core/convert/

mod.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 that cannot fail. This
//!   automatically provides an implementation of [`Into`]
//! - Implement the [`TryFrom`] trait for consuming value-to-value conversions that can fail. This
//!   automatically provides an implementation of [`TryInto`]
//!
//! The traits in this module are often used as trait bounds for generic functions such that
//! 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. In versions of Rust prior to Rust 1.41,
//! it was sometimes necessary to implement [`Into`] or [`TryInto`] directly when converting to a
//! type outside the current crate.
//!
//! # Generic Implementations
//!
//! - [`AsRef`] and [`AsMut`] auto-dereference if the inner type is a reference
//!   (but not generally for all [dereferenceable types][core::ops::Deref])
//! - [`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")]

use crate::marker::PointeeSized;

mod num;

#[unstable(feature = "float_conversions", issue = "159913")]
pub use num::FloatToFloat;
#[unstable(feature = "convert_float_to_int", issue = "67057")]
pub use num::FloatToInt;
#[unstable(feature = "integer_casts", issue = "157388")]
pub use num::{BoundedCastFromInt, CheckedCastFromInt};

/// 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 = [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(always)]
#[rustc_diagnostic_item = "convert_identity"]
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.
///
/// # Relation to `Borrow`
///
/// `AsRef` has the same signature as [`Borrow`], but [`Borrow`] is different in a few aspects:
///
/// - Unlike `AsRef`, [`Borrow`] has a blanket impl for any `T`, and can be used to accept either
///   a reference or a value. (See also note on `AsRef`'s reflexibility below.)
/// - [`Borrow`] also requires that [`Hash`], [`Eq`] and [`Ord`] for a 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`).
///
/// Note that due to historic reasons, the above currently does not hold generally for all
/// [dereferenceable types], e.g. `foo.as_ref()` will *not* work the same as
/// `Box::new(foo).as_ref()`. Instead, many smart pointers provide an `as_ref` implementation which
/// simply returns a reference to the [pointed-to value] (but do not perform a cheap
/// reference-to-reference conversion for that value). However, [`AsRef::as_ref`] should not be
/// used for the sole purpose of dereferencing; instead ['`Deref` coercion'] can be used:
///
/// [dereferenceable types]: core::ops::Deref
/// [pointed-to value]: core::ops::Deref::Target
/// ['`Deref` coercion']: core::ops::Deref#deref-coercion
///
/// ```
/// let x = Box::new(5i32);
/// // Avoid this:
/// // let y: &i32 = x.as_ref();
/// // Better just write:
/// let y: &i32 = &x;
/// ```
///
/// Types which implement [`Deref`] should consider implementing `AsRef<T>` as follows:
///
/// [`Deref`]: core::ops::Deref
///
/// ```
/// # use core::ops::Deref;
/// # struct SomeType;
/// # impl Deref for SomeType {
/// #     type Target = [u8];
/// #     fn deref(&self) -> &[u8] {
/// #         &[]
/// #     }
/// # }
/// impl<T> AsRef<T> for SomeType
/// where
///     T: ?Sized,
///     <SomeType as Deref>::Target: AsRef<T>,
/// {
///     fn as_ref(&self) -> &T {
///         self.deref().as_ref()
///     }
/// }
/// ```
///
/// # Reflexivity
///
/// Ideally, `AsRef` would be reflexive, i.e. there would be an `impl<T: ?Sized> AsRef<T> for T`
/// with [`as_ref`] simply returning its argument unchanged.
/// Such a blanket implementation is currently *not* provided due to technical restrictions of
/// Rust's type system (it would be overlapping with another existing blanket implementation for
/// `&T where T: AsRef<U>` which allows `AsRef` to auto-dereference, see "Generic Implementations"
/// above).
///
/// [`as_ref`]: AsRef::as_ref
///
/// A trivial implementation of `AsRef<T> for T` must be added explicitly for a particular type `T`
/// where needed or desired. Note, however, that not all types from `std` contain such an
/// implementation, and those cannot be added by external code due to orphan rules.
///
/// # 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.
///
/// [`&str`]: primitive@str
/// [`Borrow`]: crate::borrow::Borrow
/// [`Eq`]: crate::cmp::Eq
/// [`Ord`]: crate::cmp::Ord
/// [`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")]
#[rustc_diagnostic_item = "AsRef"]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
pub const trait AsRef<T: PointeeSized>: PointeeSized {
    /// Converts this type into a shared reference of the (usually inferred) input type.
    #[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>`].
///
/// # 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`).
///
/// Note that due to historic reasons, the above currently does not hold generally for all
/// [mutably dereferenceable types], e.g. `foo.as_mut()` will *not* work the same as
/// `Box::new(foo).as_mut()`. Instead, many smart pointers provide an `as_mut` implementation which
/// simply returns a reference to the [pointed-to value] (but do not perform a cheap
/// reference-to-reference conversion for that value). However, [`AsMut::as_mut`] should not be
/// used for the sole purpose of mutable dereferencing; instead ['`Deref` coercion'] can be used:
///
/// [mutably dereferenceable types]: core::ops::DerefMut
/// [pointed-to value]: core::ops::Deref::Target
/// ['`Deref` coercion']: core::ops::DerefMut#mutable-deref-coercion
///
/// ```
/// let mut x = Box::new(5i32);
/// // Avoid this:
/// // let y: &mut i32 = x.as_mut();
/// // Better just write:
/// let y: &mut i32 = &mut x;
/// ```
///
/// Types which implement [`DerefMut`] should consider to add an implementation of `AsMut<T>` as
/// follows:
///
/// [`DerefMut`]: core::ops::DerefMut
///
/// ```
/// # use core::ops::{Deref, DerefMut};
/// # struct SomeType;
/// # impl Deref for SomeType {
/// #     type Target = [u8];
/// #     fn deref(&self) -> &[u8] {
/// #         &[]
/// #     }
/// # }
/// # impl DerefMut for SomeType {
/// #     fn deref_mut(&mut self) -> &mut [u8] {
/// #         &mut []
/// #     }
/// # }
/// impl<T> AsMut<T> for SomeType
/// where
///     <SomeType as Deref>::Target: AsMut<T>,
/// {
///     fn as_mut(&mut self) -> &mut T {
///         self.deref_mut().as_mut()
///     }
/// }
/// ```
///
/// # Reflexivity
///
/// Ideally, `AsMut` would be reflexive, i.e. there would be an `impl<T: ?Sized> AsMut<T> for T`
/// with [`as_mut`] simply returning its argument unchanged.
/// Such a blanket implementation is currently *not* provided due to technical restrictions of
/// Rust's type system (it would be overlapping with another existing blanket implementation for
/// `&mut T where T: AsMut<U>` which allows `AsMut` to auto-dereference, see "Generic
/// Implementations" above).
///
/// [`as_mut`]: AsMut::as_mut
///
/// A trivial implementation of `AsMut<T> for T` must be added explicitly for a particular type `T`
/// where needed or desired. Note, however, that not all types from `std` contain such an
/// implementation, and those cannot be added by external code due to orphan rules.
///
/// # Examples
///
/// Using `AsMut` as trait bound for a generic function, we can accept all mutable references that
/// can be converted to type `&mut T`. Unlike [dereference], which has a single [target type],
/// there can be multiple implementations of `AsMut` for a type. In particular, `Vec<T>` implements
/// both `AsMut<Vec<T>>` and `AsMut<[T]>`.
///
/// In the following, the example functions `caesar` and `null_terminate` provide a generic
/// interface which works with any type that can be converted by cheap mutable-to-mutable conversion
/// into a byte slice (`[u8]`) or a byte vector (`Vec<u8>`), respectively.
///
/// [dereference]: core::ops::DerefMut
/// [target type]: core::ops::Deref::Target
///
/// ```
/// struct Document {
///     info: String,
///     content: Vec<u8>,
/// }
///
/// impl<T: ?Sized> AsMut<T> for Document
/// where
///     Vec<u8>: AsMut<T>,
/// {
///     fn as_mut(&mut self) -> &mut T {
///         self.content.as_mut()
///     }
/// }
///
/// fn caesar<T: AsMut<[u8]>>(data: &mut T, key: u8) {
///     for byte in data.as_mut() {
///         *byte = byte.wrapping_add(key);
///     }
/// }
///
/// fn null_terminate<T: AsMut<Vec<u8>>>(data: &mut T) {
///     // Using a non-generic inner function, which contains most of the
///     // functionality, helps to minimize monomorphization overhead.
///     fn doit(data: &mut Vec<u8>) {
///         let len = data.len();
///         if len == 0 || data[len-1] != 0 {
///             data.push(0);
///         }
///     }
///     doit(data.as_mut());
/// }
///
/// fn main() {
///     let mut v: Vec<u8> = vec![1, 2, 3];
///     caesar(&mut v, 5);
///     assert_eq!(v, [6, 7, 8]);
///     null_terminate(&mut v);
///     assert_eq!(v, [6, 7, 8, 0]);
///     let mut doc = Document {
///         info: String::from("Example"),
///         content: vec![17, 19, 8],
///     };
///     caesar(&mut doc, 1);
///     assert_eq!(doc.content, [18, 20, 9]);
///     null_terminate(&mut doc);
///     assert_eq!(doc.content, [18, 20, 9, 0]);
/// }
/// ```
///
/// Note, however, that APIs don't need to be generic. In many cases taking a `&mut [u8]` or
/// `&mut Vec<u8>`, for example, is the better choice (callers need to pass the correct type then).
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_diagnostic_item = "AsMut"]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
pub const trait AsMut<T: PointeeSized>: PointeeSized {
    /// Converts this type into a mutable reference of the (usually inferred) input type.
    #[stable(feature = "rust1", since = "1.0.0")]
    fn as_mut(