//! 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")]

#[cfg(not(feature = "ferrocene_certified"))]

use crate::error::Error;

#[cfg(not(feature = "ferrocene_certified"))]

use crate::fmt;

#[cfg(not(feature = "ferrocene_certified"))]

use crate::hash::{Hash, Hasher};

use crate::marker::PointeeSized;

#[cfg(not(feature = "ferrocene_certified"))]

mod num;

#[unstable(feature = "convert_float_to_int", issue = "67057")]

#[cfg(not(feature = "ferrocene_certified"))]

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"]

#[const_trait]

#[rustc_const_unstable(feature = "const_convert", issue = "143773")]

pub 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 work with any type that can be converted by cheap mutable-to-mutable conversion

/// into a byte slice (`[u8]`) or 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"]

#[const_trait]

#[rustc_const_unstable(feature = "const_convert", issue = "143773")]

pub 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")]

#[const_trait]

pub 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")]

#[const_trait]

pub 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")]

#[const_trait]

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`] 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")]

#[const_trait]

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")]

    #[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]

}

// 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]

}

// 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]

}

// 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)]

}

/// **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")]

#[cfg(not(feature = "ferrocene_certified"))]

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]

}

// 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]

}

////////////////////////////////////////////////////////////////////////////////

// CONCRETE IMPLS

////////////////////////////////////////////////////////////////////////////////

#[stable(feature = "rust1", since = "1.0.0")]

#[rustc_const_unstable(feature = "const_convert", issue = "143773")]

#[cfg(not(feature = "ferrocene_certified"))]

impl<T> const AsRef<[T]> for [T] {

    #[inline(always)]

}

#[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")]

#[cfg(not(feature = "ferrocene_certified"))]

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")]

#[cfg(not(feature = "ferrocene_certified"))]

impl fmt::Debug for Infallible {

    fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {

        match *self {}

    }

}

#[stable(feature = "convert_infallible", since = "1.34.0")]

#[cfg(not(feature = "ferrocene_certified"))]

impl fmt::Display for Infallible {

    fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {

        match *self {}

    }

}

#[stable(feature = "str_parse_error2", since = "1.8.0")]

#[cfg(not(feature = "ferrocene_certified"))]

impl Error for Infallible {}

#[stable(feature = "convert_infallible", since = "1.34.0")]

#[rustc_const_unstable(feature = "const_cmp", issue = "143800")]

#[cfg(not(feature = "ferrocene_certified"))]

impl const PartialEq for Infallible {

    fn eq(&self, _: &Infallible) -> bool {

        match *self {}

    }

}

#[stable(feature = "convert_infallible", since = "1.34.0")]

#[cfg(not(feature = "ferrocene_certified"))]

impl Eq for Infallible {}

#[stable(feature = "convert_infallible", since = "1.34.0")]

#[cfg(not(feature = "ferrocene_certified"))]

impl PartialOrd for Infallible {

    fn partial_cmp(&self, _other: &Self) -> Option<crate::cmp::Ordering> {

        match *self {}

    }

}

#[stable(feature = "convert_infallible", since = "1.34.0")]

#[cfg(not(feature = "ferrocene_certified"))]

impl 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")]

#[cfg(not(feature = "ferrocene_certified"))]

impl const From<!> for Infallible {

    #[inline]

    fn from(x: !) -> Self {

        x

    }

}

#[stable(feature = "convert_infallible_hash", since = "1.44.0")]

#[cfg(not(feature = "ferrocene_certified"))]

impl Hash for Infallible {

    fn hash<H: Hasher>(&self, _: &mut H) {

        match *self {}

    }

}