rust/src/libcore/convert.rs

//! 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;

/// 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 }

/// 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`.
///
/// [`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
///
/// By using trait bounds we can accept arguments of different types as long as they can be
/// converted a 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.
///
/// [`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 only implement [`Into`] if a conversion to a type outside the current crate is
/// required. Otherwise one should always prefer implementing [`From`] over [`Into`] because
/// implementing [`From`] automatically provides one with a implementation of [`Into`] thanks to
/// the blanket implementation in the standard library. [`From`] cannot do these type of
/// conversions because of Rust's orphaning rules.
///
/// **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
///
/// If the destination type is not part of the current crate
/// then you can't implement [`From`] directly.
/// For example, take this code:
///
/// ```compile_fail
/// 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 because we cannot implement a trait for a type
/// if both the trait and the type are not defined by the current crate.
/// This is due to Rust's orphaning rules. To bypass 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
///     }
/// }
/// ```
///
/// 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.
///
/// 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.
///
/// # 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#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;
}

/// 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 a 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<U>`]: trait.Into.html
/// [`from`]: trait.From.html#tymethod.from
/// [book]: ../../book/ch09-00-error-handling.html
#[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 stablized `Infallible` and `!` will be
/// equivalent.
///
/// `TryFrom<T>` can be implemented as follows:
///
/// ```
/// use std::convert::TryFrom;
///
/// struct SuperiorThanZero(i32);
///
/// impl TryFrom<i32> for SuperiorThanZero {
///     type Error = &'static str;
///
///     fn try_from(value: i32) -> Result<Self, Self::Error> {
///         if value < 0 {
///             Err("SuperiorThanZero only accepts value superior than zero!")
///         } else {
///             Ok(SuperiorThanZero(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
#[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, 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
#[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
    }
}