convert.rs.html -- source
Description: Source to the Rust file `src/libcore/convert.rs`.
Keywords: rust, rustlang, rust-lang

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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
        }
    }

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