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diff --git a/library/core/src/option.rs b/library/core/src/option.rs new file mode 100644 index 000000000..bca73cb77 --- /dev/null +++ b/library/core/src/option.rs @@ -0,0 +1,2356 @@ +//! Optional values. +//! +//! Type [`Option`] represents an optional value: every [`Option`] +//! is either [`Some`] and contains a value, or [`None`], and +//! does not. [`Option`] types are very common in Rust code, as +//! they have a number of uses: +//! +//! * Initial values +//! * Return values for functions that are not defined +//! over their entire input range (partial functions) +//! * Return value for otherwise reporting simple errors, where [`None`] is +//! returned on error +//! * Optional struct fields +//! * Struct fields that can be loaned or "taken" +//! * Optional function arguments +//! * Nullable pointers +//! * Swapping things out of difficult situations +//! +//! [`Option`]s are commonly paired with pattern matching to query the presence +//! of a value and take action, always accounting for the [`None`] case. +//! +//! ``` +//! fn divide(numerator: f64, denominator: f64) -> Option<f64> { +//! if denominator == 0.0 { +//! None +//! } else { +//! Some(numerator / denominator) +//! } +//! } +//! +//! // The return value of the function is an option +//! let result = divide(2.0, 3.0); +//! +//! // Pattern match to retrieve the value +//! match result { +//! // The division was valid +//! Some(x) => println!("Result: {x}"), +//! // The division was invalid +//! None => println!("Cannot divide by 0"), +//! } +//! ``` +//! +// +// FIXME: Show how `Option` is used in practice, with lots of methods +// +//! # Options and pointers ("nullable" pointers) +//! +//! Rust's pointer types must always point to a valid location; there are +//! no "null" references. Instead, Rust has *optional* pointers, like +//! the optional owned box, <code>[Option]<[Box\<T>]></code>. +//! +//! [Box\<T>]: ../../std/boxed/struct.Box.html +//! +//! The following example uses [`Option`] to create an optional box of +//! [`i32`]. Notice that in order to use the inner [`i32`] value, the +//! `check_optional` function first needs to use pattern matching to +//! determine whether the box has a value (i.e., it is [`Some(...)`][`Some`]) or +//! not ([`None`]). +//! +//! ``` +//! let optional = None; +//! check_optional(optional); +//! +//! let optional = Some(Box::new(9000)); +//! check_optional(optional); +//! +//! fn check_optional(optional: Option<Box<i32>>) { +//! match optional { +//! Some(p) => println!("has value {p}"), +//! None => println!("has no value"), +//! } +//! } +//! ``` +//! +//! # Representation +//! +//! Rust guarantees to optimize the following types `T` such that +//! [`Option<T>`] has the same size as `T`: +//! +//! * [`Box<U>`] +//! * `&U` +//! * `&mut U` +//! * `fn`, `extern "C" fn`[^extern_fn] +//! * [`num::NonZero*`] +//! * [`ptr::NonNull<U>`] +//! * `#[repr(transparent)]` struct around one of the types in this list. +//! +//! [^extern_fn]: this remains true for any other ABI: `extern "abi" fn` (_e.g._, `extern "system" fn`) +//! +//! [`Box<U>`]: ../../std/boxed/struct.Box.html +//! [`num::NonZero*`]: crate::num +//! [`ptr::NonNull<U>`]: crate::ptr::NonNull +//! +//! This is called the "null pointer optimization" or NPO. +//! +//! It is further guaranteed that, for the cases above, one can +//! [`mem::transmute`] from all valid values of `T` to `Option<T>` and +//! from `Some::<T>(_)` to `T` (but transmuting `None::<T>` to `T` +//! is undefined behaviour). +//! +//! # Method overview +//! +//! In addition to working with pattern matching, [`Option`] provides a wide +//! variety of different methods. +//! +//! ## Querying the variant +//! +//! The [`is_some`] and [`is_none`] methods return [`true`] if the [`Option`] +//! is [`Some`] or [`None`], respectively. +//! +//! [`is_none`]: Option::is_none +//! [`is_some`]: Option::is_some +//! +//! ## Adapters for working with references +//! +//! * [`as_ref`] converts from <code>[&][][Option]\<T></code> to <code>[Option]<[&]T></code> +//! * [`as_mut`] converts from <code>[&mut] [Option]\<T></code> to <code>[Option]<[&mut] T></code> +//! * [`as_deref`] converts from <code>[&][][Option]\<T></code> to +//! <code>[Option]<[&]T::[Target]></code> +//! * [`as_deref_mut`] converts from <code>[&mut] [Option]\<T></code> to +//! <code>[Option]<[&mut] T::[Target]></code> +//! * [`as_pin_ref`] converts from <code>[Pin]<[&][][Option]\<T>></code> to +//! <code>[Option]<[Pin]<[&]T>></code> +//! * [`as_pin_mut`] converts from <code>[Pin]<[&mut] [Option]\<T>></code> to +//! <code>[Option]<[Pin]<[&mut] T>></code> +//! +//! [&]: reference "shared reference" +//! [&mut]: reference "mutable reference" +//! [Target]: Deref::Target "ops::Deref::Target" +//! [`as_deref`]: Option::as_deref +//! [`as_deref_mut`]: Option::as_deref_mut +//! [`as_mut`]: Option::as_mut +//! [`as_pin_mut`]: Option::as_pin_mut +//! [`as_pin_ref`]: Option::as_pin_ref +//! [`as_ref`]: Option::as_ref +//! +//! ## Extracting the contained value +//! +//! These methods extract the contained value in an [`Option<T>`] when it +//! is the [`Some`] variant. If the [`Option`] is [`None`]: +//! +//! * [`expect`] panics with a provided custom message +//! * [`unwrap`] panics with a generic message +//! * [`unwrap_or`] returns the provided default value +//! * [`unwrap_or_default`] returns the default value of the type `T` +//! (which must implement the [`Default`] trait) +//! * [`unwrap_or_else`] returns the result of evaluating the provided +//! function +//! +//! [`expect`]: Option::expect +//! [`unwrap`]: Option::unwrap +//! [`unwrap_or`]: Option::unwrap_or +//! [`unwrap_or_default`]: Option::unwrap_or_default +//! [`unwrap_or_else`]: Option::unwrap_or_else +//! +//! ## Transforming contained values +//! +//! These methods transform [`Option`] to [`Result`]: +//! +//! * [`ok_or`] transforms [`Some(v)`] to [`Ok(v)`], and [`None`] to +//! [`Err(err)`] using the provided default `err` value +//! * [`ok_or_else`] transforms [`Some(v)`] to [`Ok(v)`], and [`None`] to +//! a value of [`Err`] using the provided function +//! * [`transpose`] transposes an [`Option`] of a [`Result`] into a +//! [`Result`] of an [`Option`] +//! +//! [`Err(err)`]: Err +//! [`Ok(v)`]: Ok +//! [`Some(v)`]: Some +//! [`ok_or`]: Option::ok_or +//! [`ok_or_else`]: Option::ok_or_else +//! [`transpose`]: Option::transpose +//! +//! These methods transform the [`Some`] variant: +//! +//! * [`filter`] calls the provided predicate function on the contained +//! value `t` if the [`Option`] is [`Some(t)`], and returns [`Some(t)`] +//! if the function returns `true`; otherwise, returns [`None`] +//! * [`flatten`] removes one level of nesting from an +//! [`Option<Option<T>>`] +//! * [`map`] transforms [`Option<T>`] to [`Option<U>`] by applying the +//! provided function to the contained value of [`Some`] and leaving +//! [`None`] values unchanged +//! +//! [`Some(t)`]: Some +//! [`filter`]: Option::filter +//! [`flatten`]: Option::flatten +//! [`map`]: Option::map +//! +//! These methods transform [`Option<T>`] to a value of a possibly +//! different type `U`: +//! +//! * [`map_or`] applies the provided function to the contained value of +//! [`Some`], or returns the provided default value if the [`Option`] is +//! [`None`] +//! * [`map_or_else`] applies the provided function to the contained value +//! of [`Some`], or returns the result of evaluating the provided +//! fallback function if the [`Option`] is [`None`] +//! +//! [`map_or`]: Option::map_or +//! [`map_or_else`]: Option::map_or_else +//! +//! These methods combine the [`Some`] variants of two [`Option`] values: +//! +//! * [`zip`] returns [`Some((s, o))`] if `self` is [`Some(s)`] and the +//! provided [`Option`] value is [`Some(o)`]; otherwise, returns [`None`] +//! * [`zip_with`] calls the provided function `f` and returns +//! [`Some(f(s, o))`] if `self` is [`Some(s)`] and the provided +//! [`Option`] value is [`Some(o)`]; otherwise, returns [`None`] +//! +//! [`Some(f(s, o))`]: Some +//! [`Some(o)`]: Some +//! [`Some(s)`]: Some +//! [`Some((s, o))`]: Some +//! [`zip`]: Option::zip +//! [`zip_with`]: Option::zip_with +//! +//! ## Boolean operators +//! +//! These methods treat the [`Option`] as a boolean value, where [`Some`] +//! acts like [`true`] and [`None`] acts like [`false`]. There are two +//! categories of these methods: ones that take an [`Option`] as input, and +//! ones that take a function as input (to be lazily evaluated). +//! +//! The [`and`], [`or`], and [`xor`] methods take another [`Option`] as +//! input, and produce an [`Option`] as output. Only the [`and`] method can +//! produce an [`Option<U>`] value having a different inner type `U` than +//! [`Option<T>`]. +//! +//! | method | self | input | output | +//! |---------|-----------|-----------|-----------| +//! | [`and`] | `None` | (ignored) | `None` | +//! | [`and`] | `Some(x)` | `None` | `None` | +//! | [`and`] | `Some(x)` | `Some(y)` | `Some(y)` | +//! | [`or`] | `None` | `None` | `None` | +//! | [`or`] | `None` | `Some(y)` | `Some(y)` | +//! | [`or`] | `Some(x)` | (ignored) | `Some(x)` | +//! | [`xor`] | `None` | `None` | `None` | +//! | [`xor`] | `None` | `Some(y)` | `Some(y)` | +//! | [`xor`] | `Some(x)` | `None` | `Some(x)` | +//! | [`xor`] | `Some(x)` | `Some(y)` | `None` | +//! +//! [`and`]: Option::and +//! [`or`]: Option::or +//! [`xor`]: Option::xor +//! +//! The [`and_then`] and [`or_else`] methods take a function as input, and +//! only evaluate the function when they need to produce a new value. Only +//! the [`and_then`] method can produce an [`Option<U>`] value having a +//! different inner type `U` than [`Option<T>`]. +//! +//! | method | self | function input | function result | output | +//! |--------------|-----------|----------------|-----------------|-----------| +//! | [`and_then`] | `None` | (not provided) | (not evaluated) | `None` | +//! | [`and_then`] | `Some(x)` | `x` | `None` | `None` | +//! | [`and_then`] | `Some(x)` | `x` | `Some(y)` | `Some(y)` | +//! | [`or_else`] | `None` | (not provided) | `None` | `None` | +//! | [`or_else`] | `None` | (not provided) | `Some(y)` | `Some(y)` | +//! | [`or_else`] | `Some(x)` | (not provided) | (not evaluated) | `Some(x)` | +//! +//! [`and_then`]: Option::and_then +//! [`or_else`]: Option::or_else +//! +//! This is an example of using methods like [`and_then`] and [`or`] in a +//! pipeline of method calls. Early stages of the pipeline pass failure +//! values ([`None`]) through unchanged, and continue processing on +//! success values ([`Some`]). Toward the end, [`or`] substitutes an error +//! message if it receives [`None`]. +//! +//! ``` +//! # use std::collections::BTreeMap; +//! let mut bt = BTreeMap::new(); +//! bt.insert(20u8, "foo"); +//! bt.insert(42u8, "bar"); +//! let res = [0u8, 1, 11, 200, 22] +//! .into_iter() +//! .map(|x| { +//! // `checked_sub()` returns `None` on error +//! x.checked_sub(1) +//! // same with `checked_mul()` +//! .and_then(|x| x.checked_mul(2)) +//! // `BTreeMap::get` returns `None` on error +//! .and_then(|x| bt.get(&x)) +//! // Substitute an error message if we have `None` so far +//! .or(Some(&"error!")) +//! .copied() +//! // Won't panic because we unconditionally used `Some` above +//! .unwrap() +//! }) +//! .collect::<Vec<_>>(); +//! assert_eq!(res, ["error!", "error!", "foo", "error!", "bar"]); +//! ``` +//! +//! ## Comparison operators +//! +//! If `T` implements [`PartialOrd`] then [`Option<T>`] will derive its +//! [`PartialOrd`] implementation. With this order, [`None`] compares as +//! less than any [`Some`], and two [`Some`] compare the same way as their +//! contained values would in `T`. If `T` also implements +//! [`Ord`], then so does [`Option<T>`]. +//! +//! ``` +//! assert!(None < Some(0)); +//! assert!(Some(0) < Some(1)); +//! ``` +//! +//! ## Iterating over `Option` +//! +//! An [`Option`] can be iterated over. This can be helpful if you need an +//! iterator that is conditionally empty. The iterator will either produce +//! a single value (when the [`Option`] is [`Some`]), or produce no values +//! (when the [`Option`] is [`None`]). For example, [`into_iter`] acts like +//! [`once(v)`] if the [`Option`] is [`Some(v)`], and like [`empty()`] if +//! the [`Option`] is [`None`]. +//! +//! [`Some(v)`]: Some +//! [`empty()`]: crate::iter::empty +//! [`once(v)`]: crate::iter::once +//! +//! Iterators over [`Option<T>`] come in three types: +//! +//! * [`into_iter`] consumes the [`Option`] and produces the contained +//! value +//! * [`iter`] produces an immutable reference of type `&T` to the +//! contained value +//! * [`iter_mut`] produces a mutable reference of type `&mut T` to the +//! contained value +//! +//! [`into_iter`]: Option::into_iter +//! [`iter`]: Option::iter +//! [`iter_mut`]: Option::iter_mut +//! +//! An iterator over [`Option`] can be useful when chaining iterators, for +//! example, to conditionally insert items. (It's not always necessary to +//! explicitly call an iterator constructor: many [`Iterator`] methods that +//! accept other iterators will also accept iterable types that implement +//! [`IntoIterator`], which includes [`Option`].) +//! +//! ``` +//! let yep = Some(42); +//! let nope = None; +//! // chain() already calls into_iter(), so we don't have to do so +//! let nums: Vec<i32> = (0..4).chain(yep).chain(4..8).collect(); +//! assert_eq!(nums, [0, 1, 2, 3, 42, 4, 5, 6, 7]); +//! let nums: Vec<i32> = (0..4).chain(nope).chain(4..8).collect(); +//! assert_eq!(nums, [0, 1, 2, 3, 4, 5, 6, 7]); +//! ``` +//! +//! One reason to chain iterators in this way is that a function returning +//! `impl Iterator` must have all possible return values be of the same +//! concrete type. Chaining an iterated [`Option`] can help with that. +//! +//! ``` +//! fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> { +//! // Explicit returns to illustrate return types matching +//! match do_insert { +//! true => return (0..4).chain(Some(42)).chain(4..8), +//! false => return (0..4).chain(None).chain(4..8), +//! } +//! } +//! println!("{:?}", make_iter(true).collect::<Vec<_>>()); +//! println!("{:?}", make_iter(false).collect::<Vec<_>>()); +//! ``` +//! +//! If we try to do the same thing, but using [`once()`] and [`empty()`], +//! we can't return `impl Iterator` anymore because the concrete types of +//! the return values differ. +//! +//! [`empty()`]: crate::iter::empty +//! [`once()`]: crate::iter::once +//! +//! ```compile_fail,E0308 +//! # use std::iter::{empty, once}; +//! // This won't compile because all possible returns from the function +//! // must have the same concrete type. +//! fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> { +//! // Explicit returns to illustrate return types not matching +//! match do_insert { +//! true => return (0..4).chain(once(42)).chain(4..8), +//! false => return (0..4).chain(empty()).chain(4..8), +//! } +//! } +//! ``` +//! +//! ## Collecting into `Option` +//! +//! [`Option`] implements the [`FromIterator`][impl-FromIterator] trait, +//! which allows an iterator over [`Option`] values to be collected into an +//! [`Option`] of a collection of each contained value of the original +//! [`Option`] values, or [`None`] if any of the elements was [`None`]. +//! +//! [impl-FromIterator]: Option#impl-FromIterator%3COption%3CA%3E%3E-for-Option%3CV%3E +//! +//! ``` +//! let v = [Some(2), Some(4), None, Some(8)]; +//! let res: Option<Vec<_>> = v.into_iter().collect(); +//! assert_eq!(res, None); +//! let v = [Some(2), Some(4), Some(8)]; +//! let res: Option<Vec<_>> = v.into_iter().collect(); +//! assert_eq!(res, Some(vec![2, 4, 8])); +//! ``` +//! +//! [`Option`] also implements the [`Product`][impl-Product] and +//! [`Sum`][impl-Sum] traits, allowing an iterator over [`Option`] values +//! to provide the [`product`][Iterator::product] and +//! [`sum`][Iterator::sum] methods. +//! +//! [impl-Product]: Option#impl-Product%3COption%3CU%3E%3E-for-Option%3CT%3E +//! [impl-Sum]: Option#impl-Sum%3COption%3CU%3E%3E-for-Option%3CT%3E +//! +//! ``` +//! let v = [None, Some(1), Some(2), Some(3)]; +//! let res: Option<i32> = v.into_iter().sum(); +//! assert_eq!(res, None); +//! let v = [Some(1), Some(2), Some(21)]; +//! let res: Option<i32> = v.into_iter().product(); +//! assert_eq!(res, Some(42)); +//! ``` +//! +//! ## Modifying an [`Option`] in-place +//! +//! These methods return a mutable reference to the contained value of an +//! [`Option<T>`]: +//! +//! * [`insert`] inserts a value, dropping any old contents +//! * [`get_or_insert`] gets the current value, inserting a provided +//! default value if it is [`None`] +//! * [`get_or_insert_default`] gets the current value, inserting the +//! default value of type `T` (which must implement [`Default`]) if it is +//! [`None`] +//! * [`get_or_insert_with`] gets the current value, inserting a default +//! computed by the provided function if it is [`None`] +//! +//! [`get_or_insert`]: Option::get_or_insert +//! [`get_or_insert_default`]: Option::get_or_insert_default +//! [`get_or_insert_with`]: Option::get_or_insert_with +//! [`insert`]: Option::insert +//! +//! These methods transfer ownership of the contained value of an +//! [`Option`]: +//! +//! * [`take`] takes ownership of the contained value of an [`Option`], if +//! any, replacing the [`Option`] with [`None`] +//! * [`replace`] takes ownership of the contained value of an [`Option`], +//! if any, replacing the [`Option`] with a [`Some`] containing the +//! provided value +//! +//! [`replace`]: Option::replace +//! [`take`]: Option::take +//! +//! # Examples +//! +//! Basic pattern matching on [`Option`]: +//! +//! ``` +//! let msg = Some("howdy"); +//! +//! // Take a reference to the contained string +//! if let Some(m) = &msg { +//! println!("{}", *m); +//! } +//! +//! // Remove the contained string, destroying the Option +//! let unwrapped_msg = msg.unwrap_or("default message"); +//! ``` +//! +//! Initialize a result to [`None`] before a loop: +//! +//! ``` +//! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) } +//! +//! // A list of data to search through. +//! let all_the_big_things = [ +//! Kingdom::Plant(250, "redwood"), +//! Kingdom::Plant(230, "noble fir"), +//! Kingdom::Plant(229, "sugar pine"), +//! Kingdom::Animal(25, "blue whale"), +//! Kingdom::Animal(19, "fin whale"), +//! Kingdom::Animal(15, "north pacific right whale"), +//! ]; +//! +//! // We're going to search for the name of the biggest animal, +//! // but to start with we've just got `None`. +//! let mut name_of_biggest_animal = None; +//! let mut size_of_biggest_animal = 0; +//! for big_thing in &all_the_big_things { +//! match *big_thing { +//! Kingdom::Animal(size, name) if size > size_of_biggest_animal => { +//! // Now we've found the name of some big animal +//! size_of_biggest_animal = size; +//! name_of_biggest_animal = Some(name); +//! } +//! Kingdom::Animal(..) | Kingdom::Plant(..) => () +//! } +//! } +//! +//! match name_of_biggest_animal { +//! Some(name) => println!("the biggest animal is {name}"), +//! None => println!("there are no animals :("), +//! } +//! ``` + +#![stable(feature = "rust1", since = "1.0.0")] + +use crate::iter::{self, FromIterator, FusedIterator, TrustedLen}; +use crate::marker::Destruct; +use crate::panicking::{panic, panic_str}; +use crate::pin::Pin; +use crate::{ + convert, hint, mem, + ops::{self, ControlFlow, Deref, DerefMut}, +}; + +/// The `Option` type. See [the module level documentation](self) for more. +#[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)] +#[rustc_diagnostic_item = "Option"] +#[stable(feature = "rust1", since = "1.0.0")] +pub enum Option<T> { + /// No value. + #[lang = "None"] + #[stable(feature = "rust1", since = "1.0.0")] + None, + /// Some value of type `T`. + #[lang = "Some"] + #[stable(feature = "rust1", since = "1.0.0")] + Some(#[stable(feature = "rust1", since = "1.0.0")] T), +} + +///////////////////////////////////////////////////////////////////////////// +// Type implementation +///////////////////////////////////////////////////////////////////////////// + +impl<T> Option<T> { + ///////////////////////////////////////////////////////////////////////// + // Querying the contained values + ///////////////////////////////////////////////////////////////////////// + + /// Returns `true` if the option is a [`Some`] value. + /// + /// # Examples + /// + /// ``` + /// let x: Option<u32> = Some(2); + /// assert_eq!(x.is_some(), true); + /// + /// let x: Option<u32> = None; + /// assert_eq!(x.is_some(), false); + /// ``` + #[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"] + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")] + pub const fn is_some(&self) -> bool { + matches!(*self, Some(_)) + } + + /// Returns `true` if the option is a [`Some`] and the value inside of it matches a predicate. + /// + /// # Examples + /// + /// ``` + /// #![feature(is_some_with)] + /// + /// let x: Option<u32> = Some(2); + /// assert_eq!(x.is_some_and(|&x| x > 1), true); + /// + /// let x: Option<u32> = Some(0); + /// assert_eq!(x.is_some_and(|&x| x > 1), false); + /// + /// let x: Option<u32> = None; + /// assert_eq!(x.is_some_and(|&x| x > 1), false); + /// ``` + #[must_use] + #[inline] + #[unstable(feature = "is_some_with", issue = "93050")] + pub fn is_some_and(&self, f: impl FnOnce(&T) -> bool) -> bool { + matches!(self, Some(x) if f(x)) + } + + /// Returns `true` if the option is a [`None`] value. + /// + /// # Examples + /// + /// ``` + /// let x: Option<u32> = Some(2); + /// assert_eq!(x.is_none(), false); + /// + /// let x: Option<u32> = None; + /// assert_eq!(x.is_none(), true); + /// ``` + #[must_use = "if you intended to assert that this doesn't have a value, consider \ + `.and_then(|_| panic!(\"`Option` had a value when expected `None`\"))` instead"] + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")] + pub const fn is_none(&self) -> bool { + !self.is_some() + } + + ///////////////////////////////////////////////////////////////////////// + // Adapter for working with references + ///////////////////////////////////////////////////////////////////////// + + /// Converts from `&Option<T>` to `Option<&T>`. + /// + /// # Examples + /// + /// Converts an <code>Option<[String]></code> into an <code>Option<[usize]></code>, preserving + /// the original. The [`map`] method takes the `self` argument by value, consuming the original, + /// so this technique uses `as_ref` to first take an `Option` to a reference + /// to the value inside the original. + /// + /// [`map`]: Option::map + /// [String]: ../../std/string/struct.String.html "String" + /// + /// ``` + /// let text: Option<String> = Some("Hello, world!".to_string()); + /// // First, cast `Option<String>` to `Option<&String>` with `as_ref`, + /// // then consume *that* with `map`, leaving `text` on the stack. + /// let text_length: Option<usize> = text.as_ref().map(|s| s.len()); + /// println!("still can print text: {text:?}"); + /// ``` + #[inline] + #[rustc_const_stable(feature = "const_option_basics", since = "1.48.0")] + #[stable(feature = "rust1", since = "1.0.0")] + pub const fn as_ref(&self) -> Option<&T> { + match *self { + Some(ref x) => Some(x), + None => None, + } + } + + /// Converts from `&mut Option<T>` to `Option<&mut T>`. + /// + /// # Examples + /// + /// ``` + /// let mut x = Some(2); + /// match x.as_mut() { + /// Some(v) => *v = 42, + /// None => {}, + /// } + /// assert_eq!(x, Some(42)); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_unstable(feature = "const_option", issue = "67441")] + pub const fn as_mut(&mut self) -> Option<&mut T> { + match *self { + Some(ref mut x) => Some(x), + None => None, + } + } + + /// Converts from <code>[Pin]<[&]Option\<T>></code> to <code>Option<[Pin]<[&]T>></code>. + /// + /// [&]: reference "shared reference" + #[inline] + #[must_use] + #[stable(feature = "pin", since = "1.33.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> { + match Pin::get_ref(self).as_ref() { + // SAFETY: `x` is guaranteed to be pinned because it comes from `self` + // which is pinned. + Some(x) => unsafe { Some(Pin::new_unchecked(x)) }, + None => None, + } + } + + /// Converts from <code>[Pin]<[&mut] Option\<T>></code> to <code>Option<[Pin]<[&mut] T>></code>. + /// + /// [&mut]: reference "mutable reference" + #[inline] + #[must_use] + #[stable(feature = "pin", since = "1.33.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> { + // SAFETY: `get_unchecked_mut` is never used to move the `Option` inside `self`. + // `x` is guaranteed to be pinned because it comes from `self` which is pinned. + unsafe { + match Pin::get_unchecked_mut(self).as_mut() { + Some(x) => Some(Pin::new_unchecked(x)), + None => None, + } + } + } + + ///////////////////////////////////////////////////////////////////////// + // Getting to contained values + ///////////////////////////////////////////////////////////////////////// + + /// Returns the contained [`Some`] value, consuming the `self` value. + /// + /// # Panics + /// + /// Panics if the value is a [`None`] with a custom panic message provided by + /// `msg`. + /// + /// # Examples + /// + /// ``` + /// let x = Some("value"); + /// assert_eq!(x.expect("fruits are healthy"), "value"); + /// ``` + /// + /// ```should_panic + /// let x: Option<&str> = None; + /// x.expect("fruits are healthy"); // panics with `fruits are healthy` + /// ``` + /// + /// # Recommended Message Style + /// + /// We recommend that `expect` messages are used to describe the reason you + /// _expect_ the `Option` should be `Some`. + /// + /// ```should_panic + /// # let slice: &[u8] = &[]; + /// let item = slice.get(0) + /// .expect("slice should not be empty"); + /// ``` + /// + /// **Hint**: If you're having trouble remembering how to phrase expect + /// error messages remember to focus on the word "should" as in "env + /// variable should be set by blah" or "the given binary should be available + /// and executable by the current user". + /// + /// For more detail on expect message styles and the reasoning behind our + /// recommendation please refer to the section on ["Common Message + /// Styles"](../../std/error/index.html#common-message-styles) in the [`std::error`](../../std/error/index.html) module docs. + #[inline] + #[track_caller] + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_unstable(feature = "const_option", issue = "67441")] + pub const fn expect(self, msg: &str) -> T { + match self { + Some(val) => val, + None => expect_failed(msg), + } + } + + /// Returns the contained [`Some`] value, consuming the `self` value. + /// + /// Because this function may panic, its use is generally discouraged. + /// Instead, prefer to use pattern matching and handle the [`None`] + /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or + /// [`unwrap_or_default`]. + /// + /// [`unwrap_or`]: Option::unwrap_or + /// [`unwrap_or_else`]: Option::unwrap_or_else + /// [`unwrap_or_default`]: Option::unwrap_or_default + /// + /// # Panics + /// + /// Panics if the self value equals [`None`]. + /// + /// # Examples + /// + /// ``` + /// let x = Some("air"); + /// assert_eq!(x.unwrap(), "air"); + /// ``` + /// + /// ```should_panic + /// let x: Option<&str> = None; + /// assert_eq!(x.unwrap(), "air"); // fails + /// ``` + #[inline] + #[track_caller] + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_unstable(feature = "const_option", issue = "67441")] + pub const fn unwrap(self) -> T { + match self { + Some(val) => val, + None => panic("called `Option::unwrap()` on a `None` value"), + } + } + + /// Returns the contained [`Some`] value or a provided default. + /// + /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing + /// the result of a function call, it is recommended to use [`unwrap_or_else`], + /// which is lazily evaluated. + /// + /// [`unwrap_or_else`]: Option::unwrap_or_else + /// + /// # Examples + /// + /// ``` + /// assert_eq!(Some("car").unwrap_or("bike"), "car"); + /// assert_eq!(None.unwrap_or("bike"), "bike"); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn unwrap_or(self, default: T) -> T + where + T: ~const Destruct, + { + match self { + Some(x) => x, + None => default, + } + } + + /// Returns the contained [`Some`] value or computes it from a closure. + /// + /// # Examples + /// + /// ``` + /// let k = 10; + /// assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4); + /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn unwrap_or_else<F>(self, f: F) -> T + where + F: ~const FnOnce() -> T, + F: ~const Destruct, + { + match self { + Some(x) => x, + None => f(), + } + } + + /// Returns the contained [`Some`] value or a default. + /// + /// Consumes the `self` argument then, if [`Some`], returns the contained + /// value, otherwise if [`None`], returns the [default value] for that + /// type. + /// + /// # Examples + /// + /// Converts a string to an integer, turning poorly-formed strings + /// into 0 (the default value for integers). [`parse`] converts + /// a string to any other type that implements [`FromStr`], returning + /// [`None`] on error. + /// + /// ``` + /// let good_year_from_input = "1909"; + /// let bad_year_from_input = "190blarg"; + /// let good_year = good_year_from_input.parse().ok().unwrap_or_default(); + /// let bad_year = bad_year_from_input.parse().ok().unwrap_or_default(); + /// + /// assert_eq!(1909, good_year); + /// assert_eq!(0, bad_year); + /// ``` + /// + /// [default value]: Default::default + /// [`parse`]: str::parse + /// [`FromStr`]: crate::str::FromStr + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn unwrap_or_default(self) -> T + where + T: ~const Default, + { + match self { + Some(x) => x, + None => Default::default(), + } + } + + /// Returns the contained [`Some`] value, consuming the `self` value, + /// without checking that the value is not [`None`]. + /// + /// # Safety + /// + /// Calling this method on [`None`] is *[undefined behavior]*. + /// + /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html + /// + /// # Examples + /// + /// ``` + /// let x = Some("air"); + /// assert_eq!(unsafe { x.unwrap_unchecked() }, "air"); + /// ``` + /// + /// ```no_run + /// let x: Option<&str> = None; + /// assert_eq!(unsafe { x.unwrap_unchecked() }, "air"); // Undefined behavior! + /// ``` + #[inline] + #[track_caller] + #[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const unsafe fn unwrap_unchecked(self) -> T { + debug_assert!(self.is_some()); + match self { + Some(val) => val, + // SAFETY: the safety contract must be upheld by the caller. + None => unsafe { hint::unreachable_unchecked() }, + } + } + + ///////////////////////////////////////////////////////////////////////// + // Transforming contained values + ///////////////////////////////////////////////////////////////////////// + + /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value. + /// + /// # Examples + /// + /// Converts an <code>Option<[String]></code> into an <code>Option<[usize]></code>, consuming + /// the original: + /// + /// [String]: ../../std/string/struct.String.html "String" + /// ``` + /// let maybe_some_string = Some(String::from("Hello, World!")); + /// // `Option::map` takes self *by value*, consuming `maybe_some_string` + /// let maybe_some_len = maybe_some_string.map(|s| s.len()); + /// + /// assert_eq!(maybe_some_len, Some(13)); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn map<U, F>(self, f: F) -> Option<U> + where + F: ~const FnOnce(T) -> U, + F: ~const Destruct, + { + match self { + Some(x) => Some(f(x)), + None => None, + } + } + + /// Calls the provided closure with a reference to the contained value (if [`Some`]). + /// + /// # Examples + /// + /// ``` + /// #![feature(result_option_inspect)] + /// + /// let v = vec![1, 2, 3, 4, 5]; + /// + /// // prints "got: 4" + /// let x: Option<&usize> = v.get(3).inspect(|x| println!("got: {x}")); + /// + /// // prints nothing + /// let x: Option<&usize> = v.get(5).inspect(|x| println!("got: {x}")); + /// ``` + #[inline] + #[unstable(feature = "result_option_inspect", issue = "91345")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn inspect<F>(self, f: F) -> Self + where + F: ~const FnOnce(&T), + F: ~const Destruct, + { + if let Some(ref x) = self { + f(x); + } + + self + } + + /// Returns the provided default result (if none), + /// or applies a function to the contained value (if any). + /// + /// Arguments passed to `map_or` are eagerly evaluated; if you are passing + /// the result of a function call, it is recommended to use [`map_or_else`], + /// which is lazily evaluated. + /// + /// [`map_or_else`]: Option::map_or_else + /// + /// # Examples + /// + /// ``` + /// let x = Some("foo"); + /// assert_eq!(x.map_or(42, |v| v.len()), 3); + /// + /// let x: Option<&str> = None; + /// assert_eq!(x.map_or(42, |v| v.len()), 42); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn map_or<U, F>(self, default: U, f: F) -> U + where + F: ~const FnOnce(T) -> U, + F: ~const Destruct, + U: ~const Destruct, + { + match self { + Some(t) => f(t), + None => default, + } + } + + /// Computes a default function result (if none), or + /// applies a different function to the contained value (if any). + /// + /// # Examples + /// + /// ``` + /// let k = 21; + /// + /// let x = Some("foo"); + /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3); + /// + /// let x: Option<&str> = None; + /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn map_or_else<U, D, F>(self, default: D, f: F) -> U + where + D: ~const FnOnce() -> U, + D: ~const Destruct, + F: ~const FnOnce(T) -> U, + F: ~const Destruct, + { + match self { + Some(t) => f(t), + None => default(), + } + } + + /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to + /// [`Ok(v)`] and [`None`] to [`Err(err)`]. + /// + /// Arguments passed to `ok_or` are eagerly evaluated; if you are passing the + /// result of a function call, it is recommended to use [`ok_or_else`], which is + /// lazily evaluated. + /// + /// [`Ok(v)`]: Ok + /// [`Err(err)`]: Err + /// [`Some(v)`]: Some + /// [`ok_or_else`]: Option::ok_or_else + /// + /// # Examples + /// + /// ``` + /// let x = Some("foo"); + /// assert_eq!(x.ok_or(0), Ok("foo")); + /// + /// let x: Option<&str> = None; + /// assert_eq!(x.ok_or(0), Err(0)); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn ok_or<E>(self, err: E) -> Result<T, E> + where + E: ~const Destruct, + { + match self { + Some(v) => Ok(v), + None => Err(err), + } + } + + /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to + /// [`Ok(v)`] and [`None`] to [`Err(err())`]. + /// + /// [`Ok(v)`]: Ok + /// [`Err(err())`]: Err + /// [`Some(v)`]: Some + /// + /// # Examples + /// + /// ``` + /// let x = Some("foo"); + /// assert_eq!(x.ok_or_else(|| 0), Ok("foo")); + /// + /// let x: Option<&str> = None; + /// assert_eq!(x.ok_or_else(|| 0), Err(0)); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn ok_or_else<E, F>(self, err: F) -> Result<T, E> + where + F: ~const FnOnce() -> E, + F: ~const Destruct, + { + match self { + Some(v) => Ok(v), + None => Err(err()), + } + } + + /// Converts from `Option<T>` (or `&Option<T>`) to `Option<&T::Target>`. + /// + /// Leaves the original Option in-place, creating a new one with a reference + /// to the original one, additionally coercing the contents via [`Deref`]. + /// + /// # Examples + /// + /// ``` + /// let x: Option<String> = Some("hey".to_owned()); + /// assert_eq!(x.as_deref(), Some("hey")); + /// + /// let x: Option<String> = None; + /// assert_eq!(x.as_deref(), None); + /// ``` + #[stable(feature = "option_deref", since = "1.40.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn as_deref(&self) -> Option<&T::Target> + where + T: ~const Deref, + { + match self.as_ref() { + Some(t) => Some(t.deref()), + None => None, + } + } + + /// Converts from `Option<T>` (or `&mut Option<T>`) to `Option<&mut T::Target>`. + /// + /// Leaves the original `Option` in-place, creating a new one containing a mutable reference to + /// the inner type's [`Deref::Target`] type. + /// + /// # Examples + /// + /// ``` + /// let mut x: Option<String> = Some("hey".to_owned()); + /// assert_eq!(x.as_deref_mut().map(|x| { + /// x.make_ascii_uppercase(); + /// x + /// }), Some("HEY".to_owned().as_mut_str())); + /// ``` + #[stable(feature = "option_deref", since = "1.40.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn as_deref_mut(&mut self) -> Option<&mut T::Target> + where + T: ~const DerefMut, + { + match self.as_mut() { + Some(t) => Some(t.deref_mut()), + None => None, + } + } + + ///////////////////////////////////////////////////////////////////////// + // Iterator constructors + ///////////////////////////////////////////////////////////////////////// + + /// Returns an iterator over the possibly contained value. + /// + /// # Examples + /// + /// ``` + /// let x = Some(4); + /// assert_eq!(x.iter().next(), Some(&4)); + /// + /// let x: Option<u32> = None; + /// assert_eq!(x.iter().next(), None); + /// ``` + #[inline] + #[rustc_const_unstable(feature = "const_option", issue = "67441")] + #[stable(feature = "rust1", since = "1.0.0")] + pub const fn iter(&self) -> Iter<'_, T> { + Iter { inner: Item { opt: self.as_ref() } } + } + + /// Returns a mutable iterator over the possibly contained value. + /// + /// # Examples + /// + /// ``` + /// let mut x = Some(4); + /// match x.iter_mut().next() { + /// Some(v) => *v = 42, + /// None => {}, + /// } + /// assert_eq!(x, Some(42)); + /// + /// let mut x: Option<u32> = None; + /// assert_eq!(x.iter_mut().next(), None); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + pub fn iter_mut(&mut self) -> IterMut<'_, T> { + IterMut { inner: Item { opt: self.as_mut() } } + } + + ///////////////////////////////////////////////////////////////////////// + // Boolean operations on the values, eager and lazy + ///////////////////////////////////////////////////////////////////////// + + /// Returns [`None`] if the option is [`None`], otherwise returns `optb`. + /// + /// # Examples + /// + /// ``` + /// let x = Some(2); + /// let y: Option<&str> = None; + /// assert_eq!(x.and(y), None); + /// + /// let x: Option<u32> = None; + /// let y = Some("foo"); + /// assert_eq!(x.and(y), None); + /// + /// let x = Some(2); + /// let y = Some("foo"); + /// assert_eq!(x.and(y), Some("foo")); + /// + /// let x: Option<u32> = None; + /// let y: Option<&str> = None; + /// assert_eq!(x.and(y), None); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn and<U>(self, optb: Option<U>) -> Option<U> + where + T: ~const Destruct, + U: ~const Destruct, + { + match self { + Some(_) => optb, + None => None, + } + } + + /// Returns [`None`] if the option is [`None`], otherwise calls `f` with the + /// wrapped value and returns the result. + /// + /// Some languages call this operation flatmap. + /// + /// # Examples + /// + /// ``` + /// fn sq_then_to_string(x: u32) -> Option<String> { + /// x.checked_mul(x).map(|sq| sq.to_string()) + /// } + /// + /// assert_eq!(Some(2).and_then(sq_then_to_string), Some(4.to_string())); + /// assert_eq!(Some(1_000_000).and_then(sq_then_to_string), None); // overflowed! + /// assert_eq!(None.and_then(sq_then_to_string), None); + /// ``` + /// + /// Often used to chain fallible operations that may return [`None`]. + /// + /// ``` + /// let arr_2d = [["A0", "A1"], ["B0", "B1"]]; + /// + /// let item_0_1 = arr_2d.get(0).and_then(|row| row.get(1)); + /// assert_eq!(item_0_1, Some(&"A1")); + /// + /// let item_2_0 = arr_2d.get(2).and_then(|row| row.get(0)); + /// assert_eq!(item_2_0, None); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn and_then<U, F>(self, f: F) -> Option<U> + where + F: ~const FnOnce(T) -> Option<U>, + F: ~const Destruct, + { + match self { + Some(x) => f(x), + None => None, + } + } + + /// Returns [`None`] if the option is [`None`], otherwise calls `predicate` + /// with the wrapped value and returns: + /// + /// - [`Some(t)`] if `predicate` returns `true` (where `t` is the wrapped + /// value), and + /// - [`None`] if `predicate` returns `false`. + /// + /// This function works similar to [`Iterator::filter()`]. You can imagine + /// the `Option<T>` being an iterator over one or zero elements. `filter()` + /// lets you decide which elements to keep. + /// + /// # Examples + /// + /// ```rust + /// fn is_even(n: &i32) -> bool { + /// n % 2 == 0 + /// } + /// + /// assert_eq!(None.filter(is_even), None); + /// assert_eq!(Some(3).filter(is_even), None); + /// assert_eq!(Some(4).filter(is_even), Some(4)); + /// ``` + /// + /// [`Some(t)`]: Some + #[inline] + #[stable(feature = "option_filter", since = "1.27.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn filter<P>(self, predicate: P) -> Self + where + T: ~const Destruct, + P: ~const FnOnce(&T) -> bool, + P: ~const Destruct, + { + if let Some(x) = self { + if predicate(&x) { + return Some(x); + } + } + None + } + + /// Returns the option if it contains a value, otherwise returns `optb`. + /// + /// Arguments passed to `or` are eagerly evaluated; if you are passing the + /// result of a function call, it is recommended to use [`or_else`], which is + /// lazily evaluated. + /// + /// [`or_else`]: Option::or_else + /// + /// # Examples + /// + /// ``` + /// let x = Some(2); + /// let y = None; + /// assert_eq!(x.or(y), Some(2)); + /// + /// let x = None; + /// let y = Some(100); + /// assert_eq!(x.or(y), Some(100)); + /// + /// let x = Some(2); + /// let y = Some(100); + /// assert_eq!(x.or(y), Some(2)); + /// + /// let x: Option<u32> = None; + /// let y = None; + /// assert_eq!(x.or(y), None); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn or(self, optb: Option<T>) -> Option<T> + where + T: ~const Destruct, + { + match self { + Some(x) => Some(x), + None => optb, + } + } + + /// Returns the option if it contains a value, otherwise calls `f` and + /// returns the result. + /// + /// # Examples + /// + /// ``` + /// fn nobody() -> Option<&'static str> { None } + /// fn vikings() -> Option<&'static str> { Some("vikings") } + /// + /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians")); + /// assert_eq!(None.or_else(vikings), Some("vikings")); + /// assert_eq!(None.or_else(nobody), None); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn or_else<F>(self, f: F) -> Option<T> + where + F: ~const FnOnce() -> Option<T>, + F: ~const Destruct, + { + match self { + Some(x) => Some(x), + None => f(), + } + } + + /// Returns [`Some`] if exactly one of `self`, `optb` is [`Some`], otherwise returns [`None`]. + /// + /// # Examples + /// + /// ``` + /// let x = Some(2); + /// let y: Option<u32> = None; + /// assert_eq!(x.xor(y), Some(2)); + /// + /// let x: Option<u32> = None; + /// let y = Some(2); + /// assert_eq!(x.xor(y), Some(2)); + /// + /// let x = Some(2); + /// let y = Some(2); + /// assert_eq!(x.xor(y), None); + /// + /// let x: Option<u32> = None; + /// let y: Option<u32> = None; + /// assert_eq!(x.xor(y), None); + /// ``` + #[inline] + #[stable(feature = "option_xor", since = "1.37.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn xor(self, optb: Option<T>) -> Option<T> + where + T: ~const Destruct, + { + match (self, optb) { + (Some(a), None) => Some(a), + (None, Some(b)) => Some(b), + _ => None, + } + } + + ///////////////////////////////////////////////////////////////////////// + // Entry-like operations to insert a value and return a reference + ///////////////////////////////////////////////////////////////////////// + + /// Inserts `value` into the option, then returns a mutable reference to it. + /// + /// If the option already contains a value, the old value is dropped. + /// + /// See also [`Option::get_or_insert`], which doesn't update the value if + /// the option already contains [`Some`]. + /// + /// # Example + /// + /// ``` + /// let mut opt = None; + /// let val = opt.insert(1); + /// assert_eq!(*val, 1); + /// assert_eq!(opt.unwrap(), 1); + /// let val = opt.insert(2); + /// assert_eq!(*val, 2); + /// *val = 3; + /// assert_eq!(opt.unwrap(), 3); + /// ``` + #[must_use = "if you intended to set a value, consider assignment instead"] + #[inline] + #[stable(feature = "option_insert", since = "1.53.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn insert(&mut self, value: T) -> &mut T + where + T: ~const Destruct, + { + *self = Some(value); + + // SAFETY: the code above just filled the option + unsafe { self.as_mut().unwrap_unchecked() } + } + + /// Inserts `value` into the option if it is [`None`], then + /// returns a mutable reference to the contained value. + /// + /// See also [`Option::insert`], which updates the value even if + /// the option already contains [`Some`]. + /// + /// # Examples + /// + /// ``` + /// let mut x = None; + /// + /// { + /// let y: &mut u32 = x.get_or_insert(5); + /// assert_eq!(y, &5); + /// + /// *y = 7; + /// } + /// + /// assert_eq!(x, Some(7)); + /// ``` + #[inline] + #[stable(feature = "option_entry", since = "1.20.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn get_or_insert(&mut self, value: T) -> &mut T + where + T: ~const Destruct, + { + if let None = *self { + *self = Some(value); + } + + // SAFETY: a `None` variant for `self` would have been replaced by a `Some` + // variant in the code above. + unsafe { self.as_mut().unwrap_unchecked() } + } + + /// Inserts the default value into the option if it is [`None`], then + /// returns a mutable reference to the contained value. + /// + /// # Examples + /// + /// ``` + /// #![feature(option_get_or_insert_default)] + /// + /// let mut x = None; + /// + /// { + /// let y: &mut u32 = x.get_or_insert_default(); + /// assert_eq!(y, &0); + /// + /// *y = 7; + /// } + /// + /// assert_eq!(x, Some(7)); + /// ``` + #[inline] + #[unstable(feature = "option_get_or_insert_default", issue = "82901")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn get_or_insert_default(&mut self) -> &mut T + where + T: ~const Default, + { + const fn default<T: ~const Default>() -> T { + T::default() + } + + self.get_or_insert_with(default) + } + + /// Inserts a value computed from `f` into the option if it is [`None`], + /// then returns a mutable reference to the contained value. + /// + /// # Examples + /// + /// ``` + /// let mut x = None; + /// + /// { + /// let y: &mut u32 = x.get_or_insert_with(|| 5); + /// assert_eq!(y, &5); + /// + /// *y = 7; + /// } + /// + /// assert_eq!(x, Some(7)); + /// ``` + #[inline] + #[stable(feature = "option_entry", since = "1.20.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn get_or_insert_with<F>(&mut self, f: F) -> &mut T + where + F: ~const FnOnce() -> T, + F: ~const Destruct, + { + if let None = *self { + // the compiler isn't smart enough to know that we are not dropping a `T` + // here and wants us to ensure `T` can be dropped at compile time. + mem::forget(mem::replace(self, Some(f()))) + } + + // SAFETY: a `None` variant for `self` would have been replaced by a `Some` + // variant in the code above. + unsafe { self.as_mut().unwrap_unchecked() } + } + + ///////////////////////////////////////////////////////////////////////// + // Misc + ///////////////////////////////////////////////////////////////////////// + + /// Takes the value out of the option, leaving a [`None`] in its place. + /// + /// # Examples + /// + /// ``` + /// let mut x = Some(2); + /// let y = x.take(); + /// assert_eq!(x, None); + /// assert_eq!(y, Some(2)); + /// + /// let mut x: Option<u32> = None; + /// let y = x.take(); + /// assert_eq!(x, None); + /// assert_eq!(y, None); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_unstable(feature = "const_option", issue = "67441")] + pub const fn take(&mut self) -> Option<T> { + // FIXME replace `mem::replace` by `mem::take` when the latter is const ready + mem::replace(self, None) + } + + /// Replaces the actual value in the option by the value given in parameter, + /// returning the old value if present, + /// leaving a [`Some`] in its place without deinitializing either one. + /// + /// # Examples + /// + /// ``` + /// let mut x = Some(2); + /// let old = x.replace(5); + /// assert_eq!(x, Some(5)); + /// assert_eq!(old, Some(2)); + /// + /// let mut x = None; + /// let old = x.replace(3); + /// assert_eq!(x, Some(3)); + /// assert_eq!(old, None); + /// ``` + #[inline] + #[rustc_const_unstable(feature = "const_option", issue = "67441")] + #[stable(feature = "option_replace", since = "1.31.0")] + pub const fn replace(&mut self, value: T) -> Option<T> { + mem::replace(self, Some(value)) + } + + /// Returns `true` if the option is a [`Some`] value containing the given value. + /// + /// # Examples + /// + /// ``` + /// #![feature(option_result_contains)] + /// + /// let x: Option<u32> = Some(2); + /// assert_eq!(x.contains(&2), true); + /// + /// let x: Option<u32> = Some(3); + /// assert_eq!(x.contains(&2), false); + /// + /// let x: Option<u32> = None; + /// assert_eq!(x.contains(&2), false); + /// ``` + #[must_use] + #[inline] + #[unstable(feature = "option_result_contains", issue = "62358")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn contains<U>(&self, x: &U) -> bool + where + U: ~const PartialEq<T>, + { + match self { + Some(y) => x.eq(y), + None => false, + } + } + + /// Zips `self` with another `Option`. + /// + /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some((s, o))`. + /// Otherwise, `None` is returned. + /// + /// # Examples + /// + /// ``` + /// let x = Some(1); + /// let y = Some("hi"); + /// let z = None::<u8>; + /// + /// assert_eq!(x.zip(y), Some((1, "hi"))); + /// assert_eq!(x.zip(z), None); + /// ``` + #[stable(feature = "option_zip_option", since = "1.46.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn zip<U>(self, other: Option<U>) -> Option<(T, U)> + where + T: ~const Destruct, + U: ~const Destruct, + { + match (self, other) { + (Some(a), Some(b)) => Some((a, b)), + _ => None, + } + } + + /// Zips `self` and another `Option` with function `f`. + /// + /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`. + /// Otherwise, `None` is returned. + /// + /// # Examples + /// + /// ``` + /// #![feature(option_zip)] + /// + /// #[derive(Debug, PartialEq)] + /// struct Point { + /// x: f64, + /// y: f64, + /// } + /// + /// impl Point { + /// fn new(x: f64, y: f64) -> Self { + /// Self { x, y } + /// } + /// } + /// + /// let x = Some(17.5); + /// let y = Some(42.7); + /// + /// assert_eq!(x.zip_with(y, Point::new), Some(Point { x: 17.5, y: 42.7 })); + /// assert_eq!(x.zip_with(None, Point::new), None); + /// ``` + #[unstable(feature = "option_zip", issue = "70086")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R> + where + F: ~const FnOnce(T, U) -> R, + F: ~const Destruct, + T: ~const Destruct, + U: ~const Destruct, + { + match (self, other) { + (Some(a), Some(b)) => Some(f(a, b)), + _ => None, + } + } +} + +impl<T, U> Option<(T, U)> { + /// Unzips an option containing a tuple of two options. + /// + /// If `self` is `Some((a, b))` this method returns `(Some(a), Some(b))`. + /// Otherwise, `(None, None)` is returned. + /// + /// # Examples + /// + /// ``` + /// #![feature(unzip_option)] + /// + /// let x = Some((1, "hi")); + /// let y = None::<(u8, u32)>; + /// + /// assert_eq!(x.unzip(), (Some(1), Some("hi"))); + /// assert_eq!(y.unzip(), (None, None)); + /// ``` + #[inline] + #[unstable(feature = "unzip_option", issue = "87800", reason = "recently added")] + pub const fn unzip(self) -> (Option<T>, Option<U>) { + match self { + Some((a, b)) => (Some(a), Some(b)), + None => (None, None), + } + } +} + +impl<T> Option<&T> { + /// Maps an `Option<&T>` to an `Option<T>` by copying the contents of the + /// option. + /// + /// # Examples + /// + /// ``` + /// let x = 12; + /// let opt_x = Some(&x); + /// assert_eq!(opt_x, Some(&12)); + /// let copied = opt_x.copied(); + /// assert_eq!(copied, Some(12)); + /// ``` + #[must_use = "`self` will be dropped if the result is not used"] + #[stable(feature = "copied", since = "1.35.0")] + #[rustc_const_unstable(feature = "const_option", issue = "67441")] + pub const fn copied(self) -> Option<T> + where + T: Copy, + { + // FIXME: this implementation, which sidesteps using `Option::map` since it's not const + // ready yet, should be reverted when possible to avoid code repetition + match self { + Some(&v) => Some(v), + None => None, + } + } + + /// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the + /// option. + /// + /// # Examples + /// + /// ``` + /// let x = 12; + /// let opt_x = Some(&x); + /// assert_eq!(opt_x, Some(&12)); + /// let cloned = opt_x.cloned(); + /// assert_eq!(cloned, Some(12)); + /// ``` + #[must_use = "`self` will be dropped if the result is not used"] + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_unstable(feature = "const_option_cloned", issue = "91582")] + pub const fn cloned(self) -> Option<T> + where + T: ~const Clone, + { + match self { + Some(t) => Some(t.clone()), + None => None, + } + } +} + +impl<T> Option<&mut T> { + /// Maps an `Option<&mut T>` to an `Option<T>` by copying the contents of the + /// option. + /// + /// # Examples + /// + /// ``` + /// let mut x = 12; + /// let opt_x = Some(&mut x); + /// assert_eq!(opt_x, Some(&mut 12)); + /// let copied = opt_x.copied(); + /// assert_eq!(copied, Some(12)); + /// ``` + #[must_use = "`self` will be dropped if the result is not used"] + #[stable(feature = "copied", since = "1.35.0")] + #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")] + pub const fn copied(self) -> Option<T> + where + T: Copy, + { + match self { + Some(&mut t) => Some(t), + None => None, + } + } + + /// Maps an `Option<&mut T>` to an `Option<T>` by cloning the contents of the + /// option. + /// + /// # Examples + /// + /// ``` + /// let mut x = 12; + /// let opt_x = Some(&mut x); + /// assert_eq!(opt_x, Some(&mut 12)); + /// let cloned = opt_x.cloned(); + /// assert_eq!(cloned, Some(12)); + /// ``` + #[must_use = "`self` will be dropped if the result is not used"] + #[stable(since = "1.26.0", feature = "option_ref_mut_cloned")] + #[rustc_const_unstable(feature = "const_option_cloned", issue = "91582")] + pub const fn cloned(self) -> Option<T> + where + T: ~const Clone, + { + match self { + Some(t) => Some(t.clone()), + None => None, + } + } +} + +impl<T, E> Option<Result<T, E>> { + /// Transposes an `Option` of a [`Result`] into a [`Result`] of an `Option`. + /// + /// [`None`] will be mapped to <code>[Ok]\([None])</code>. + /// <code>[Some]\([Ok]\(\_))</code> and <code>[Some]\([Err]\(\_))</code> will be mapped to + /// <code>[Ok]\([Some]\(\_))</code> and <code>[Err]\(\_)</code>. + /// + /// # Examples + /// + /// ``` + /// #[derive(Debug, Eq, PartialEq)] + /// struct SomeErr; + /// + /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5)); + /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5)); + /// assert_eq!(x, y.transpose()); + /// ``` + #[inline] + #[stable(feature = "transpose_result", since = "1.33.0")] + #[rustc_const_unstable(feature = "const_option", issue = "67441")] + pub const fn transpose(self) -> Result<Option<T>, E> { + match self { + Some(Ok(x)) => Ok(Some(x)), + Some(Err(e)) => Err(e), + None => Ok(None), + } + } +} + +// This is a separate function to reduce the code size of .expect() itself. +#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))] +#[cfg_attr(feature = "panic_immediate_abort", inline)] +#[cold] +#[track_caller] +#[rustc_const_unstable(feature = "const_option", issue = "67441")] +const fn expect_failed(msg: &str) -> ! { + panic_str(msg) +} + +///////////////////////////////////////////////////////////////////////////// +// Trait implementations +///////////////////////////////////////////////////////////////////////////// + +#[stable(feature = "rust1", since = "1.0.0")] +#[rustc_const_unstable(feature = "const_clone", issue = "91805")] +impl<T> const Clone for Option<T> +where + T: ~const Clone + ~const Destruct, +{ + #[inline] + fn clone(&self) -> Self { + match self { + Some(x) => Some(x.clone()), + None => None, + } + } + + #[inline] + fn clone_from(&mut self, source: &Self) { + match (self, source) { + (Some(to), Some(from)) => to.clone_from(from), + (to, from) => *to = from.clone(), + } + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +#[rustc_const_unstable(feature = "const_default_impls", issue = "87864")] +impl<T> const Default for Option<T> { + /// Returns [`None`][Option::None]. + /// + /// # Examples + /// + /// ``` + /// let opt: Option<u32> = Option::default(); + /// assert!(opt.is_none()); + /// ``` + #[inline] + fn default() -> Option<T> { + None + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<T> IntoIterator for Option<T> { + type Item = T; + type IntoIter = IntoIter<T>; + + /// Returns a consuming iterator over the possibly contained value. + /// + /// # Examples + /// + /// ``` + /// let x = Some("string"); + /// let v: Vec<&str> = x.into_iter().collect(); + /// assert_eq!(v, ["string"]); + /// + /// let x = None; + /// let v: Vec<&str> = x.into_iter().collect(); + /// assert!(v.is_empty()); + /// ``` + #[inline] + fn into_iter(self) -> IntoIter<T> { + IntoIter { inner: Item { opt: self } } + } +} + +#[stable(since = "1.4.0", feature = "option_iter")] +impl<'a, T> IntoIterator for &'a Option<T> { + type Item = &'a T; + type IntoIter = Iter<'a, T>; + + fn into_iter(self) -> Iter<'a, T> { + self.iter() + } +} + +#[stable(since = "1.4.0", feature = "option_iter")] +impl<'a, T> IntoIterator for &'a mut Option<T> { + type Item = &'a mut T; + type IntoIter = IterMut<'a, T>; + + fn into_iter(self) -> IterMut<'a, T> { + self.iter_mut() + } +} + +#[stable(since = "1.12.0", feature = "option_from")] +#[rustc_const_unstable(feature = "const_convert", issue = "88674")] +impl<T> const From<T> for Option<T> { + /// Moves `val` into a new [`Some`]. + /// + /// # Examples + /// + /// ``` + /// let o: Option<u8> = Option::from(67); + /// + /// assert_eq!(Some(67), o); + /// ``` + fn from(val: T) -> Option<T> { + Some(val) + } +} + +#[stable(feature = "option_ref_from_ref_option", since = "1.30.0")] +#[rustc_const_unstable(feature = "const_convert", issue = "88674")] +impl<'a, T> const From<&'a Option<T>> for Option<&'a T> { + /// Converts from `&Option<T>` to `Option<&T>`. + /// + /// # Examples + /// + /// Converts an <code>[Option]<[String]></code> into an <code>[Option]<[usize]></code>, preserving + /// the original. The [`map`] method takes the `self` argument by value, consuming the original, + /// so this technique uses `from` to first take an [`Option`] to a reference + /// to the value inside the original. + /// + /// [`map`]: Option::map + /// [String]: ../../std/string/struct.String.html "String" + /// + /// ``` + /// let s: Option<String> = Some(String::from("Hello, Rustaceans!")); + /// let o: Option<usize> = Option::from(&s).map(|ss: &String| ss.len()); + /// + /// println!("Can still print s: {s:?}"); + /// + /// assert_eq!(o, Some(18)); + /// ``` + fn from(o: &'a Option<T>) -> Option<&'a T> { + o.as_ref() + } +} + +#[stable(feature = "option_ref_from_ref_option", since = "1.30.0")] +#[rustc_const_unstable(feature = "const_convert", issue = "88674")] +impl<'a, T> const From<&'a mut Option<T>> for Option<&'a mut T> { + /// Converts from `&mut Option<T>` to `Option<&mut T>` + /// + /// # Examples + /// + /// ``` + /// let mut s = Some(String::from("Hello")); + /// let o: Option<&mut String> = Option::from(&mut s); + /// + /// match o { + /// Some(t) => *t = String::from("Hello, Rustaceans!"), + /// None => (), + /// } + /// + /// assert_eq!(s, Some(String::from("Hello, Rustaceans!"))); + /// ``` + fn from(o: &'a mut Option<T>) -> Option<&'a mut T> { + o.as_mut() + } +} + +///////////////////////////////////////////////////////////////////////////// +// The Option Iterators +///////////////////////////////////////////////////////////////////////////// + +#[derive(Clone, Debug)] +struct Item<A> { + opt: Option<A>, +} + +impl<A> Iterator for Item<A> { + type Item = A; + + #[inline] + fn next(&mut self) -> Option<A> { + self.opt.take() + } + + #[inline] + fn size_hint(&self) -> (usize, Option<usize>) { + match self.opt { + Some(_) => (1, Some(1)), + None => (0, Some(0)), + } + } +} + +impl<A> DoubleEndedIterator for Item<A> { + #[inline] + fn next_back(&mut self) -> Option<A> { + self.opt.take() + } +} + +impl<A> ExactSizeIterator for Item<A> {} +impl<A> FusedIterator for Item<A> {} +unsafe impl<A> TrustedLen for Item<A> {} + +/// An iterator over a reference to the [`Some`] variant of an [`Option`]. +/// +/// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none. +/// +/// This `struct` is created by the [`Option::iter`] function. +#[stable(feature = "rust1", since = "1.0.0")] +#[derive(Debug)] +pub struct Iter<'a, A: 'a> { + inner: Item<&'a A>, +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<'a, A> Iterator for Iter<'a, A> { + type Item = &'a A; + + #[inline] + fn next(&mut self) -> Option<&'a A> { + self.inner.next() + } + #[inline] + fn size_hint(&self) -> (usize, Option<usize>) { + self.inner.size_hint() + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<'a, A> DoubleEndedIterator for Iter<'a, A> { + #[inline] + fn next_back(&mut self) -> Option<&'a A> { + self.inner.next_back() + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<A> ExactSizeIterator for Iter<'_, A> {} + +#[stable(feature = "fused", since = "1.26.0")] +impl<A> FusedIterator for Iter<'_, A> {} + +#[unstable(feature = "trusted_len", issue = "37572")] +unsafe impl<A> TrustedLen for Iter<'_, A> {} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<A> Clone for Iter<'_, A> { + #[inline] + fn clone(&self) -> Self { + Iter { inner: self.inner.clone() } + } +} + +/// An iterator over a mutable reference to the [`Some`] variant of an [`Option`]. +/// +/// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none. +/// +/// This `struct` is created by the [`Option::iter_mut`] function. +#[stable(feature = "rust1", since = "1.0.0")] +#[derive(Debug)] +pub struct IterMut<'a, A: 'a> { + inner: Item<&'a mut A>, +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<'a, A> Iterator for IterMut<'a, A> { + type Item = &'a mut A; + + #[inline] + fn next(&mut self) -> Option<&'a mut A> { + self.inner.next() + } + #[inline] + fn size_hint(&self) -> (usize, Option<usize>) { + self.inner.size_hint() + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<'a, A> DoubleEndedIterator for IterMut<'a, A> { + #[inline] + fn next_back(&mut self) -> Option<&'a mut A> { + self.inner.next_back() + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<A> ExactSizeIterator for IterMut<'_, A> {} + +#[stable(feature = "fused", since = "1.26.0")] +impl<A> FusedIterator for IterMut<'_, A> {} +#[unstable(feature = "trusted_len", issue = "37572")] +unsafe impl<A> TrustedLen for IterMut<'_, A> {} + +/// An iterator over the value in [`Some`] variant of an [`Option`]. +/// +/// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none. +/// +/// This `struct` is created by the [`Option::into_iter`] function. +#[derive(Clone, Debug)] +#[stable(feature = "rust1", since = "1.0.0")] +pub struct IntoIter<A> { + inner: Item<A>, +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<A> Iterator for IntoIter<A> { + type Item = A; + + #[inline] + fn next(&mut self) -> Option<A> { + self.inner.next() + } + #[inline] + fn size_hint(&self) -> (usize, Option<usize>) { + self.inner.size_hint() + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<A> DoubleEndedIterator for IntoIter<A> { + #[inline] + fn next_back(&mut self) -> Option<A> { + self.inner.next_back() + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<A> ExactSizeIterator for IntoIter<A> {} + +#[stable(feature = "fused", since = "1.26.0")] +impl<A> FusedIterator for IntoIter<A> {} + +#[unstable(feature = "trusted_len", issue = "37572")] +unsafe impl<A> TrustedLen for IntoIter<A> {} + +///////////////////////////////////////////////////////////////////////////// +// FromIterator +///////////////////////////////////////////////////////////////////////////// + +#[stable(feature = "rust1", since = "1.0.0")] +impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> { + /// Takes each element in the [`Iterator`]: if it is [`None`][Option::None], + /// no further elements are taken, and the [`None`][Option::None] is + /// returned. Should no [`None`][Option::None] occur, a container of type + /// `V` containing the values of each [`Option`] is returned. + /// + /// # Examples + /// + /// Here is an example which increments every integer in a vector. + /// We use the checked variant of `add` that returns `None` when the + /// calculation would result in an overflow. + /// + /// ``` + /// let items = vec![0_u16, 1, 2]; + /// + /// let res: Option<Vec<u16>> = items + /// .iter() + /// .map(|x| x.checked_add(1)) + /// .collect(); + /// + /// assert_eq!(res, Some(vec![1, 2, 3])); + /// ``` + /// + /// As you can see, this will return the expected, valid items. + /// + /// Here is another example that tries to subtract one from another list + /// of integers, this time checking for underflow: + /// + /// ``` + /// let items = vec![2_u16, 1, 0]; + /// + /// let res: Option<Vec<u16>> = items + /// .iter() + /// .map(|x| x.checked_sub(1)) + /// .collect(); + /// + /// assert_eq!(res, None); + /// ``` + /// + /// Since the last element is zero, it would underflow. Thus, the resulting + /// value is `None`. + /// + /// Here is a variation on the previous example, showing that no + /// further elements are taken from `iter` after the first `None`. + /// + /// ``` + /// let items = vec![3_u16, 2, 1, 10]; + /// + /// let mut shared = 0; + /// + /// let res: Option<Vec<u16>> = items + /// .iter() + /// .map(|x| { shared += x; x.checked_sub(2) }) + /// .collect(); + /// + /// assert_eq!(res, None); + /// assert_eq!(shared, 6); + /// ``` + /// + /// Since the third element caused an underflow, no further elements were taken, + /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16. + #[inline] + fn from_iter<I: IntoIterator<Item = Option<A>>>(iter: I) -> Option<V> { + // FIXME(#11084): This could be replaced with Iterator::scan when this + // performance bug is closed. + + iter::try_process(iter.into_iter(), |i| i.collect()) + } +} + +#[unstable(feature = "try_trait_v2", issue = "84277")] +#[rustc_const_unstable(feature = "const_convert", issue = "88674")] +impl<T> const ops::Try for Option<T> { + type Output = T; + type Residual = Option<convert::Infallible>; + + #[inline] + fn from_output(output: Self::Output) -> Self { + Some(output) + } + + #[inline] + fn branch(self) -> ControlFlow<Self::Residual, Self::Output> { + match self { + Some(v) => ControlFlow::Continue(v), + None => ControlFlow::Break(None), + } + } +} + +#[unstable(feature = "try_trait_v2", issue = "84277")] +#[rustc_const_unstable(feature = "const_convert", issue = "88674")] +impl<T> const ops::FromResidual for Option<T> { + #[inline] + fn from_residual(residual: Option<convert::Infallible>) -> Self { + match residual { + None => None, + } + } +} + +#[unstable(feature = "try_trait_v2_yeet", issue = "96374")] +impl<T> ops::FromResidual<ops::Yeet<()>> for Option<T> { + #[inline] + fn from_residual(ops::Yeet(()): ops::Yeet<()>) -> Self { + None + } +} + +#[unstable(feature = "try_trait_v2_residual", issue = "91285")] +impl<T> ops::Residual<T> for Option<convert::Infallible> { + type TryType = Option<T>; +} + +impl<T> Option<Option<T>> { + /// Converts from `Option<Option<T>>` to `Option<T>`. + /// + /// # Examples + /// + /// Basic usage: + /// + /// ``` + /// let x: Option<Option<u32>> = Some(Some(6)); + /// assert_eq!(Some(6), x.flatten()); + /// + /// let x: Option<Option<u32>> = Some(None); + /// assert_eq!(None, x.flatten()); + /// + /// let x: Option<Option<u32>> = None; + /// assert_eq!(None, x.flatten()); + /// ``` + /// + /// Flattening only removes one level of nesting at a time: + /// + /// ``` + /// let x: Option<Option<Option<u32>>> = Some(Some(Some(6))); + /// assert_eq!(Some(Some(6)), x.flatten()); + /// assert_eq!(Some(6), x.flatten().flatten()); + /// ``` + #[inline] + #[stable(feature = "option_flattening", since = "1.40.0")] + #[rustc_const_unstable(feature = "const_option", issue = "67441")] + pub const fn flatten(self) -> Option<T> { + match self { + Some(inner) => inner, + None => None, + } + } +} |