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// Copyright 2014 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. //! 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. //! //! - Impl the `As*` traits for reference-to-reference conversions //! - Impl 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>` or `TryFrom<T>` rather than //! `Into<U>` or `TryInto<U>`, 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 impl //! //! - `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. #![stable(feature = "rust1", since = "1.0.0")] /// A cheap, reference-to-reference conversion. /// /// `AsRef` is very similar to, but different than, [`Borrow`]. See /// [the book][book] for more. /// /// [book]: ../../book/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 /// /// # 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); /// ``` /// /// # Generic Impls /// /// - `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`) /// #[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. /// /// **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 /// /// # 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); /// ``` /// /// # Generic Impls /// /// - `AsMut` 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`) /// #[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. /// /// **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. /// /// # 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); /// ``` /// /// # Generic Impls /// /// - `[From<T>][From] for U` implies `Into<U> for T` /// - [`into()`] is reflexive, which means that `Into<T> for T` is implemented /// /// [`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; } /// Construct `Self` via a conversion. /// /// **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>`]. /// /// # Examples /// /// [`String`] implements `From<&str>`: /// /// ``` /// let string = "hello".to_string(); /// let other_string = String::from("hello"); /// /// assert_eq!(string, other_string); /// ``` /// # Generic impls /// /// - `From<T> for U` implies `[Into<U>] for T` /// - [`from()`] is reflexive, which means that `From<T> for T` is implemented /// /// [`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 #[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. /// /// [`TryFrom`]: trait.TryFrom.html #[unstable(feature = "try_from", issue = "33417")] pub trait TryInto<T>: Sized { /// The type returned in the event of a conversion error. type Err; /// Performs the conversion. fn try_into(self) -> Result<T, Self::Err>; } /// 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 Err; /// Performs the conversion. fn try_from(T) -> Result<Self, Self::Err>; } //////////////////////////////////////////////////////////////////////////////// // GENERIC IMPLS //////////////////////////////////////////////////////////////////////////////// // As lifts over & #[stable(feature = "rust1", since = "1.0.0")] impl<'a, T: ?Sized, U: ?Sized> AsRef<U> for &'a 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<'a, T: ?Sized, U: ?Sized> AsRef<U> for &'a mut T where T: AsRef<U> { fn as_ref(&self) -> &U { <T as AsRef<U>>::as_ref(*self) } } // FIXME (#23442): 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<'a, T: ?Sized, U: ?Sized> AsMut<U> for &'a mut T where T: AsMut<U> { fn as_mut(&mut self) -> &mut U { (*self).as_mut() } } // FIXME (#23442): 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 Err = U::Err; fn try_into(self) -> Result<U, U::Err> { U::try_from(self) } } //////////////////////////////////////////////////////////////////////////////// // 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 } }