), None);
 838     ///
 839     /// let x: Option<u32> = None;
 840     /// let y: Option<u32> = None;
 841     /// assert_eq!(x.xor(y), None);
 842     /// ```
 843     #[inline]
 844     #[stable(feature = "option_xor", since = "1.37.0")]
 845     pub fn xor(self, optb: Option<T>) -> Option<T> {
 846         match (self, optb) {
 847             (Some(a), None) => Some(a),
 848             (None, Some(b)) => Some(b),
 849             _ => None,
 850         }
 851     }
 852
 853     /////////////////////////////////////////////////////////////////////////
 854     // Entry-like operations to insert if None and return a reference
 855     /////////////////////////////////////////////////////////////////////////
 856
 857     /// Inserts `value` into the option if it is [`None`], then
 858     /// returns a mutable reference to the contained value.
 859     ///
 860     /// # Examples
 861     ///
 862     /// ```
 863     /// let mut x = None;
 864     ///
 865     /// {
 866     ///     let y: &mut u32 = x.get_or_insert(5);
 867     ///     assert_eq!(y, &5);
 868     ///
 869     ///     *y = 7;
 870     /// }
 871     ///
 872     /// assert_eq!(x, Some(7));
 873     /// ```
 874     #[inline]
 875     #[stable(feature = "option_entry", since = "1.20.0")]
 876     pub fn get_or_insert(&mut self, value: T) -> &mut T {
 877         self.get_or_insert_with(|| value)
 878     }
 879
 880     /// Inserts the default value into the option if it is [`None`], then
 881     /// returns a mutable reference to the contained value.
 882     ///
 883     /// # Examples
 884     ///
 885     /// ```
 886     /// #![feature(option_get_or_insert_default)]
 887     ///
 888     /// let mut x = None;
 889     ///
 890     /// {
 891     ///     let y: &mut u32 = x.get_or_insert_default();
 892     ///     assert_eq!(y, &0);
 893     ///
 894     ///     *y = 7;
 895     /// }
 896     ///
 897     /// assert_eq!(x, Some(7));
 898     /// ```
 899     #[inline]
 900     #[unstable(feature = "option_get_or_insert_default", issue = "82901")]
 901     pub fn get_or_insert_default(&mut self) -> &mut T
 902     where
 903         T: Default,
 904     {
 905         self.get_or_insert_with(Default::default)
 906     }
 907
 908     /// Inserts a value computed from `f` into the option if it is [`None`],
 909     /// then returns a mutable reference to the contained value.
 910     ///
 911     /// # Examples
 912     ///
 913     /// ```
 914     /// let mut x = None;
 915     ///
 916     /// {
 917     ///     let y: &mut u32 = x.get_or_insert_with(|| 5);
 918     ///     assert_eq!(y, &5);
 919     ///
 920     ///     *y = 7;
 921     /// }
 922     ///
 923     /// assert_eq!(x, Some(7));
 924     /// ```
 925     #[inline]
 926     #[stable(feature = "option_entry", since = "1.20.0")]
 927     pub fn get_or_insert_with<F: FnOnce() -> T>(&mut self, f: F) -> &mut T {
 928         if let None = *self {
 929             *self = Some(f());
 930         }
 931
 932         match self {
 933             Some(v) => v,
 934             // SAFETY: a `None` variant for `self` would have been replaced by a `Some`
 935             // variant in the code above.
 936             None => unsafe { hint::unreachable_unchecked() },
 937         }
 938     }
 939
 940     /////////////////////////////////////////////////////////////////////////
 941     // Misc
 942     /////////////////////////////////////////////////////////////////////////
 943
 944     /// Takes the value out of the option, leaving a [`None`] in its place.
 945     ///
 946     /// # Examples
 947     ///
 948     /// ```
 949     /// let mut x = Some(2);
 950     /// let y = x.take();
 951     /// assert_eq!(x, None);
 952     /// assert_eq!(y, Some(2));
 953     ///
 954     /// let mut x: Option<u32> = None;
 955     /// let y = x.take();
 956     /// assert_eq!(x, None);
 957     /// assert_eq!(y, None);
 958     /// ```
 959     #[inline]
 960     #[stable(feature = "rust1", since = "1.0.0")]
 961     pub fn take(&mut self) -> Option<T> {
 962         mem::take(self)
 963     }
 964
 965     /// Replaces the actual value in the option by the value given in parameter,
 966     /// returning the old value if present,
 967     /// leaving a [`Some`] in its place without deinitializing either one.
 968     ///
 969     /// # Examples
 970     ///
 971     /// ```
 972     /// let mut x = Some(2);
 973     /// let old = x.replace(5);
 974     /// assert_eq!(x, Some(5));
 975     /// assert_eq!(old, Some(2));
 976     ///
 977     /// let mut x = None;
 978     /// let old = x.replace(3);
 979     /// assert_eq!(x, Some(3));
 980     /// assert_eq!(old, None);
 981     /// ```
 982     #[inline]
 983     #[stable(feature = "option_replace", since = "1.31.0")]
 984     pub fn replace(&mut self, value: T) -> Option<T> {
 985         mem::replace(self, Some(value))
 986     }
 987
 988     /// Zips `self` with another `Option`.
 989     ///
 990     /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some((s, o))`.
 991     /// Otherwise, `None` is returned.
 992     ///
 993     /// # Examples
 994     ///
 995     /// ```
 996     /// let x = Some(1);
 997     /// let y = Some("hi");
 998     /// let z = None::<u8>;
 999     ///
1000     /// assert_eq!(x.zip(y), Some((1, "hi")));
1001     /// assert_eq!(x.zip(z), None);
1002     /// ```
1003     #[stable(feature = "option_zip_option", since = "1.46.0")]
1004     pub fn zip<U>(self, other: Option<U>) -> Option<(T, U)> {
1005         match (self, other) {
1006             (Some(a), Some(b)) => Some((a, b)),
1007             _ => None,
1008         }
1009     }
1010
1011     /// Zips `self` and another `Option` with function `f`.
1012     ///
1013     /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`.
1014     /// Otherwise, `None` is returned.
1015     ///
1016     /// # Examples
1017     ///
1018     /// ```
1019     /// #![feature(option_zip)]
1020     ///
1021     /// #[derive(Debug, PartialEq)]
1022     /// struct Point {
1023     ///     x: f64,
1024     ///     y: f64,
1025     /// }
1026     ///
1027     /// impl Point {
1028     ///     fn new(x: f64, y: f64) -> Self {
1029     ///         Self { x, y }
1030     ///     }
1031     /// }
1032     ///
1033     /// let x = Some(17.5);
1034     /// let y = Some(42.7);
1035     ///
1036     /// assert_eq!(x.zip_with(y, Point::new), Some(Point { x: 17.5, y: 42.7 }));
1037     /// assert_eq!(x.zip_with(None, Point::new), None);
1038     /// ```
1039     #[unstable(feature = "option_zip", issue = "70086")]
1040     pub fn zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R>
1041     where
1042         F: FnOnce(T, U) -> R,
1043     {
1044         Some(f(self?, other?))
1045     }
1046 }
1047
1048 impl<T: Copy> Option<&T> {
1049     /// Maps an `Option<&T>` to an `Option<T>` by copying the contents of the
1050     /// option.
1051     ///
1052     /// # Examples
1053     ///
1054     /// ```
1055     /// let x = 12;
1056     /// let opt_x = Some(&x);
1057     /// assert_eq!(opt_x, Some(&12));
1058     /// let copied = opt_x.copied();
1059     /// assert_eq!(copied, Some(12));
1060     /// ```
1061     #[stable(feature = "copied", since = "1.35.0")]
1062     pub fn copied(self) -> Option<T> {
1063         self.map(|&t| t)
1064     }
1065 }
1066
1067 impl<T: Copy> Option<&mut T> {
1068     /// Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the
1069     /// option.
1070     ///
1071     /// # Examples
1072     ///
1073     /// ```
1074     /// let mut x = 12;
1075     /// let opt_x = Some(&mut x);
1076     /// assert_eq!(opt_x, Some(&mut 12));
1077     /// let copied = opt_x.copied();
1078     /// assert_eq!(copied, Some(12));
1079     /// ```
1080     #[stable(feature = "copied", since = "1.35.0")]
1081     pub fn copied(self) -> Option<T> {
1082         self.map(|&mut t| t)
1083     }
1084 }
1085
1086 impl<T: Clone> Option<&T> {
1087     /// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the
1088     /// option.
1089     ///
1090     /// # Examples
1091     ///
1092     /// ```
1093     /// let x = 12;
1094     /// let opt_x = Some(&x);
1095     /// assert_eq!(opt_x, Some(&12));
1096     /// let cloned = opt_x.cloned();
1097     /// assert_eq!(cloned, Some(12));
1098     /// ```
1099     #[stable(feature = "rust1", since = "1.0.0")]
1100     pub fn cloned(self) -> Option<T> {
1101         self.map(|t| t.clone())
1102     }
1103 }
1104
1105 impl<T: Clone> Option<&mut T> {
1106     /// Maps an `Option<&mut T>` to an `Option<T>` by cloning the contents of the
1107     /// option.
1108     ///
1109     /// # Examples
1110     ///
1111     /// ```
1112     /// let mut x = 12;
1113     /// let opt_x = Some(&mut x);
1114     /// assert_eq!(opt_x, Some(&mut 12));
1115     /// let cloned = opt_x.cloned();
1116     /// assert_eq!(cloned, Some(12));
1117     /// ```
1118     #[stable(since = "1.26.0", feature = "option_ref_mut_cloned")]
1119     pub fn cloned(self) -> Option<T> {
1120         self.map(|t| t.clone())
1121     }
1122 }
1123
1124 impl<T: fmt::Debug> Option<T> {
1125     /// Consumes `self` while expecting [`None`] and returning nothing.
1126     ///
1127     /// # Panics
1128     ///
1129     /// Panics if the value is a [`Some`], with a panic message including the
1130     /// passed message, and the content of the [`Some`].
1131     ///
1132     /// # Examples
1133     ///
1134     /// ```
1135     /// #![feature(option_expect_none)]
1136     ///
1137     /// use std::collections::HashMap;
1138     /// let mut squares = HashMap::new();
1139     /// for i in -10..=10 {
1140     ///     // This will not panic, since all keys are unique.
1141     ///     squares.insert(i, i * i).expect_none("duplicate key");
1142     /// }
1143     /// ```
1144     ///
1145     /// ```should_panic
1146     /// #![feature(option_expect_none)]
1147     ///
1148     /// use std::collections::HashMap;
1149     /// let mut sqrts = HashMap::new();
1150     /// for i in -10..=10 {
1151     ///     // This will panic, since both negative and positive `i` will
1152     ///     // insert the same `i * i` key, returning the old `Some(i)`.
1153     ///     sqrts.insert(i * i, i).expect_none("duplicate key");
1154     /// }
1155     /// ```
1156     #[inline]
1157     #[track_caller]
1158     #[unstable(feature = "option_expect_none", reason = "newly added", issue = "62633")]
1159     pub fn expect_none(self, msg: &str) {
1160         if let Some(val) = self {
1161             expect_none_failed(msg, &val);
1162         }
1163     }
1164
1165     /// Consumes `self` while expecting [`None`] and returning nothing.
1166     ///
1167     /// # Panics
1168     ///
1169     /// Panics if the value is a [`Some`], with a custom panic message provided
1170     /// by the [`Some`]'s value.
1171     ///
1172     /// [`Some(v)`]: Some
1173     ///
1174     /// # Examples
1175     ///
1176     /// ```
1177     /// #![feature(option_unwrap_none)]
1178     ///
1179     /// use std::collections::HashMap;
1180     /// let mut squares = HashMap::new();
1181     /// for i in -10..=10 {
1182     ///     // This will not panic, since all keys are unique.
1183     ///     squares.insert(i, i * i).unwrap_none();
1184     /// }
1185     /// ```
1186     ///
1187     /// ```should_panic
1188     /// #![feature(option_unwrap_none)]
1189     ///
1190     /// use std::collections::HashMap;
1191     /// let mut sqrts = HashMap::new();
1192     /// for i in -10..=10 {
1193     ///     // This will panic, since both negative and positive `i` will
1194     ///     // insert the same `i * i` key, returning the old `Some(i)`.
1195     ///     sqrts.insert(i * i, i).unwrap_none();
1196     /// }
1197     /// ```
1198     #[inline]
1199     #[track_caller]
1200     #[unstable(feature = "option_unwrap_none", reason = "newly added", issue = "62633")]
1201     pub fn unwrap_none(self) {
1202         if let Some(val) = self {
1203             expect_none_failed("called `Option::unwrap_none()` on a `Some` value", &val);
1204         }
1205     }
1206 }
1207
1208 impl<T: Default> Option<T> {
1209     /// Returns the contained [`Some`] value or a default
1210     ///
1211     /// Consumes the `self` argument then, if [`Some`], returns the contained
1212     /// value, otherwise if [`None`], returns the [default value] for that
1213     /// type.
1214     ///
1215     /// # Examples
1216     ///
1217     /// Converts a string to an integer, turning poorly-formed strings
1218     /// into 0 (the default value for integers). [`parse`] converts
1219     /// a string to any other type that implements [`FromStr`], returning
1220     /// [`None`] on error.
1221     ///
1222     /// ```
1223     /// let good_year_from_input = "1909";
1224     /// let bad_year_from_input = "190blarg";
1225     /// let good_year = good_year_from_input.parse().ok().unwrap_or_default();
1226     /// let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();
1227     ///
1228     /// assert_eq!(1909, good_year);
1229     /// assert_eq!(0, bad_year);
1230     /// ```
1231     ///
1232     /// [default value]: Default::default
1233     /// [`parse`]: str::parse
1234     /// [`FromStr`]: crate::str::FromStr
1235     #[inline]
1236     #[stable(feature = "rust1", since = "1.0.0")]
1237     pub fn unwrap_or_default(self) -> T {
1238         match self {
1239             Some(x) => x,
1240             None => Default::default(),
1241         }
1242     }
1243 }
1244
1245 impl<T: Deref> Option<T> {
1246     /// Converts from `Option<T>` (or `&Option<T>`) to `Option<&T::Target>`.
1247     ///
1248     /// Leaves the original Option in-place, creating a new one with a reference
1249     /// to the original one, additionally coercing the contents via [`Deref`].
1250     ///
1251     /// # Examples
1252     ///
1253     /// ```
1254     /// let x: Option<String> = Some("hey".to_owned());
1255     /// assert_eq!(x.as_deref(), Some("hey"));
1256     ///
1257     /// let x: Option<String> = None;
1258     /// assert_eq!(x.as_deref(), None);
1259     /// ```
1260     #[stable(feature = "option_deref", since = "1.40.0")]
1261     pub fn as_deref(&self) -> Option<&T::Target> {
1262         self.as_ref().map(|t| t.deref())
1263     }
1264 }
1265
1266 impl<T: DerefMut> Option<T> {
1267     /// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`.
1268     ///
1269     /// Leaves the original `Option` in-place, creating a new one containing a mutable reference to
1270     /// the inner type's `Deref::Target` type.
1271     ///
1272     /// # Examples
1273     ///
1274     /// ```
1275     /// let mut x: Option<String> = Some("hey".to_owned());
1276     /// assert_eq!(x.as_deref_mut().map(|x| {
1277     ///     x.make_ascii_uppercase();
1278     ///     x
1279     /// }), Some("HEY".to_owned().as_mut_str()));
1280     /// ```
1281     #[stable(feature = "option_deref", since = "1.40.0")]
1282     pub fn as_deref_mut(&mut self) -> Option<&mut T::Target> {
1283         self.as_mut().map(|t| t.deref_mut())
1284     }
1285 }
1286
1287 impl<T, E> Option<Result<T, E>> {
1288     /// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`.
1289     ///
1290     /// [`None`] will be mapped to [`Ok`]`(`[`None`]`)`.
1291     /// [`Some`]`(`[`Ok`]`(_))` and [`Some`]`(`[`Err`]`(_))` will be mapped to
1292     /// [`Ok`]`(`[`Some`]`(_))` and [`Err`]`(_)`.
1293     ///
1294     /// # Examples
1295     ///
1296     /// ```
1297     /// #[derive(Debug, Eq, PartialEq)]
1298     /// struct SomeErr;
1299     ///
1300     /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1301     /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1302     /// assert_eq!(x, y.transpose());
1303     /// ```
1304     #[inline]
1305     #[stable(feature = "transpose_result", since = "1.33.0")]
1306     #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1307     pub const fn transpose(self) -> Result<Option<T>, E> {
1308         match self {
1309             Some(Ok(x)) => Ok(Some(x)),
1310             Some(Err(e)) => Err(e),
1311             None => Ok(None),
1312         }
1313     }
1314 }
1315
1316 // This is a separate function to reduce the code size of .expect() itself.
1317 #[inline(never)]
1318 #[cold]
1319 #[track_caller]
1320 fn expect_failed(msg: &str) -> ! {
1321     panic!("{}", msg)
1322 }
1323
1324 // This is a separate function to reduce the code size of .expect_none() itself.
1325 #[inline(never)]
1326 #[cold]
1327 #[track_caller]
1328 fn expect_none_failed(msg: &str, value: &dyn fmt::Debug) -> ! {
1329     panic!("{}: {:?}", msg, value)
1330 }
1331
1332 /////////////////////////////////////////////////////////////////////////////
1333 // Trait implementations
1334 /////////////////////////////////////////////////////////////////////////////
1335
1336 #[stable(feature = "rust1", since = "1.0.0")]
1337 impl<T: Clone> Clone for Option<T> {
1338     #[inline]
1339     fn clone(&self) -> Self {
1340         match self {
1341             Some(x) => Some(x.clone()),
1342             None => None,
1343         }
1344     }
1345
1346     #[inline]
1347     fn clone_from(&mut self, source: &Self) {
1348         match (self, source) {
1349             (Some(to), Some(from)) => to.clone_from(from),
1350             (to, from) => *to = from.clone(),
1351         }
1352     }
1353 }
1354
1355 #[stable(feature = "rust1", since = "1.0.0")]
1356 impl<T> Default for Option<T> {
1357     /// Returns [`None`][Option::None].
1358     ///
1359     /// # Examples
1360     ///
1361     /// ```
1362     /// let opt: Option<u32> = Option::default();
1363     /// assert!(opt.is_none());
1364     /// ```
1365     #[inline]
1366     fn default() -> Option<T> {
1367         None
1368     }
1369 }
1370
1371 #[stable(feature = "rust1", since = "1.0.0")]
1372 impl<T> IntoIterator for Option<T> {
1373     type Item = T;
1374     type IntoIter = IntoIter<T>;
1375
1376     /// Returns a consuming iterator over the possibly contained value.
1377     ///
1378     /// # Examples
1379     ///
1380     /// ```
1381     /// let x = Some("string");
1382     /// let v: Vec<&str> = x.into_iter().collect();
1383     /// assert_eq!(v, ["string"]);
1384     ///
1385     /// let x = None;
1386     /// let v: Vec<&str> = x.into_iter().collect();
1387     /// assert!(v.is_empty());
1388     /// ```
1389     #[inline]
1390     fn into_iter(self) -> IntoIter<T> {
1391         IntoIter { inner: Item { opt: self } }
1392     }
1393 }
1394
1395 #[stable(since = "1.4.0", feature = "option_iter")]
1396 impl<'a, T> IntoIterator for &'a Option<T> {
1397     type Item = &'a T;
1398     type IntoIter = Iter<'a, T>;
1399
1400     fn into_iter(self) -> Iter<'a, T> {
1401         self.iter()
1402     }
1403 }
1404
1405 #[stable(since = "1.4.0", feature = "option_iter")]
1406 impl<'a, T> IntoIterator for &'a mut Option<T> {
1407     type Item = &'a mut T;
1408     type IntoIter = IterMut<'a, T>;
1409
1410     fn into_iter(self) -> IterMut<'a, T> {
1411         self.iter_mut()
1412     }
1413 }
1414
1415 #[stable(since = "1.12.0", feature = "option_from")]
1416 impl<T> From<T> for Option<T> {
1417     /// Copies `val` into a new `Some`.
1418     ///
1419     /// # Examples
1420     ///
1421     /// ```
1422     /// let o: Option<u8> = Option::from(67);
1423     ///
1424     /// assert_eq!(Some(67), o);
1425     /// ```
1426     fn from(val: T) -> Option<T> {
1427         Some(val)
1428     }
1429 }
1430
1431 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1432 impl<'a, T> From<&'a Option<T>> for Option<&'a T> {
1433     /// Converts from `&Option<T>` to `Option<&T>`.
1434     ///
1435     /// # Examples
1436     ///
1437     /// Converts an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, preserving the original.
1438     /// The [`map`] method takes the `self` argument by value, consuming the original,
1439     /// so this technique uses `as_ref` to first take an `Option` to a reference
1440     /// to the value inside the original.
1441     ///
1442     /// [`map`]: Option::map
1443     /// [`String`]: ../../std/string/struct.String.html
1444     ///
1445     /// ```
1446     /// let s: Option<String> = Some(String::from("Hello, Rustaceans!"));
1447     /// let o: Option<usize> = Option::from(&s).map(|ss: &String| ss.len());
1448     ///
1449     /// println!("Can still print s: {:?}", s);
1450     ///
1451     /// assert_eq!(o, Some(18));
1452     /// ```
1453     fn from(o: &'a Option<T>) -> Option<&'a T> {
1454         o.as_ref()
1455     }
1456 }
1457
1458 #[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
1459 impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T> {
1460     /// Converts from `&mut Option<T>` to `Option<&mut T>`
1461     ///
1462     /// # Examples
1463     ///
1464     /// ```
1465     /// let mut s = Some(String::from("Hello"));
1466     /// let o: Option<&mut String> = Option::from(&mut s);
1467     ///
1468     /// match o {
1469     ///     Some(t) => *t = String::from("Hello, Rustaceans!"),
1470     ///     None => (),
1471     /// }
1472     ///
1473     /// assert_eq!(s, Some(String::from("Hello, Rustaceans!")));
1474     /// ```
1475     fn from(o: &'a mut Option<T>) -> Option<&'a mut T> {
1476         o.as_mut()
1477     }
1478 }
1479
1480 /////////////////////////////////////////////////////////////////////////////
1481 // The Option Iterators
1482 /////////////////////////////////////////////////////////////////////////////
1483
1484 #[derive(Clone, Debug)]
1485 struct Item<A> {
1486     opt: Option<A>,
1487 }
1488
1489 impl<A> Iterator for Item<A> {
1490     type Item = A;
1491
1492     #[inline]
1493     fn next(&mut self) -> Option<A> {
1494         self.opt.take()
1495     }
1496
1497     #[inline]
1498     fn size_hint(&self) -> (usize, Option<usize>) {
1499         match self.opt {
1500             Some(_) => (1, Some(1)),
1501             None => (0, Some(0)),
1502         }
1503     }
1504 }
1505
1506 impl<A> DoubleEndedIterator for Item<A> {
1507     #[inline]
1508     fn next_back(&mut self) -> Option<A> {
1509         self.opt.take()
1510     }
1511 }
1512
1513 impl<A> ExactSizeIterator for Item<A> {}
1514 impl<A> FusedIterator for Item<A> {}
1515 unsafe impl<A> TrustedLen for Item<A> {}
1516
1517 /// An iterator over a reference to the [`Some`] variant of an [`Option`].
1518 ///
1519 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1520 ///
1521 /// This `struct` is created by the [`Option::iter`] function.
1522 #[stable(feature = "rust1", since = "1.0.0")]
1523 #[derive(Debug)]
1524 pub struct Iter<'a, A: 'a> {
1525     inner: Item<&'a A>,
1526 }
1527
1528 #[stable(feature = "rust1", since = "1.0.0")]
1529 impl<'a, A> Iterator for Iter<'a, A> {
1530     type Item = &'a A;
1531
1532     #[inline]
1533     fn next(&mut self) -> Option<&'a A> {
1534         self.inner.next()
1535     }
1536     #[inline]
1537     fn size_hint(&self) -> (usize, Option<usize>) {
1538         self.inner.size_hint()
1539     }
1540 }
1541
1542 #[stable(feature = "rust1", since = "1.0.0")]
1543 impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
1544     #[inline]
1545     fn next_back(&mut self) -> Option<&'a A> {
1546         self.inner.next_back()
1547     }
1548 }
1549
1550 #[stable(feature = "rust1", since = "1.0.0")]
1551 impl<A> ExactSizeIterator for Iter<'_, A> {}
1552
1553 #[stable(feature = "fused", since = "1.26.0")]
1554 impl<A> FusedIterator for Iter<'_, A> {}
1555
1556 #[unstable(feature = "trusted_len", issue = "37572")]
1557 unsafe impl<A> TrustedLen for Iter<'_, A> {}
1558
1559 #[stable(feature = "rust1", since = "1.0.0")]
1560 impl<A> Clone for Iter<'_, A> {
1561     #[inline]
1562     fn clone(&self) -> Self {
1563         Iter { inner: self.inner.clone() }
1564     }
1565 }
1566
1567 /// An iterator over a mutable reference to the [`Some`] variant of an [`Option`].
1568 ///
1569 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1570 ///
1571 /// This `struct` is created by the [`Option::iter_mut`] function.
1572 #[stable(feature = "rust1", since = "1.0.0")]
1573 #[derive(Debug)]
1574 pub struct IterMut<'a, A: 'a> {
1575     inner: Item<&'a mut A>,
1576 }
1577
1578 #[stable(feature = "rust1", since = "1.0.0")]
1579 impl<'a, A> Iterator for IterMut<'a, A> {
1580     type Item = &'a mut A;
1581
1582     #[inline]
1583     fn next(&mut self) -> Option<&'a mut A> {
1584         self.inner.next()
1585     }
1586     #[inline]
1587     fn size_hint(&self) -> (usize, Option<usize>) {
1588         self.inner.size_hint()
1589     }
1590 }
1591
1592 #[stable(feature = "rust1", since = "1.0.0")]
1593 impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
1594     #[inline]
1595     fn next_back(&mut self) -> Option<&'a mut A> {
1596         self.inner.next_back()
1597     }
1598 }
1599
1600 #[stable(feature = "rust1", since = "1.0.0")]
1601 impl<A> ExactSizeIterator for IterMut<'_, A> {}
1602
1603 #[stable(feature = "fused", since = "1.26.0")]
1604 impl<A> FusedIterator for IterMut<'_, A> {}
1605 #[unstable(feature = "trusted_len", issue = "37572")]
1606 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
1607
1608 /// An iterator over the value in [`Some`] variant of an [`Option`].
1609 ///
1610 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1611 ///
1612 /// This `struct` is created by the [`Option::into_iter`] function.
1613 #[derive(Clone, Debug)]
1614 #[stable(feature = "rust1", since = "1.0.0")]
1615 pub struct IntoIter<A> {
1616     inner: Item<A>,
1617 }
1618
1619 #[stable(feature = "rust1", since = "1.0.0")]
1620 impl<A> Iterator for IntoIter<A> {
1621     type Item = A;
1622
1623     #[inline]
1624     fn next(&mut self) -> Option<A> {
1625         self.inner.next()
1626     }
1627     #[inline]
1628     fn size_hint(&self) -> (usize, Option<usize>) {
1629         self.inner.size_hint()
1630     }
1631 }
1632
1633 #[stable(feature = "rust1", since = "1.0.0")]
1634 impl<A> DoubleEndedIterator for IntoIter<A> {
1635     #[inline]
1636     fn next_back(&mut self) -> Option<A> {
1637         self.inner.next_back()
1638     }
1639 }
1640
1641 #[stable(feature = "rust1", since = "1.0.0")]
1642 impl<A> ExactSizeIterator for IntoIter<A> {}
1643
1644 #[stable(feature = "fused", since = "1.26.0")]
1645 impl<A> FusedIterator for IntoIter<A> {}
1646
1647 #[unstable(feature = "trusted_len", issue = "37572")]
1648 unsafe impl<A> TrustedLen for IntoIter<A> {}
1649
1650 /////////////////////////////////////////////////////////////////////////////
1651 // FromIterator
1652 /////////////////////////////////////////////////////////////////////////////
1653
1654 #[stable(feature = "rust1", since = "1.0.0")]
1655 impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
1656     /// Takes each element in the [`Iterator`]: if it is [`None`][Option::None],
1657     /// no further elements are taken, and the [`None`][Option::None] is
1658     /// returned. Should no [`None`][Option::None] occur, a container with the
1659     /// values of each [`Option`] is returned.
1660     ///
1661     /// # Examples
1662     ///
1663     /// Here is an example which increments every integer in a vector.
1664     /// We use the checked variant of `add` that returns `None` when the
1665     /// calculation would result in an overflow.
1666     ///
1667     /// ```
1668     /// let items = vec![0_u16, 1, 2];
1669     ///
1670     /// let res: Option<Vec<u16>> = items
1671     ///     .iter()
1672     ///     .map(|x| x.checked_add(1))
1673     ///     .collect();
1674     ///
1675     /// assert_eq!(res, Some(vec![1, 2, 3]));
1676     /// ```
1677     ///
1678     /// As you can see, this will return the expected, valid items.
1679     ///
1680     /// Here is another example that tries to subtract one from another list
1681     /// of integers, this time checking for underflow:
1682     ///
1683     /// ```
1684     /// let items = vec![2_u16, 1, 0];
1685     ///
1686     /// let res: Option<Vec<u16>> = items
1687     ///     .iter()
1688     ///     .map(|x| x.checked_sub(1))
1689     ///     .collect();
1690     ///
1691     /// assert_eq!(res, None);
1692     /// ```
1693     ///
1694     /// Since the last element is zero, it would underflow. Thus, the resulting
1695     /// value is `None`.
1696     ///
1697     /// Here is a variation on the previous example, showing that no
1698     /// further elements are taken from `iter` after the first `None`.
1699     ///
1700     /// ```
1701     /// let items = vec![3_u16, 2, 1, 10];
1702     ///
1703     /// let mut shared = 0;
1704     ///
1705     /// let res: Option<Vec<u16>> = items
1706     ///     .iter()
1707     ///     .map(|x| { shared += x; x.checked_sub(2) })
1708     ///     .collect();
1709     ///
1710     /// assert_eq!(res, None);
1711     /// assert_eq!(shared, 6);
1712     /// ```
1713     ///
1714     /// Since the third element caused an underflow, no further elements were taken,
1715     /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
1716     #[inline]
1717     fn from_iter<I: IntoIterator<Item = Option<A>>>(iter: I) -> Option<V> {
1718         // FIXME(#11084): This could be replaced with Iterator::scan when this
1719         // performance bug is closed.
1720
1721         iter.into_iter().map(|x| x.ok_or(())).collect::<Result<_, _>>().ok()
1722     }
1723 }
1724
1725 /// The error type that results from applying the try operator (`?`) to a `None` value. If you wish
1726 /// to allow `x?` (where `x` is an `Option<T>`) to be converted into your error type, you can
1727 /// implement `impl From<NoneError>` for `YourErrorType`. In that case, `x?` within a function that
1728 /// returns `Result<_, YourErrorType>` will translate a `None` value into an `Err` result.
1729 #[rustc_diagnostic_item = "none_error"]
1730 #[unstable(feature = "try_trait", issue = "42327")]
1731 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
1732 pub struct NoneError;
1733
1734 #[unstable(feature = "try_trait", issue = "42327")]
1735 impl<T> ops::Try for Option<T> {
1736     type Ok = T;
1737     type Error = NoneError;
1738
1739     #[inline]
1740     fn into_result(self) -> Result<T, NoneError> {
1741         self.ok_or(NoneError)
1742     }
1743
1744     #[inline]
1745     fn from_ok(v: T) -> Self {
1746         Some(v)
1747     }
1748
1749     #[inline]
1750     fn from_error(_: NoneError) -> Self {
1751         None
1752     }
1753 }
1754
1755 impl<T> Option<Option<T>> {
1756     /// Converts from `Option<Option<T>>` to `Option<T>`
1757     ///
1758     /// # Examples
1759     ///
1760     /// Basic usage:
1761     ///
1762     /// ```
1763     /// let x: Option<Option<u32>> = Some(Some(6));
1764     /// assert_eq!(Some(6), x.flatten());
1765     ///
1766     /// let x: Option<Option<u32>> = Some(None);
1767     /// assert_eq!(None, x.flatten());
1768     ///
1769     /// let x: Option<Option<u32>> = None;
1770     /// assert_eq!(None, x.flatten());
1771     /// ```
1772     ///
1773     /// Flattening only removes one level of nesting at a time:
1774     ///
1775     /// ```
1776     /// let x: Option<Option<Option<u32>>> = Some(Some(Some(6)));
1777     /// assert_eq!(Some(Some(6)), x.flatten());
1778     /// assert_eq!(Some(6), x.flatten().flatten());
1779     /// ```
1780     #[inline]
1781     #[stable(feature = "option_flattening", since = "1.40.0")]
1782     #[rustc_const_unstable(feature = "const_option", issue = "67441")]
1783     pub const fn flatten(self) -> Option<T> {
1784         match self {
1785             Some(inner) => inner,
1786             None => None,
1787         }
1788     }
1789 }