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.
    //!
    //! - 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")]

    use fmt;

    /// A type used as the error type for implementations of fallible conversion
    /// traits in cases where conversions cannot actually fail.
    ///
    /// Because `Infallible` has no variants, a value of this type can never exist.
    /// It is used only to satisfy trait signatures that expect an error type, and
    /// signals to both the compiler and the user that the error case is impossible.
    #[unstable(feature = "try_from", issue = "33417")]
    #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
    pub enum Infallible {}

    #[unstable(feature = "try_from", issue = "33417")]
    impl fmt::Display for Infallible {
        fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
            match *self {
            }
        }
    }
    /// 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 goal is to simply convert into a reference
    /// - Use `Borrow` when goal is related to writing code that is agnostic to the
    ///   type of borrow and if is 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 reference or a mutable
    ///   reference (e.g.: `foo.as_ref()` will work the same if `foo` has type
    ///   `&mut Foo` or `&&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::io::{self, Read};
    /// 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 file = std::fs::File::open("test")?;
    ///     let mut contents = String::new();
    ///     file.read_to_string(&mut contents)?;
    ///     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<'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 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 = Infallible;

        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
        }
    }

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