library/core/src/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
//! - 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. When targeting a version prior to Rust 1.41, it
//! may be 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::error::Error;
use crate::fmt;
use crate::hash::{Hash, Hasher};
use crate::marker::PointeeSized;

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 = [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(&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:
///
/// ```
/// # #![allow(non_local_definitions)]
/// 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);
/// ```
///
/// [`String`]: ../../std/string/struct.String.html
/// [`Vec`]: ../../std/vec/struct.Vec.html
#[rustc_diagnostic_item = "Into"]
#[stable(feature = "rust1", since = "1.0.0")]
#[doc(search_unbox)]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
pub const trait Into<T>: Sized {
    /// Converts this type into the (usually inferred) input type.
    #[must_use]
    #[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`] when targeting a version prior to Rust 1.41 and converting to a type
/// outside the current crate.
/// `From` was not able to do these types of conversions in earlier versions because of Rust's
/// orphaning rules.
/// See [`Into`] for more details.
///
/// 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.
///
/// The `From` trait 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>`.
/// `From` simplifies error handling by allowing a function to return a single error type
/// that encapsulates multiple error types. See the "Examples" section and [the book][book] for more
/// details.
///
/// **Note: This trait must not fail**. The `From` trait is intended for perfect conversions.
/// If the conversion can fail or is not perfect, 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
///
/// # When to implement `From`
///
/// While there's no technical restrictions on which conversions can be done using
/// a `From` implementation, the general expectation is that the conversions
/// should typically be restricted as follows:
///
/// * The conversion is *infallible*: if the conversion can fail, use [`TryFrom`]
///   instead; don't provide a `From` impl that panics.
///
/// * The conversion is *lossless*: semantically, it should not lose or discard
///   information. For example, `i32: From<u16>` exists, where the original
///   value can be recovered using `u16: TryFrom<i32>`.  And `String: From<&str>`
///   exists, where you can get something equivalent to the original value via
///   `Deref`.  But `From` cannot be used to convert from `u32` to `u16`, since
///   that cannot succeed in a lossless way.  (There's some wiggle room here for
///   information not considered semantically relevant.  For example,
///   `Box<[T]>: From<Vec<T>>` exists even though it might not preserve capacity,
///   like how two vectors can be equal despite differing capacities.)
///
/// * The conversion is *value-preserving*: the conceptual kind and meaning of
///   the resulting value is the same, even though the Rust type and technical
///   representation might be different.  For example `-1_i8 as u8` is *lossless*,
///   since `as` casting back can recover the original value, but that conversion
///   is *not* available via `From` because `-1` and `255` are different conceptual
///   values (despite being identical bit patterns technically).  But
///   `f32: From<i16>` *is* available because `1_i16` and `1.0_f32` are conceptually
///   the same real number (despite having very different bit patterns technically).
///   `String: From<char>` is available because they're both *text*, but
///   `String: From<u32>` is *not* available, since `1` (a number) and `"1"`
///   (text) are too different.  (Converting values to text is instead covered
///   by the [`Display`](crate::fmt::Display) trait.)
///
/// * The conversion is *obvious*: it's the only reasonable conversion between
///   the two types.  Otherwise it's better to have it be a named method or
///   constructor, like how [`str::as_bytes`] is a method and how integers have
///   methods like [`u32::from_ne_bytes`], [`u32::from_le_bytes`], and
///   [`u32::from_be_bytes`], none of which are `From` implementations.  Whereas
///   there's only one reasonable way to wrap an [`Ipv6Addr`](crate::net::Ipv6Addr)
///   into an [`IpAddr`](crate::net::IpAddr), thus `IpAddr: From<Ipv6Addr>` exists.
///
/// # 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 with `From::from`.
///
/// ```
/// 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)
/// }
/// ```
///
/// [`String`]: ../../std/string/struct.String.html
/// [`from`]: From::from
/// [book]: ../../book/ch09-00-error-handling.html
#[rustc_diagnostic_item = "From"]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_on_unimplemented(on(
    all(Self = "&str", T = "alloc::string::String"),
    note = "to coerce a `{T}` into a `{Self}`, use `&*` as a prefix",
))]
#[doc(search_unbox)]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
pub const trait From<T>: Sized {
    /// Converts to this type from the input type.
    #[rustc_diagnostic_item = "from_fn"]
    #[must_use]
    #[stable(feature = "rust1", since = "1.0.0")]
    fn from(value: 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`].
///
/// Prefer using [`TryInto`] over [`TryFrom`] when specifying trait bounds on a generic function
/// to ensure that types that only implement [`TryInto`] can be used as well.
///
/// # Implementing `TryInto`
///
/// This suffers the same restrictions and reasoning as implementing
/// [`Into`], see there for details.
#[rustc_diagnostic_item = "TryInto"]
#[stable(feature = "try_from", since = "1.34.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
pub const 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`] 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.
///
/// Prefer using [`TryInto`] over [`TryFrom`] when specifying trait bounds on a generic function
/// to ensure that types that only implement [`TryInto`] can be used as well.
///
/// `TryFrom<T>` can be implemented as follows:
///
/// ```
/// 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 values greater than zero!")
///         } else {
///             Ok(GreaterThanZero(value))
///         }
///     }
/// }
/// ```
///
/// # Examples
///
/// As described, [`i32`] implements `TryFrom<`[`i64`]`>`:
///
/// ```
/// 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`]: TryFrom::try_from
#[rustc_diagnostic_item = "TryFrom"]
#[stable(feature = "try_from", since = "1.34.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
pub const 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")]
    #[rustc_diagnostic_item = "try_from_fn"]
    fn try_from(value: T) -> Result<Self, Self::Error>;
}

////////////////////////////////////////////////////////////////////////////////
// GENERIC IMPLS
////////////////////////////////////////////////////////////////////////////////

// As lifts over &
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
impl<T: PointeeSized, U: PointeeSized> const AsRef<U> for &T
where
    T: [const] AsRef<U>,
{
    #[inline]
    fn as_ref(&self) -> &U {
        <T as AsRef<U>>::as_ref(*self)
    }
}

// As lifts over &mut
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
impl<T: PointeeSized, U: PointeeSized> const AsRef<U> for &mut T
where
    T: [const] AsRef<U>,
{
    #[inline]
    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")]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
impl<T: PointeeSized, U: PointeeSized> const AsMut<U> for &mut T
where
    T: [const] AsMut<U>,
{
    #[inline]
    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")]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
impl<T, U> const Into<U> for T
where
    U: [const] From<T>,
{
    /// Calls `U::from(self)`.
    ///
    /// That is, this conversion is whatever the implementation of
    /// <code>[From]&lt;T&gt; for U</code> chooses to do.
    #[inline]
    #[track_caller]
    fn into(self) -> U {
        U::from(self)
    }
}

// From (and thus Into) is reflexive
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
impl<T> const From<T> for T {
    /// Returns the argument unchanged.
    #[inline(always)]
    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")]
#[rustc_reservation_impl = "permitting this impl would forbid us from adding \
                            `impl<T> From<!> for T` later; see rust-lang/rust#64715 for details"]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
impl<T> const From<!> for T {
    fn from(t: !) -> T {
        t
    }
}

// TryFrom implies TryInto
#[stable(feature = "try_from", since = "1.34.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
impl<T, U> const TryInto<U> for T
where
    U: [const] TryFrom<T>,
{
    type Error = U::Error;

    #[inline]
    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")]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
impl<T, U> const TryFrom<U> for T
where
    U: [const] Into<T>,
{
    type Error = Infallible;

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

////////////////////////////////////////////////////////////////////////////////
// CONCRETE IMPLS
////////////////////////////////////////////////////////////////////////////////

#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
impl<T> const AsRef<[T]> for [T] {
    #[inline(always)]
    fn as_ref(&self) -> &[T] {
        self
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
impl<T> const AsMut<[T]> for [T] {
    #[inline(always)]
    fn as_mut(&mut self) -> &mut [T] {
        self
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
impl const AsRef<str> for str {
    #[inline(always)]
    fn as_ref(&self) -> &str {
        self
    }
}

#[stable(feature = "as_mut_str_for_str", since = "1.51.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
impl const AsMut<str> for str {
    #[inline(always)]
    fn as_mut(&mut self) -> &mut 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-fledged type: in the position of a function’s return type.
/// Specifically, it is possible to have 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.
#[stable(feature = "convert_infallible", since = "1.34.0")]
#[derive(Copy)]
pub enum Infallible {}

#[stable(feature = "convert_infallible", since = "1.34.0")]
#[rustc_const_unstable(feature = "const_clone", issue = "142757")]
impl const 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 = "str_parse_error2", since = "1.8.0")]
impl Error for Infallible {}

#[stable(feature = "convert_infallible", since = "1.34.0")]
#[rustc_const_unstable(feature = "const_cmp", issue = "143800")]
impl const PartialEq for Infallible {
    fn eq(&self, _: &Infallible) -> bool {
        match *self {}
    }
}

#[stable(feature = "convert_infallible", since = "1.34.0")]
#[rustc_const_unstable(feature = "const_cmp", issue = "143800")]
impl const Eq for Infallible {}

#[stable(feature = "convert_infallible", since = "1.34.0")]
#[rustc_const_unstable(feature = "const_cmp", issue = "143800")]
impl const PartialOrd for Infallible {
    fn partial_cmp(&self, _other: &Self) -> Option<crate::cmp::Ordering> {
        match *self {}
    }
}

#[stable(feature = "convert_infallible", since = "1.34.0")]
#[rustc_const_unstable(feature = "const_cmp", issue = "143800")]
impl const Ord for Infallible {
    fn cmp(&self, _other: &Self) -> crate::cmp::Ordering {
        match *self {}
    }
}

#[stable(feature = "convert_infallible", since = "1.34.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
impl const From<!> for Infallible {
    #[inline]
    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 {}
    }
}