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diff --git a/library/core/src/convert/mod.rs b/library/core/src/convert/mod.rs new file mode 100644 index 000000000..b30c8a4ae --- /dev/null +++ b/library/core/src/convert/mod.rs @@ -0,0 +1,755 @@ +//! 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 +//! - [`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::fmt; +use crate::hash::{Hash, Hasher}; + +mod num; + +#[unstable(feature = "convert_float_to_int", issue = "67057")] +pub use num::FloatToInt; + +/// The identity function. +/// +/// Two things are important to note about this function: +/// +/// - It is not always equivalent to a closure like `|x| x`, since the +/// closure may coerce `x` into a different type. +/// +/// - It moves the input `x` passed to the function. +/// +/// While it might seem strange to have a function that just returns back the +/// input, there are some interesting uses. +/// +/// # Examples +/// +/// Using `identity` to do nothing in a sequence of other, interesting, +/// functions: +/// +/// ```rust +/// use std::convert::identity; +/// +/// fn manipulation(x: u32) -> u32 { +/// // Let's pretend that adding one is an interesting function. +/// x + 1 +/// } +/// +/// let _arr = &[identity, manipulation]; +/// ``` +/// +/// Using `identity` as a "do nothing" base case in a conditional: +/// +/// ```rust +/// use std::convert::identity; +/// +/// # let condition = true; +/// # +/// # fn manipulation(x: u32) -> u32 { x + 1 } +/// # +/// let do_stuff = if condition { manipulation } else { identity }; +/// +/// // Do more interesting stuff... +/// +/// let _results = do_stuff(42); +/// ``` +/// +/// Using `identity` to keep the `Some` variants of an iterator of `Option<T>`: +/// +/// ```rust +/// use std::convert::identity; +/// +/// let iter = [Some(1), None, Some(3)].into_iter(); +/// let filtered = iter.filter_map(identity).collect::<Vec<_>>(); +/// assert_eq!(vec![1, 3], filtered); +/// ``` +#[stable(feature = "convert_id", since = "1.33.0")] +#[rustc_const_stable(feature = "const_identity", since = "1.33.0")] +#[inline] +pub const fn identity<T>(x: T) -> T { + x +} + +/// Used to do a cheap reference-to-reference conversion. +/// +/// This trait is similar to [`AsMut`] which is used for converting between mutable references. +/// If you need to do a costly conversion it is better to implement [`From`] with type +/// `&T` or write a custom function. +/// +/// `AsRef` has the same signature as [`Borrow`], but [`Borrow`] is different in 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. +/// - [`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`) +/// +/// # 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")] +#[cfg_attr(not(test), rustc_diagnostic_item = "AsRef")] +pub trait AsRef<T: ?Sized> { + /// 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`) +/// +/// # 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")] +#[cfg_attr(not(test), rustc_diagnostic_item = "AsMut")] +pub trait AsMut<T: ?Sized> { + /// 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: +/// +/// ``` +/// 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")] +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 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**. 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 +/// +/// # 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) +/// } +/// ``` +/// +/// [`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 = "std::string::String"), + note = "to coerce a `{T}` into a `{Self}`, use `&*` as a prefix", +))] +pub trait From<T>: Sized { + /// Converts to this type from the input type. + #[lang = "from"] + #[must_use] + #[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. +#[rustc_diagnostic_item = "TryInto"] +#[stable(feature = "try_from", since = "1.34.0")] +pub trait TryInto<T>: Sized { + /// The type returned in the event of a conversion error. + #[stable(feature = "try_from", since = "1.34.0")] + type Error; + + /// Performs the conversion. + #[stable(feature = "try_from", since = "1.34.0")] + fn try_into(self) -> Result<T, Self::Error>; +} + +/// Simple and safe type conversions that may fail in a controlled +/// way under some circumstances. It is the reciprocal of [`TryInto`]. +/// +/// This is useful when you are doing a type conversion that may +/// trivially succeed but may also need special handling. +/// For example, there is no way to convert an [`i64`] into an [`i32`] +/// using the [`From`] trait, because an [`i64`] may contain a value +/// that an [`i32`] cannot represent and so the conversion would lose data. +/// This might be handled by truncating the [`i64`] to an [`i32`] (essentially +/// giving the [`i64`]'s value modulo [`i32::MAX`]) or by simply returning +/// [`i32::MAX`], or by some other method. The [`From`] trait is intended +/// for perfect conversions, so the `TryFrom` trait informs the +/// programmer when a type conversion could go bad and lets them +/// decide how to handle it. +/// +/// # Generic Implementations +/// +/// - `TryFrom<T> for U` implies [`TryInto`]`<U> for T` +/// - [`try_from`] is reflexive, which means that `TryFrom<T> for T` +/// is implemented and cannot fail -- the associated `Error` type for +/// calling `T::try_from()` on a value of type `T` is [`Infallible`]. +/// When the [`!`] type is stabilized [`Infallible`] and [`!`] will be +/// equivalent. +/// +/// `TryFrom<T>` can be implemented as follows: +/// +/// ``` +/// struct GreaterThanZero(i32); +/// +/// impl TryFrom<i32> for GreaterThanZero { +/// type Error = &'static str; +/// +/// fn try_from(value: i32) -> Result<Self, Self::Error> { +/// if value <= 0 { +/// Err("GreaterThanZero only accepts value superior than zero!") +/// } else { +/// Ok(GreaterThanZero(value)) +/// } +/// } +/// } +/// ``` +/// +/// # Examples +/// +/// As described, [`i32`] implements `TryFrom<`[`i64`]`>`: +/// +/// ``` +/// 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")] +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")] +#[rustc_const_unstable(feature = "const_convert", issue = "88674")] +impl<T: ?Sized, U: ?Sized> 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 = "88674")] +impl<T: ?Sized, U: ?Sized> 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 = "88674")] +impl<T: ?Sized, U: ?Sized> 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 = "88674")] +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]<T> for U</code> chooses to do. + 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 = "88674")] +impl<T> const From<T> for T { + /// Returns the argument unchanged. + fn from(t: T) -> T { + t + } +} + +/// **Stability note:** This impl does not yet exist, but we are +/// "reserving space" to add it in the future. See +/// [rust-lang/rust#64715][#64715] for details. +/// +/// [#64715]: https://github.com/rust-lang/rust/issues/64715 +#[stable(feature = "convert_infallible", since = "1.34.0")] +#[allow(unused_attributes)] // FIXME(#58633): do a principled fix instead. +#[rustc_reservation_impl = "permitting this impl would forbid us from adding \ + `impl<T> From<!> for T` later; see rust-lang/rust#64715 for details"] +#[rustc_const_unstable(feature = "const_convert", issue = "88674")] +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 = "88674")] +impl<T, U> const TryInto<U> for T +where + U: ~const 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")] +#[rustc_const_unstable(feature = "const_convert", issue = "88674")] +impl<T, U> const TryFrom<U> for T +where + U: ~const 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 + } +} + +#[stable(feature = "as_mut_str_for_str", since = "1.51.0")] +impl AsMut<str> for str { + #[inline] + 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 = "91805")] +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 = "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")] +#[rustc_const_unstable(feature = "const_convert", issue = "88674")] +impl const From<!> for Infallible { + fn from(x: !) -> Self { + x + } +} + +#[stable(feature = "convert_infallible_hash", since = "1.44.0")] +impl Hash for Infallible { + fn hash<H: Hasher>(&self, _: &mut H) { + match *self {} + } +} |