| //! Traits for conversions between types. |
| //! |
| //! The traits in this module provide a general way to talk about conversions |
| //! from one type to another. They follow the standard Rust conventions of |
| //! `as`/`into`/`from`. |
| //! |
| //! Like many traits, these are often used as bounds for generic functions, to |
| //! support arguments of multiple types. |
| //! |
| //! - Implement the `As*` traits for reference-to-reference conversions |
| //! - Implement the [`Into`] trait when you want to consume the value in the conversion |
| //! - The [`From`] trait is the most flexible, useful for value _and_ reference conversions |
| //! - The [`TryFrom`] and [`TryInto`] traits behave like [`From`] and [`Into`], but allow for the |
| //! conversion to fail |
| //! |
| //! As a library author, you should prefer implementing [`From<T>`][`From`] or |
| //! [`TryFrom<T>`][`TryFrom`] rather than [`Into<U>`][`Into`] or [`TryInto<U>`][`TryInto`], |
| //! as [`From`] and [`TryFrom`] provide greater flexibility and offer |
| //! equivalent [`Into`] or [`TryInto`] implementations for free, thanks to a |
| //! blanket implementation in the standard library. |
| //! |
| //! # Generic Implementations |
| //! |
| //! - [`AsRef`] and [`AsMut`] auto-dereference if the inner type is a reference |
| //! - [`From`]`<U> for T` implies [`Into`]`<T> for U` |
| //! - [`TryFrom`]`<U> for T` implies [`TryInto`]`<T> for U` |
| //! - [`From`] and [`Into`] are reflexive, which means that all types can |
| //! `into` themselves and `from` themselves |
| //! |
| //! See each trait for usage examples. |
| //! |
| //! [`Into`]: trait.Into.html |
| //! [`From`]: trait.From.html |
| //! [`TryFrom`]: trait.TryFrom.html |
| //! [`TryInto`]: trait.TryInto.html |
| //! [`AsRef`]: trait.AsRef.html |
| //! [`AsMut`]: trait.AsMut.html |
| |
| #![stable(feature = "rust1", since = "1.0.0")] |
| |
| /// An identity function. |
| /// |
| /// Two things are important to note about this function: |
| /// |
| /// - It is not always equivalent to a closure like `|x| x` since the |
| /// closure may coerce `x` into a different type. |
| /// |
| /// - It moves the input `x` passed to the function. |
| /// |
| /// While it might seem strange to have a function that just returns back the |
| /// input, there are some interesting uses. |
| /// |
| /// # Examples |
| /// |
| /// Using `identity` to do nothing among other interesting functions: |
| /// |
| /// ```rust |
| /// use std::convert::identity; |
| /// |
| /// fn manipulation(x: u32) -> u32 { |
| /// // Let's assume that this function does something interesting. |
| /// x + 1 |
| /// } |
| /// |
| /// let _arr = &[identity, manipulation]; |
| /// ``` |
| /// |
| /// Using `identity` to get a function that changes nothing in a conditional: |
| /// |
| /// ```rust |
| /// use std::convert::identity; |
| /// |
| /// # let condition = true; |
| /// |
| /// # fn manipulation(x: u32) -> u32 { x + 1 } |
| /// |
| /// let do_stuff = if condition { manipulation } else { identity }; |
| /// |
| /// // do more interesting stuff.. |
| /// |
| /// let _results = do_stuff(42); |
| /// ``` |
| /// |
| /// Using `identity` to keep the `Some` variants of an iterator of `Option<T>`: |
| /// |
| /// ```rust |
| /// use std::convert::identity; |
| /// |
| /// let iter = vec![Some(1), None, Some(3)].into_iter(); |
| /// let filtered = iter.filter_map(identity).collect::<Vec<_>>(); |
| /// assert_eq!(vec![1, 3], filtered); |
| /// ``` |
| #[stable(feature = "convert_id", since = "1.33.0")] |
| #[inline] |
| pub const fn identity<T>(x: T) -> T { x } |
| |
| /// A cheap reference-to-reference conversion. Used to convert a value to a |
| /// reference value within generic code. |
| /// |
| /// `AsRef` is very similar to, but serves a slightly different purpose than, |
| /// [`Borrow`]. |
| /// |
| /// `AsRef` is to be used when wishing to convert to a reference of another |
| /// type. |
| /// `Borrow` is more related to the notion of taking the reference. It is |
| /// useful when wishing to abstract over the type of reference |
| /// (`&T`, `&mut T`) or allow both the referenced and owned type to be treated |
| /// in the same manner. |
| /// |
| /// The key difference between the two traits is the intention: |
| /// |
| /// - Use `AsRef` when the goal is to simply convert into a reference |
| /// - Use `Borrow` when the goal is related to writing code that is agnostic to |
| /// the type of borrow and whether it is a reference or value |
| /// |
| /// See [the book][book] for a more detailed comparison. |
| /// |
| /// [book]: ../../book/first-edition/borrow-and-asref.html |
| /// [`Borrow`]: ../../std/borrow/trait.Borrow.html |
| /// |
| /// **Note: this trait must not fail**. If the conversion can fail, use a |
| /// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`]. |
| /// |
| /// [`Option<T>`]: ../../std/option/enum.Option.html |
| /// [`Result<T, E>`]: ../../std/result/enum.Result.html |
| /// |
| /// # Generic Implementations |
| /// |
| /// - `AsRef` auto-dereferences if the inner type is a reference or a mutable |
| /// reference (e.g.: `foo.as_ref()` will work the same if `foo` has type |
| /// `&mut Foo` or `&&mut Foo`) |
| /// |
| /// # Examples |
| /// |
| /// Both [`String`] and `&str` implement `AsRef<str>`: |
| /// |
| /// [`String`]: ../../std/string/struct.String.html |
| /// |
| /// ``` |
| /// fn is_hello<T: AsRef<str>>(s: T) { |
| /// assert_eq!("hello", s.as_ref()); |
| /// } |
| /// |
| /// let s = "hello"; |
| /// is_hello(s); |
| /// |
| /// let s = "hello".to_string(); |
| /// is_hello(s); |
| /// ``` |
| /// |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub trait AsRef<T: ?Sized> { |
| /// Performs the conversion. |
| #[stable(feature = "rust1", since = "1.0.0")] |
| fn as_ref(&self) -> &T; |
| } |
| |
| /// A cheap, mutable reference-to-mutable reference conversion. |
| /// |
| /// This trait is similar to `AsRef` but used for converting between mutable |
| /// references. |
| /// |
| /// **Note: this trait must not fail**. If the conversion can fail, use a |
| /// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`]. |
| /// |
| /// [`Option<T>`]: ../../std/option/enum.Option.html |
| /// [`Result<T, E>`]: ../../std/result/enum.Result.html |
| /// |
| /// # Generic Implementations |
| /// |
| /// - `AsMut` auto-dereferences if the inner type is a mutable reference |
| /// (e.g.: `foo.as_mut()` will work the same if `foo` has type `&mut Foo` |
| /// or `&mut &mut Foo`) |
| /// |
| /// # Examples |
| /// |
| /// [`Box<T>`] implements `AsMut<T>`: |
| /// |
| /// [`Box<T>`]: ../../std/boxed/struct.Box.html |
| /// |
| /// ``` |
| /// fn add_one<T: AsMut<u64>>(num: &mut T) { |
| /// *num.as_mut() += 1; |
| /// } |
| /// |
| /// let mut boxed_num = Box::new(0); |
| /// add_one(&mut boxed_num); |
| /// assert_eq!(*boxed_num, 1); |
| /// ``` |
| /// |
| /// |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub trait AsMut<T: ?Sized> { |
| /// Performs the conversion. |
| #[stable(feature = "rust1", since = "1.0.0")] |
| fn as_mut(&mut self) -> &mut T; |
| } |
| |
| /// A conversion that consumes `self`, which may or may not be expensive. The |
| /// reciprocal of [`From`][From]. |
| /// |
| /// **Note: this trait must not fail**. If the conversion can fail, use |
| /// [`TryInto`] or a dedicated method which returns an [`Option<T>`] or a |
| /// [`Result<T, E>`]. |
| /// |
| /// Library authors should not directly implement this trait, but should prefer |
| /// implementing the [`From`][From] trait, which offers greater flexibility and |
| /// provides an equivalent `Into` implementation for free, thanks to a blanket |
| /// implementation in the standard library. |
| /// |
| /// # Generic Implementations |
| /// |
| /// - [`From<T>`][From]` for U` implies `Into<U> for T` |
| /// - [`into`] is reflexive, which means that `Into<T> for T` is implemented |
| /// |
| /// # Implementing `Into` |
| /// |
| /// There is one exception to implementing `Into`, and it's kind of esoteric. |
| /// If the destination type is not part of the current crate, and it uses a |
| /// generic variable, then you can't implement `From` directly. For example, |
| /// take this crate: |
| /// |
| /// ```compile_fail |
| /// struct Wrapper<T>(Vec<T>); |
| /// impl<T> From<Wrapper<T>> for Vec<T> { |
| /// fn from(w: Wrapper<T>) -> Vec<T> { |
| /// w.0 |
| /// } |
| /// } |
| /// ``` |
| /// |
| /// To fix this, you can implement `Into` directly: |
| /// |
| /// ``` |
| /// struct Wrapper<T>(Vec<T>); |
| /// impl<T> Into<Vec<T>> for Wrapper<T> { |
| /// fn into(self) -> Vec<T> { |
| /// self.0 |
| /// } |
| /// } |
| /// ``` |
| /// |
| /// This won't always allow the conversion: for example, `try!` and `?` |
| /// always use `From`. However, in most cases, people use `Into` to do the |
| /// conversions, and this will allow that. |
| /// |
| /// In almost all cases, you should try to implement `From`, then fall back |
| /// to `Into` if `From` can't be implemented. |
| /// |
| /// # Examples |
| /// |
| /// [`String`] implements `Into<Vec<u8>>`: |
| /// |
| /// ``` |
| /// fn is_hello<T: Into<Vec<u8>>>(s: T) { |
| /// let bytes = b"hello".to_vec(); |
| /// assert_eq!(bytes, s.into()); |
| /// } |
| /// |
| /// let s = "hello".to_string(); |
| /// is_hello(s); |
| /// ``` |
| /// |
| /// [`TryInto`]: trait.TryInto.html |
| /// [`Option<T>`]: ../../std/option/enum.Option.html |
| /// [`Result<T, E>`]: ../../std/result/enum.Result.html |
| /// [`String`]: ../../std/string/struct.String.html |
| /// [From]: trait.From.html |
| /// [`into`]: trait.Into.html#tymethod.into |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub trait Into<T>: Sized { |
| /// Performs the conversion. |
| #[stable(feature = "rust1", since = "1.0.0")] |
| fn into(self) -> T; |
| } |
| |
| /// Simple and safe type conversions in to `Self`. It is the reciprocal of |
| /// `Into`. |
| /// |
| /// This trait is useful when performing error handling as described by |
| /// [the book][book] and is closely related to the `?` operator. |
| /// |
| /// When constructing a function that is capable of failing the return type |
| /// will generally be of the form `Result<T, E>`. |
| /// |
| /// The `From` trait allows for simplification of error handling by providing a |
| /// means of returning a single error type that encapsulates numerous possible |
| /// erroneous situations. |
| /// |
| /// This trait is not limited to error handling, rather the general case for |
| /// this trait would be in any type conversions to have an explicit definition |
| /// of how they are performed. |
| /// |
| /// **Note: this trait must not fail**. If the conversion can fail, use |
| /// [`TryFrom`] or a dedicated method which returns an [`Option<T>`] or a |
| /// [`Result<T, E>`]. |
| /// |
| /// # Generic Implementations |
| /// |
| /// - `From<T> for U` implies [`Into<U>`]` for T` |
| /// - [`from`] is reflexive, which means that `From<T> for T` is implemented |
| /// |
| /// # Examples |
| /// |
| /// [`String`] implements `From<&str>`: |
| /// |
| /// ``` |
| /// let string = "hello".to_string(); |
| /// let other_string = String::from("hello"); |
| /// |
| /// assert_eq!(string, other_string); |
| /// ``` |
| /// |
| /// An example usage for error handling: |
| /// |
| /// ``` |
| /// use std::fs; |
| /// use std::io; |
| /// use std::num; |
| /// |
| /// enum CliError { |
| /// IoError(io::Error), |
| /// ParseError(num::ParseIntError), |
| /// } |
| /// |
| /// impl From<io::Error> for CliError { |
| /// fn from(error: io::Error) -> Self { |
| /// CliError::IoError(error) |
| /// } |
| /// } |
| /// |
| /// impl From<num::ParseIntError> for CliError { |
| /// fn from(error: num::ParseIntError) -> Self { |
| /// CliError::ParseError(error) |
| /// } |
| /// } |
| /// |
| /// fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> { |
| /// let mut contents = fs::read_to_string(&file_name)?; |
| /// let num: i32 = contents.trim().parse()?; |
| /// Ok(num) |
| /// } |
| /// ``` |
| /// |
| /// [`TryFrom`]: trait.TryFrom.html |
| /// [`Option<T>`]: ../../std/option/enum.Option.html |
| /// [`Result<T, E>`]: ../../std/result/enum.Result.html |
| /// [`String`]: ../../std/string/struct.String.html |
| /// [`Into<U>`]: trait.Into.html |
| /// [`from`]: trait.From.html#tymethod.from |
| /// [book]: ../../book/first-edition/error-handling.html |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub trait From<T>: Sized { |
| /// Performs the conversion. |
| #[stable(feature = "rust1", since = "1.0.0")] |
| fn from(_: T) -> Self; |
| } |
| |
| /// An attempted conversion that consumes `self`, which may or may not be |
| /// expensive. |
| /// |
| /// Library authors should not directly implement this trait, but should prefer |
| /// implementing the [`TryFrom`] trait, which offers greater flexibility and |
| /// provides an equivalent `TryInto` implementation for free, thanks to a |
| /// blanket implementation in the standard library. For more information on this, |
| /// see the documentation for [`Into`]. |
| /// |
| /// [`TryFrom`]: trait.TryFrom.html |
| /// [`Into`]: trait.Into.html |
| #[unstable(feature = "try_from", issue = "33417")] |
| pub trait TryInto<T>: Sized { |
| /// The type returned in the event of a conversion error. |
| type Error; |
| |
| /// Performs the conversion. |
| fn try_into(self) -> Result<T, Self::Error>; |
| } |
| |
| /// Attempt to construct `Self` via a conversion. |
| #[unstable(feature = "try_from", issue = "33417")] |
| pub trait TryFrom<T>: Sized { |
| /// The type returned in the event of a conversion error. |
| type Error; |
| |
| /// Performs the conversion. |
| fn try_from(value: T) -> Result<Self, Self::Error>; |
| } |
| |
| //////////////////////////////////////////////////////////////////////////////// |
| // GENERIC IMPLS |
| //////////////////////////////////////////////////////////////////////////////// |
| |
| // As lifts over & |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T: ?Sized, U: ?Sized> AsRef<U> for &T where T: AsRef<U> |
| { |
| fn as_ref(&self) -> &U { |
| <T as AsRef<U>>::as_ref(*self) |
| } |
| } |
| |
| // As lifts over &mut |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T: ?Sized, U: ?Sized> AsRef<U> for &mut T where T: AsRef<U> |
| { |
| fn as_ref(&self) -> &U { |
| <T as AsRef<U>>::as_ref(*self) |
| } |
| } |
| |
| // FIXME (#45742): replace the above impls for &/&mut with the following more general one: |
| // // As lifts over Deref |
| // impl<D: ?Sized + Deref, U: ?Sized> AsRef<U> for D where D::Target: AsRef<U> { |
| // fn as_ref(&self) -> &U { |
| // self.deref().as_ref() |
| // } |
| // } |
| |
| // AsMut lifts over &mut |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T: ?Sized, U: ?Sized> AsMut<U> for &mut T where T: AsMut<U> |
| { |
| fn as_mut(&mut self) -> &mut U { |
| (*self).as_mut() |
| } |
| } |
| |
| // FIXME (#45742): replace the above impl for &mut with the following more general one: |
| // // AsMut lifts over DerefMut |
| // impl<D: ?Sized + Deref, U: ?Sized> AsMut<U> for D where D::Target: AsMut<U> { |
| // fn as_mut(&mut self) -> &mut U { |
| // self.deref_mut().as_mut() |
| // } |
| // } |
| |
| // From implies Into |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T, U> Into<U> for T where U: From<T> |
| { |
| fn into(self) -> U { |
| U::from(self) |
| } |
| } |
| |
| // From (and thus Into) is reflexive |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T> From<T> for T { |
| fn from(t: T) -> T { t } |
| } |
| |
| |
| // TryFrom implies TryInto |
| #[unstable(feature = "try_from", issue = "33417")] |
| impl<T, U> TryInto<U> for T where U: TryFrom<T> |
| { |
| type Error = U::Error; |
| |
| fn try_into(self) -> Result<U, U::Error> { |
| U::try_from(self) |
| } |
| } |
| |
| // Infallible conversions are semantically equivalent to fallible conversions |
| // with an uninhabited error type. |
| #[unstable(feature = "try_from", issue = "33417")] |
| impl<T, U> TryFrom<U> for T where T: From<U> { |
| type Error = !; |
| |
| fn try_from(value: U) -> Result<Self, Self::Error> { |
| Ok(T::from(value)) |
| } |
| } |
| |
| //////////////////////////////////////////////////////////////////////////////// |
| // CONCRETE IMPLS |
| //////////////////////////////////////////////////////////////////////////////// |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T> AsRef<[T]> for [T] { |
| fn as_ref(&self) -> &[T] { |
| self |
| } |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T> AsMut<[T]> for [T] { |
| fn as_mut(&mut self) -> &mut [T] { |
| self |
| } |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl AsRef<str> for str { |
| #[inline] |
| fn as_ref(&self) -> &str { |
| self |
| } |
| } |