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

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    //! Traits for conversions between types.
    //!
    //! The traits in this module provide a way to convert from one type to another type.
    //! Each trait serves a different purpose:
    //!
    //! - Implement the [`AsRef`] trait for cheap reference-to-reference conversions
    //! - Implement the [`AsMut`] trait for cheap mutable-to-mutable conversions
    //! - Implement the [`From`] trait for consuming value-to-value conversions
    //! - Implement the [`Into`] trait for consuming value-to-value conversions to types
    //!   outside the current crate
    //! - The [`TryFrom`] and [`TryInto`] traits behave like [`From`] and [`Into`],
    //!   but should be implemented when the conversion can fail.
    //!
    //! The traits in this module are often used as trait bounds for generic functions such that to
    //! arguments of multiple types are supported. See the documentation of each trait for examples.
    //!
    //! As a library author, you should always 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. Only implement [`Into`] or [`TryInto`]
    //! when a conversion to a type outside the current crate is required.
    //!
    //! # 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 crate::fmt;
    use crate::hash::{Hash, Hasher};

    mod num;

    #[unstable(feature = "convert_float_to_int", issue = "67057")]
    pub use num::FloatToInt;

    /// The 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 in a sequence of other, interesting,
    /// functions:
    ///
    /// ```rust
    /// use std::convert::identity;
    ///
    /// fn manipulation(x: u32) -> u32 {
    ///     // Let's pretend that adding one is an interesting function.
    ///     x + 1
    /// }
    ///
    /// let _arr = &[identity, manipulation];
    /// ```
    ///
    /// Using `identity` as a "do nothing" base case 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")]
    #[rustc_const_stable(feature = "const_identity", since = "1.33.0")]
    #[inline]
    pub const fn identity<T>(x: T) -> T {
        x
    }

    /// Used to do a cheap reference-to-reference conversion.
    ///
    /// This trait is similar to [`AsMut`] which is used for converting between mutable references.
    /// If you need to do a costly conversion it is better to implement [`From`] with type
    /// `&T` or write a custom function.
    ///
    /// `AsRef` has the same signature as [`Borrow`], but [`Borrow`] is different in few aspects:
    ///
    /// - Unlike `AsRef`, [`Borrow`] has a blanket impl for any `T`, and can be used to accept either
    ///   a reference or a value.
    /// - [`Borrow`] also requires that [`Hash`], [`Eq`] and [`Ord`] for borrowed value are
    ///   equivalent to those of the owned value. For this reason, if you want to
    ///   borrow only a single field of a struct you can implement `AsRef`, but not [`Borrow`].
    ///
    /// **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>`].
    ///
    /// # 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
    ///
    /// By using trait bounds we can accept arguments of different types as long as they can be
    /// converted to the specified type `T`.
    ///
    /// For example: By creating a generic function that takes an `AsRef<str>` we express that we
    /// want to accept all references that can be converted to [`&str`] as an argument.
    /// Since both [`String`] and [`&str`] implement `AsRef<str>` we can accept both as input argument.
    ///
    /// [`Option<T>`]: ../../std/option/enum.Option.html
    /// [`Result<T, E>`]: ../../std/result/enum.Result.html
    /// [`Borrow`]: ../../std/borrow/trait.Borrow.html
    /// [`Hash`]: ../../std/hash/trait.Hash.html
    /// [`Eq`]: ../../std/cmp/trait.Eq.html
    /// [`Ord`]: ../../std/cmp/trait.Ord.html
    /// [`&str`]: ../../std/primitive.str.html
    /// [`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;
    }

    /// Used to do a cheap mutable-to-mutable reference conversion.
    ///
    /// This trait is similar to [`AsRef`] but used for converting between mutable
    /// references. If you need to do a costly conversion it is better to
    /// implement [`From`] with type `&mut T` or write a custom function.
    ///
    /// **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
    ///
    /// Using `AsMut` as trait bound for a generic function we can accept all mutable references
    /// that can be converted to type `&mut T`. Because [`Box<T>`] implements `AsMut<T>` we can
    /// write a function `add_one` that takes all arguments that can be converted to `&mut u64`.
    /// Because [`Box<T>`] implements `AsMut<T>`, `add_one` accepts arguments of type
    /// `&mut Box<u64>` as well:
    ///
    /// ```
    /// 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);
    /// ```
    ///
    /// [`Box<T>`]: ../../std/boxed/struct.Box.html
    #[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 value-to-value conversion that consumes the input value. The
    /// opposite of [`From`].
    ///
    /// One should avoid implementing [`Into`] and implement [`From`] instead.
    /// Implementing [`From`] automatically provides one with an implementation of [`Into`]
    /// thanks to the blanket implementation in the standard library.
    ///
    /// Prefer using [`Into`] over [`From`] when specifying trait bounds on a generic function
    /// to ensure that types that only implement [`Into`] can be used as well.
    ///
    /// **Note: This trait must not fail**. If the conversion can fail, use [`TryInto`].
    ///
    /// # Generic Implementations
    ///
    /// - [`From`]`<T> for U` implies `Into<U> for T`
    /// - [`Into`] is reflexive, which means that `Into<T> for T` is implemented
    ///
    /// # Implementing [`Into`] for conversions to external types in old versions of Rust
    ///
    /// Prior to Rust 1.41, if the destination type was not part of the current crate
    /// then you couldn't implement [`From`] directly.
    /// For example, take this code:
    ///
    /// ```
    /// struct Wrapper<T>(Vec<T>);
    /// impl<T> From<Wrapper<T>> for Vec<T> {
    ///     fn from(w: Wrapper<T>) -> Vec<T> {
    ///         w.0
    ///     }
    /// }
    /// ```
    /// This will fail to compile in older versions of the language because Rust's orphaning rules
    /// used to be a little bit more strict. To bypass this, you could implement [`Into`] directly:
    ///
    /// ```
    /// struct Wrapper<T>(Vec<T>);
    /// impl<T> Into<Vec<T>> for Wrapper<T> {
    ///     fn into(self) -> Vec<T> {
    ///         self.0
    ///     }
    /// }
    /// ```
    ///
    /// It is important to understand that [`Into`] does not provide a [`From`] implementation
    /// (as [`From`] does with [`Into`]). Therefore, you should always try to implement [`From`]
    /// and then fall back to [`Into`] if [`From`] can't be implemented.
    ///
    /// # Examples
    ///
    /// [`String`] implements [`Into`]`<`[`Vec`]`<`[`u8`]`>>`:
    ///
    /// In order to express that we want a generic function to take all arguments that can be
    /// converted to a specified type `T`, we can use a trait bound of [`Into`]`<T>`.
    /// For example: The function `is_hello` takes all arguments that can be converted into a
    /// [`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
    /// [`Vec`]: ../../std/vec/struct.Vec.html
    #[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;
    }

    /// Used to do value-to-value conversions while consuming the input value. It is the reciprocal of
    /// [`Into`].
    ///
    /// One should always prefer implementing `From` over [`Into`]
    /// because implementing `From` automatically provides one with an implementation of [`Into`]
    /// thanks to the blanket implementation in the standard library.
    ///
    /// Only implement [`Into`] if a conversion to a type outside the current crate is required.
    /// `From` cannot do these type of conversions because of Rust's orphaning rules.
    /// See [`Into`] for more details.
    ///
    /// Prefer using [`Into`] over using `From` when specifying trait bounds on a generic function.
    /// This way, types that directly implement [`Into`] can be used as arguments as well.
    ///
    /// The `From` is also very useful when performing error handling. When constructing a function
    /// that is capable of failing, the return type will generally be of the form `Result<T, E>`.
    /// The `From` trait simplifies error handling by allowing a function to return a single error type
    /// that encapsulate multiple error types. See the "Examples" section and [the book][book] for more
    /// details.
    ///
    /// **Note: This trait must not fail**. If the conversion can fail, use [`TryFrom`].
    ///
    /// # 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>`:
    ///
    /// An explicit conversion from a `&str` to a String is done as follows:
    ///
    /// ```
    /// let string = "hello".to_string();
    /// let other_string = String::from("hello");
    ///
    /// assert_eq!(string, other_string);
    /// ```
    ///
    /// While performing error handling it is often useful to implement `From` for your own error type.
    /// By converting underlying error types to our own custom error type that encapsulates the
    /// underlying error type, we can return a single error type without losing information on the
    /// underlying cause. The '?' operator automatically converts the underlying error type to our
    /// custom error type by calling `Into<CliError>::into` which is automatically provided when
    /// implementing `From`. The compiler then infers which implementation of `Into` should be used.
    ///
    /// ```
    /// 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`]: trait.Into.html
    /// [`from`]: trait.From.html#tymethod.from
    /// [book]: ../../book/ch09-00-error-handling.html
    #[rustc_diagnostic_item = "from_trait"]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_on_unimplemented(on(
        all(_Self = "&str", T = "std::string::String"),
        note = "to coerce a `{T}` into a `{Self}`, use `&*` as a prefix",
    ))]
    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 usually 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`].
    ///
    /// # Implementing `TryInto`
    ///
    /// This suffers the same restrictions and reasoning as implementing
    /// [`Into`], see there for details.
    ///
    /// [`TryFrom`]: trait.TryFrom.html
    /// [`Into`]: trait.Into.html
    #[stable(feature = "try_from", since = "1.34.0")]
    pub trait TryInto<T>: Sized {
        /// The type returned in the event of a conversion error.
        #[stable(feature = "try_from", since = "1.34.0")]
        type Error;

        /// Performs the conversion.
        #[stable(feature = "try_from", since = "1.34.0")]
        fn try_into(self) -> Result<T, Self::Error>;
    }

    /// Simple and safe type conversions that may fail in a controlled
    /// way under some circumstances. It is the reciprocal of [`TryInto`].
    ///
    /// This is useful when you are doing a type conversion that may
    /// trivially succeed but may also need special handling.
    /// For example, there is no way to convert an [`i64`] into an [`i32`]
    /// using the [`From`] trait, because an [`i64`] may contain a value
    /// that an [`i32`] cannot represent and so the conversion would lose data.
    /// This might be handled by truncating the [`i64`] to an [`i32`] (essentially
    /// giving the [`i64`]'s value modulo [`i32::MAX`]) or by simply returning
    /// [`i32::MAX`], or by some other method.  The [`From`] trait is intended
    /// for perfect conversions, so the `TryFrom` trait informs the
    /// programmer when a type conversion could go bad and lets them
    /// decide how to handle it.
    ///
    /// # Generic Implementations
    ///
    /// - `TryFrom<T> for U` implies [`TryInto`]`<U> for T`
    /// - [`try_from`] is reflexive, which means that `TryFrom<T> for T`
    /// is implemented and cannot fail -- the associated `Error` type for
    /// calling `T::try_from()` on a value of type `T` is [`Infallible`].
    /// When the [`!`] type is stabilized [`Infallible`] and [`!`] will be
    /// equivalent.
    ///
    /// `TryFrom<T>` can be implemented as follows:
    ///
    /// ```
    /// use std::convert::TryFrom;
    ///
    /// struct GreaterThanZero(i32);
    ///
    /// impl TryFrom<i32> for GreaterThanZero {
    ///     type Error = &'static str;
    ///
    ///     fn try_from(value: i32) -> Result<Self, Self::Error> {
    ///         if value <= 0 {
    ///             Err("GreaterThanZero only accepts value superior than zero!")
    ///         } else {
    ///             Ok(GreaterThanZero(value))
    ///         }
    ///     }
    /// }
    /// ```
    ///
    /// # Examples
    ///
    /// As described, [`i32`] implements `TryFrom<`[`i64`]`>`:
    ///
    /// ```
    /// use std::convert::TryFrom;
    ///
    /// let big_number = 1_000_000_000_000i64;
    /// // Silently truncates `big_number`, requires detecting
    /// // and handling the truncation after the fact.
    /// let smaller_number = big_number as i32;
    /// assert_eq!(smaller_number, -727379968);
    ///
    /// // Returns an error because `big_number` is too big to
    /// // fit in an `i32`.
    /// let try_smaller_number = i32::try_from(big_number);
    /// assert!(try_smaller_number.is_err());
    ///
    /// // Returns `Ok(3)`.
    /// let try_successful_smaller_number = i32::try_from(3);
    /// assert!(try_successful_smaller_number.is_ok());
    /// ```
    ///
    /// [`try_from`]: trait.TryFrom.html#tymethod.try_from
    /// [`TryInto`]: trait.TryInto.html
    /// [`i32::MAX`]: ../../std/i32/constant.MAX.html
    /// [`!`]: ../../std/primitive.never.html
    /// [`Infallible`]: enum.Infallible.html
    #[stable(feature = "try_from", since = "1.34.0")]
    pub trait TryFrom<T>: Sized {
        /// The type returned in the event of a conversion error.
        #[stable(feature = "try_from", since = "1.34.0")]
        type Error;

        /// Performs the conversion.
        #[stable(feature = "try_from", since = "1.34.0")]
        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<Target: AsRef<U>>, U: ?Sized> AsRef<U> for D {
    //     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<Target: AsMut<U>>, U: ?Sized> AsMut<U> for D {
    //     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
        }
    }

    /// **Stability note:** This impl does not yet exist, but we are
    /// "reserving space" to add it in the future. See
    /// [rust-lang/rust#64715][#64715] for details.
    ///
    /// [#64715]: https://github.com/rust-lang/rust/issues/64715
    #[stable(feature = "convert_infallible", since = "1.34.0")]
    #[allow(unused_attributes)] // FIXME(#58633): do a principled fix instead.
    #[rustc_reservation_impl = "permitting this impl would forbid us from adding \
                                `impl<T> From<!> for T` later; see rust-lang/rust#64715 for details"]
    impl<T> From<!> for T {
        fn from(t: !) -> T {
            t
        }
    }

    // TryFrom implies TryInto
    #[stable(feature = "try_from", since = "1.34.0")]
    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.
    #[stable(feature = "try_from", since = "1.34.0")]
    impl<T, U> TryFrom<U> for T
    where
        U: Into<T>,
    {
        type Error = Infallible;

        fn try_from(value: U) -> Result<Self, Self::Error> {
            Ok(U::into(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
        }
    }

    ////////////////////////////////////////////////////////////////////////////////
    // THE NO-ERROR ERROR TYPE
    ////////////////////////////////////////////////////////////////////////////////

    /// The error type for errors that can never happen.
    ///
    /// Since this enum has no variant, a value of this type can never actually exist.
    /// This can be useful for generic APIs that use [`Result`] and parameterize the error type,
    /// to indicate that the result is always [`Ok`].
    ///
    /// For example, the [`TryFrom`] trait (conversion that returns a [`Result`])
    /// has a blanket implementation for all types where a reverse [`Into`] implementation exists.
    ///
    /// ```ignore (illustrates std code, duplicating the impl in a doctest would be an error)
    /// impl<T, U> TryFrom<U> for T where U: Into<T> {
    ///     type Error = Infallible;
    ///
    ///     fn try_from(value: U) -> Result<Self, Infallible> {
    ///         Ok(U::into(value))  // Never returns `Err`
    ///     }
    /// }
    /// ```
    ///
    /// # Future compatibility
    ///
    /// This enum has the same role as [the `!` “never” type][never],
    /// which is unstable in this version of Rust.
    /// When `!` is stabilized, we plan to make `Infallible` a type alias to it:
    ///
    /// ```ignore (illustrates future std change)
    /// pub type Infallible = !;
    /// ```
    ///
    /// … and eventually deprecate `Infallible`.
    ///
    ///
    /// However there is one case where `!` syntax can be used
    /// before `!` is stabilized as a full-fleged type: in the position of a function’s return type.
    /// Specifically, it is possible implementations for two different function pointer types:
    ///
    /// ```
    /// trait MyTrait {}
    /// impl MyTrait for fn() -> ! {}
    /// impl MyTrait for fn() -> std::convert::Infallible {}
    /// ```
    ///
    /// With `Infallible` being an enum, this code is valid.
    /// However when `Infallible` becomes an alias for the never type,
    /// the two `impl`s will start to overlap
    /// and therefore will be disallowed by the language’s trait coherence rules.
    ///
    /// [`Ok`]: ../result/enum.Result.html#variant.Ok
    /// [`Result`]: ../result/enum.Result.html
    /// [`TryFrom`]: trait.TryFrom.html
    /// [`Into`]: trait.Into.html
    /// [never]: ../../std/primitive.never.html
    #[stable(feature = "convert_infallible", since = "1.34.0")]
    #[derive(Copy)]
    pub enum Infallible {}

    #[stable(feature = "convert_infallible", since = "1.34.0")]
    impl Clone for Infallible {
        fn clone(&self) -> Infallible {
            match *self {}
        }
    }

    #[stable(feature = "convert_infallible", since = "1.34.0")]
    impl fmt::Debug for Infallible {
        fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
            match *self {}
        }
    }

    #[stable(feature = "convert_infallible", since = "1.34.0")]
    impl fmt::Display for Infallible {
        fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
            match *self {}
        }
    }

    #[stable(feature = "convert_infallible", since = "1.34.0")]
    impl PartialEq for Infallible {
        fn eq(&self, _: &Infallible) -> bool {
            match *self {}
        }
    }

    #[stable(feature = "convert_infallible", since = "1.34.0")]
    impl Eq for Infallible {}

    #[stable(feature = "convert_infallible", since = "1.34.0")]
    impl PartialOrd for Infallible {
        fn partial_cmp(&self, _other: &Self) -> Option<crate::cmp::Ordering> {
            match *self {}
        }
    }

    #[stable(feature = "convert_infallible", since = "1.34.0")]
    impl Ord for Infallible {
        fn cmp(&self, _other: &Self) -> crate::cmp::Ordering {
            match *self {}
        }
    }

    #[stable(feature = "convert_infallible", since = "1.34.0")]
    impl From<!> for Infallible {
        fn from(x: !) -> Self {
            x
        }
    }

    #[stable(feature = "convert_infallible_hash", since = "1.44.0")]
    impl Hash for Infallible {
        fn hash<H: Hasher>(&self, _: &mut H) {
            match *self {}
        }
    }