| //! Traits for conversions between types. |
| //! |
| //! The traits in this module provide a way to convert from one type to another type. |
| //! Each trait serves a different purpose: |
| //! |
| //! - Implement the [`AsRef`] trait for cheap reference-to-reference conversions |
| //! - Implement the [`AsMut`] trait for cheap mutable-to-mutable conversions |
| //! - Implement the [`From`] trait for consuming value-to-value conversions |
| //! - Implement the [`Into`] trait for consuming value-to-value conversions to types |
| //! outside the current crate |
| //! - The [`TryFrom`] and [`TryInto`] traits behave like [`From`] and [`Into`], |
| //! but should be implemented when the conversion can fail. |
| //! |
| //! The traits in this module are often used as trait bounds for generic functions such that to |
| //! arguments of multiple types are supported. See the documentation of each trait for examples. |
| //! |
| //! As a library author, you should always prefer implementing [`From<T>`][`From`] or |
| //! [`TryFrom<T>`][`TryFrom`] rather than [`Into<U>`][`Into`] or [`TryInto<U>`][`TryInto`], |
| //! as [`From`] and [`TryFrom`] provide greater flexibility and offer |
| //! equivalent [`Into`] or [`TryInto`] implementations for free, thanks to a |
| //! blanket implementation in the standard library. Only implement [`Into`] or [`TryInto`] |
| //! when a conversion to a type outside the current crate is required. |
| //! |
| //! # Generic Implementations |
| //! |
| //! - [`AsRef`] and [`AsMut`] auto-dereference if the inner type is a reference |
| //! - [`From`]`<U> for T` implies [`Into`]`<T> for U` |
| //! - [`TryFrom`]`<U> for T` implies [`TryInto`]`<T> for U` |
| //! - [`From`] and [`Into`] are reflexive, which means that all types can |
| //! `into` themselves and `from` themselves |
| //! |
| //! See each trait for usage examples. |
| //! |
| //! [`Into`]: trait.Into.html |
| //! [`From`]: trait.From.html |
| //! [`TryFrom`]: trait.TryFrom.html |
| //! [`TryInto`]: trait.TryInto.html |
| //! [`AsRef`]: trait.AsRef.html |
| //! [`AsMut`]: trait.AsMut.html |
| |
| #![stable(feature = "rust1", since = "1.0.0")] |
| |
| use crate::fmt; |
| |
| /// An identity function. |
| /// |
| /// Two things are important to note about this function: |
| /// |
| /// - It is not always equivalent to a closure like `|x| x` since the |
| /// closure may coerce `x` into a different type. |
| /// |
| /// - It moves the input `x` passed to the function. |
| /// |
| /// While it might seem strange to have a function that just returns back the |
| /// input, there are some interesting uses. |
| /// |
| /// # Examples |
| /// |
| /// Using `identity` to do nothing among other interesting functions: |
| /// |
| /// ```rust |
| /// use std::convert::identity; |
| /// |
| /// fn manipulation(x: u32) -> u32 { |
| /// // Let's assume that this function does something interesting. |
| /// x + 1 |
| /// } |
| /// |
| /// let _arr = &[identity, manipulation]; |
| /// ``` |
| /// |
| /// Using `identity` to get a function that changes nothing in a conditional: |
| /// |
| /// ```rust |
| /// use std::convert::identity; |
| /// |
| /// # let condition = true; |
| /// |
| /// # fn manipulation(x: u32) -> u32 { x + 1 } |
| /// |
| /// let do_stuff = if condition { manipulation } else { identity }; |
| /// |
| /// // do more interesting stuff.. |
| /// |
| /// let _results = do_stuff(42); |
| /// ``` |
| /// |
| /// Using `identity` to keep the `Some` variants of an iterator of `Option<T>`: |
| /// |
| /// ```rust |
| /// use std::convert::identity; |
| /// |
| /// let iter = vec![Some(1), None, Some(3)].into_iter(); |
| /// let filtered = iter.filter_map(identity).collect::<Vec<_>>(); |
| /// assert_eq!(vec![1, 3], filtered); |
| /// ``` |
| #[stable(feature = "convert_id", since = "1.33.0")] |
| #[inline] |
| pub const fn identity<T>(x: T) -> T { x } |
| |
| /// Used to do a cheap reference-to-reference conversion. |
| /// |
| /// This trait is similar to [`AsMut`] which is used for converting between mutable references. |
| /// If you need to do a costly conversion it is better to implement [`From`] with type |
| /// `&T` or write a custom function. |
| /// |
| /// `AsRef` has the same signature as [`Borrow`], but `Borrow` is different in few aspects: |
| /// |
| /// - Unlike `AsRef`, `Borrow` has a blanket impl for any `T`, and can be used to accept either |
| /// a reference or a value. |
| /// - `Borrow` also requires that `Hash`, `Eq` and `Ord` for borrowed value are |
| /// equivalent to those of the owned value. For this reason, if you want to |
| /// borrow only a single field of a struct you can implement `AsRef`, but not `Borrow`. |
| /// |
| /// [`Borrow`]: ../../std/borrow/trait.Borrow.html |
| /// |
| /// **Note: This trait must not fail**. If the conversion can fail, use a |
| /// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`]. |
| /// |
| /// [`Option<T>`]: ../../std/option/enum.Option.html |
| /// [`Result<T, E>`]: ../../std/result/enum.Result.html |
| /// |
| /// # Generic Implementations |
| /// |
| /// - `AsRef` auto-dereferences if the inner type is a reference or a mutable |
| /// reference (e.g.: `foo.as_ref()` will work the same if `foo` has type |
| /// `&mut Foo` or `&&mut Foo`) |
| /// |
| /// # Examples |
| /// |
| /// By using trait bounds we can accept arguments of different types as long as they can be |
| /// converted 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. |
| /// |
| /// [`String`]: ../../std/string/struct.String.html |
| /// |
| /// ``` |
| /// fn is_hello<T: AsRef<str>>(s: T) { |
| /// assert_eq!("hello", s.as_ref()); |
| /// } |
| /// |
| /// let s = "hello"; |
| /// is_hello(s); |
| /// |
| /// let s = "hello".to_string(); |
| /// is_hello(s); |
| /// ``` |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub trait AsRef<T: ?Sized> { |
| /// Performs the conversion. |
| #[stable(feature = "rust1", since = "1.0.0")] |
| fn as_ref(&self) -> &T; |
| } |
| |
| /// Used to do a cheap mutable-to-mutable reference conversion. |
| /// |
| /// This trait is similar to [`AsRef`] but used for converting between mutable |
| /// references. If you need to do a costly conversion it is better to |
| /// implement [`From`] with type `&mut T` or write a custom function. |
| /// |
| /// **Note: This trait must not fail**. If the conversion can fail, use a |
| /// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`]. |
| /// |
| /// [`Option<T>`]: ../../std/option/enum.Option.html |
| /// [`Result<T, E>`]: ../../std/result/enum.Result.html |
| /// |
| /// # Generic Implementations |
| /// |
| /// - `AsMut` auto-dereferences if the inner type is a mutable reference |
| /// (e.g.: `foo.as_mut()` will work the same if `foo` has type `&mut Foo` |
| /// or `&mut &mut Foo`) |
| /// |
| /// # Examples |
| /// |
| /// Using `AsMut` as trait bound for a generic function we can accept all mutable references |
| /// that can be converted to type `&mut T`. Because [`Box<T>`] implements `AsMut<T>` we can |
| /// write a function `add_one` that takes all arguments that can be converted to `&mut u64`. |
| /// Because [`Box<T>`] implements `AsMut<T>`, `add_one` accepts arguments of type |
| /// `&mut Box<u64>` as well: |
| /// |
| /// ``` |
| /// fn add_one<T: AsMut<u64>>(num: &mut T) { |
| /// *num.as_mut() += 1; |
| /// } |
| /// |
| /// let mut boxed_num = Box::new(0); |
| /// add_one(&mut boxed_num); |
| /// assert_eq!(*boxed_num, 1); |
| /// ``` |
| /// |
| /// [`Box<T>`]: ../../std/boxed/struct.Box.html |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub trait AsMut<T: ?Sized> { |
| /// Performs the conversion. |
| #[stable(feature = "rust1", since = "1.0.0")] |
| fn as_mut(&mut self) -> &mut T; |
| } |
| |
| /// A value-to-value conversion that consumes the input value. The |
| /// opposite of [`From`]. |
| /// |
| /// One should only implement [`Into`] if a conversion to a type outside the current crate is |
| /// required. Otherwise one should always prefer implementing [`From`] over [`Into`] because |
| /// implementing [`From`] automatically provides one with a implementation of [`Into`] thanks to |
| /// the blanket implementation in the standard library. [`From`] cannot do these type of |
| /// conversions because of Rust's orphaning rules. |
| /// |
| /// **Note: This trait must not fail**. If the conversion can fail, use [`TryInto`]. |
| /// |
| /// # Generic Implementations |
| /// |
| /// - [`From`]`<T> for U` implies `Into<U> for T` |
| /// - [`Into`] is reflexive, which means that `Into<T> for T` is implemented |
| /// |
| /// # Implementing [`Into`] for conversions to external types |
| /// |
| /// If the destination type is not part of the current crate |
| /// then you can't implement [`From`] directly. |
| /// For example, take this code: |
| /// |
| /// ```compile_fail |
| /// struct Wrapper<T>(Vec<T>); |
| /// impl<T> From<Wrapper<T>> for Vec<T> { |
| /// fn from(w: Wrapper<T>) -> Vec<T> { |
| /// w.0 |
| /// } |
| /// } |
| /// ``` |
| /// This will fail to compile because we cannot implement a trait for a type |
| /// if both the trait and the type are not defined by the current crate. |
| /// This is due to Rust's orphaning rules. To bypass this, you can implement [`Into`] directly: |
| /// |
| /// ``` |
| /// struct Wrapper<T>(Vec<T>); |
| /// impl<T> Into<Vec<T>> for Wrapper<T> { |
| /// fn into(self) -> Vec<T> { |
| /// self.0 |
| /// } |
| /// } |
| /// ``` |
| /// |
| /// It is important to understand that [`Into`] does not provide a [`From`] implementation |
| /// (as [`From`] does with [`Into`]). Therefore, you should always try to implement [`From`] |
| /// and then fall back to [`Into`] if [`From`] can't be implemented. |
| /// |
| /// Prefer using [`Into`] over [`From`] when specifying trait bounds on a generic function |
| /// to ensure that types that only implement [`Into`] can be used as well. |
| /// |
| /// # Examples |
| /// |
| /// [`String`] implements `Into<Vec<u8>>`: |
| /// |
| /// In order to express that we want a generic function to take all arguments that can be |
| /// converted to a specified type `T`, we can use a trait bound of [`Into`]`<T>`. |
| /// For example: The function `is_hello` takes all arguments that can be converted into a |
| /// `Vec<u8>`. |
| /// |
| /// ``` |
| /// fn is_hello<T: Into<Vec<u8>>>(s: T) { |
| /// let bytes = b"hello".to_vec(); |
| /// assert_eq!(bytes, s.into()); |
| /// } |
| /// |
| /// let s = "hello".to_string(); |
| /// is_hello(s); |
| /// ``` |
| /// |
| /// [`TryInto`]: trait.TryInto.html |
| /// [`Option<T>`]: ../../std/option/enum.Option.html |
| /// [`Result<T, E>`]: ../../std/result/enum.Result.html |
| /// [`String`]: ../../std/string/struct.String.html |
| /// [`From`]: trait.From.html |
| /// [`Into`]: trait.Into.html |
| #[stable(feature = "rust1", since = "1.0.0")] |
| pub trait Into<T>: Sized { |
| /// Performs the conversion. |
| #[stable(feature = "rust1", since = "1.0.0")] |
| fn into(self) -> T; |
| } |
| |
| /// Used to do value-to-value conversions while consuming the input value. It is the reciprocal of |
| /// [`Into`]. |
| /// |
| /// One should always prefer implementing `From` over [`Into`] |
| /// because implementing `From` automatically provides one with a implementation of [`Into`] |
| /// thanks to the blanket implementation in the standard library. |
| /// |
| /// Only implement [`Into`] if a conversion to a type outside the current crate is required. |
| /// `From` cannot do these type of conversions because of Rust's orphaning rules. |
| /// See [`Into`] for more details. |
| /// |
| /// Prefer using [`Into`] over using `From` when specifying trait bounds on a generic function. |
| /// This way, types that directly implement [`Into`] can be used as arguments as well. |
| /// |
| /// The `From` is also very useful when performing error handling. When constructing a function |
| /// that is capable of failing, the return type will generally be of the form `Result<T, E>`. |
| /// The `From` trait simplifies error handling by allowing a function to return a single error type |
| /// that encapsulate multiple error types. See the "Examples" section and [the book][book] for more |
| /// details. |
| /// |
| /// **Note: This trait must not fail**. If the conversion can fail, use [`TryFrom`]. |
| /// |
| /// # Generic Implementations |
| /// |
| /// - `From<T> for U` implies [`Into`]`<U> for T` |
| /// - `From` is reflexive, which means that `From<T> for T` is implemented |
| /// |
| /// # Examples |
| /// |
| /// [`String`] implements `From<&str>`: |
| /// |
| /// An explicit conversion from a `&str` to a String is done as follows: |
| /// |
| /// ``` |
| /// let string = "hello".to_string(); |
| /// let other_string = String::from("hello"); |
| /// |
| /// assert_eq!(string, other_string); |
| /// ``` |
| /// |
| /// While performing error handling it is often useful to implement `From` for your own error type. |
| /// By converting underlying error types to our own custom error type that encapsulates the |
| /// underlying error type, we can return a single error type without losing information on the |
| /// underlying cause. The '?' operator automatically converts the underlying error type to our |
| /// custom error type by calling `Into<CliError>::into` which is automatically provided when |
| /// implementing `From`. The compiler then infers which implementation of `Into` should be used. |
| /// |
| /// ``` |
| /// use std::fs; |
| /// use std::io; |
| /// use std::num; |
| /// |
| /// enum CliError { |
| /// IoError(io::Error), |
| /// ParseError(num::ParseIntError), |
| /// } |
| /// |
| /// impl From<io::Error> for CliError { |
| /// fn from(error: io::Error) -> Self { |
| /// CliError::IoError(error) |
| /// } |
| /// } |
| /// |
| /// impl From<num::ParseIntError> for CliError { |
| /// fn from(error: num::ParseIntError) -> Self { |
| /// CliError::ParseError(error) |
| /// } |
| /// } |
| /// |
| /// fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> { |
| /// let mut contents = fs::read_to_string(&file_name)?; |
| /// let num: i32 = contents.trim().parse()?; |
| /// Ok(num) |
| /// } |
| /// ``` |
| /// |
| /// [`TryFrom`]: trait.TryFrom.html |
| /// [`Option<T>`]: ../../std/option/enum.Option.html |
| /// [`Result<T, E>`]: ../../std/result/enum.Result.html |
| /// [`String`]: ../../std/string/struct.String.html |
| /// [`Into`]: trait.Into.html |
| /// [`from`]: trait.From.html#tymethod.from |
| /// [book]: ../../book/ch09-00-error-handling.html |
| #[stable(feature = "rust1", since = "1.0.0")] |
| #[rustc_on_unimplemented( |
| on( |
| all(_Self="&str", T="std::string::String"), |
| note="to coerce a `{T}` into a `{Self}`, use `&*` as a prefix", |
| ) |
| )] |
| pub trait From<T>: Sized { |
| /// Performs the conversion. |
| #[stable(feature = "rust1", since = "1.0.0")] |
| fn from(_: T) -> Self; |
| } |
| |
| /// An attempted conversion that consumes `self`, which may or may not be |
| /// expensive. |
| /// |
| /// Library authors should usually not directly implement this trait, |
| /// but should prefer implementing the [`TryFrom`] trait, which offers |
| /// greater flexibility and provides an equivalent `TryInto` |
| /// implementation for free, thanks to a blanket implementation in the |
| /// standard library. For more information on this, see the |
| /// documentation for [`Into`]. |
| /// |
| /// # Implementing `TryInto` |
| /// |
| /// This suffers the same restrictions and reasoning as implementing |
| /// [`Into`], see there for details. |
| /// |
| /// [`TryFrom`]: trait.TryFrom.html |
| /// [`Into`]: trait.Into.html |
| #[stable(feature = "try_from", since = "1.34.0")] |
| pub trait TryInto<T>: Sized { |
| /// The type returned in the event of a conversion error. |
| #[stable(feature = "try_from", since = "1.34.0")] |
| type Error; |
| |
| /// Performs the conversion. |
| #[stable(feature = "try_from", since = "1.34.0")] |
| fn try_into(self) -> Result<T, Self::Error>; |
| } |
| |
| /// Simple and safe type conversions that may fail in a controlled |
| /// way under some circumstances. It is the reciprocal of [`TryInto`]. |
| /// |
| /// This is useful when you are doing a type conversion that may |
| /// trivially succeed but may also need special handling. |
| /// For example, there is no way to convert an `i64` into an `i32` |
| /// using the [`From`] trait, because an `i64` may contain a value |
| /// that an `i32` cannot represent and so the conversion would lose data. |
| /// This might be handled by truncating the `i64` to an `i32` (essentially |
| /// giving the `i64`'s value modulo `i32::MAX`) or by simply returning |
| /// `i32::MAX`, or by some other method. The `From` trait is intended |
| /// for perfect conversions, so the `TryFrom` trait informs the |
| /// programmer when a type conversion could go bad and lets them |
| /// decide how to handle it. |
| /// |
| /// # Generic Implementations |
| /// |
| /// - `TryFrom<T> for U` implies [`TryInto`]`<U> for T` |
| /// - [`try_from`] is reflexive, which means that `TryFrom<T> for T` |
| /// is implemented and cannot fail -- the associated `Error` type for |
| /// calling `T::try_from()` on a value of type `T` is `Infallible`. |
| /// When the `!` type is stablized `Infallible` and `!` will be |
| /// equivalent. |
| /// |
| /// `TryFrom<T>` can be implemented as follows: |
| /// |
| /// ``` |
| /// use std::convert::TryFrom; |
| /// |
| /// struct SuperiorThanZero(i32); |
| /// |
| /// impl TryFrom<i32> for SuperiorThanZero { |
| /// type Error = &'static str; |
| /// |
| /// fn try_from(value: i32) -> Result<Self, Self::Error> { |
| /// if value < 0 { |
| /// Err("SuperiorThanZero only accepts value superior than zero!") |
| /// } else { |
| /// Ok(SuperiorThanZero(value)) |
| /// } |
| /// } |
| /// } |
| /// ``` |
| /// |
| /// # Examples |
| /// |
| /// As described, [`i32`] implements `TryFrom<i64>`: |
| /// |
| /// ``` |
| /// use std::convert::TryFrom; |
| /// |
| /// let big_number = 1_000_000_000_000i64; |
| /// // Silently truncates `big_number`, requires detecting |
| /// // and handling the truncation after the fact. |
| /// let smaller_number = big_number as i32; |
| /// assert_eq!(smaller_number, -727379968); |
| /// |
| /// // Returns an error because `big_number` is too big to |
| /// // fit in an `i32`. |
| /// let try_smaller_number = i32::try_from(big_number); |
| /// assert!(try_smaller_number.is_err()); |
| /// |
| /// // Returns `Ok(3)`. |
| /// let try_successful_smaller_number = i32::try_from(3); |
| /// assert!(try_successful_smaller_number.is_ok()); |
| /// ``` |
| /// |
| /// [`try_from`]: trait.TryFrom.html#tymethod.try_from |
| /// [`TryInto`]: trait.TryInto.html |
| #[stable(feature = "try_from", since = "1.34.0")] |
| pub trait TryFrom<T>: Sized { |
| /// The type returned in the event of a conversion error. |
| #[stable(feature = "try_from", since = "1.34.0")] |
| type Error; |
| |
| /// Performs the conversion. |
| #[stable(feature = "try_from", since = "1.34.0")] |
| fn try_from(value: T) -> Result<Self, Self::Error>; |
| } |
| |
| //////////////////////////////////////////////////////////////////////////////// |
| // GENERIC IMPLS |
| //////////////////////////////////////////////////////////////////////////////// |
| |
| // As lifts over & |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T: ?Sized, U: ?Sized> AsRef<U> for &T where T: AsRef<U> |
| { |
| fn as_ref(&self) -> &U { |
| <T as AsRef<U>>::as_ref(*self) |
| } |
| } |
| |
| // As lifts over &mut |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T: ?Sized, U: ?Sized> AsRef<U> for &mut T where T: AsRef<U> |
| { |
| fn as_ref(&self) -> &U { |
| <T as AsRef<U>>::as_ref(*self) |
| } |
| } |
| |
| // FIXME (#45742): replace the above impls for &/&mut with the following more general one: |
| // // As lifts over Deref |
| // impl<D: ?Sized + Deref, U: ?Sized> AsRef<U> for D where D::Target: AsRef<U> { |
| // fn as_ref(&self) -> &U { |
| // self.deref().as_ref() |
| // } |
| // } |
| |
| // AsMut lifts over &mut |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T: ?Sized, U: ?Sized> AsMut<U> for &mut T where T: AsMut<U> |
| { |
| fn as_mut(&mut self) -> &mut U { |
| (*self).as_mut() |
| } |
| } |
| |
| // FIXME (#45742): replace the above impl for &mut with the following more general one: |
| // // AsMut lifts over DerefMut |
| // impl<D: ?Sized + Deref, U: ?Sized> AsMut<U> for D where D::Target: AsMut<U> { |
| // fn as_mut(&mut self) -> &mut U { |
| // self.deref_mut().as_mut() |
| // } |
| // } |
| |
| // From implies Into |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T, U> Into<U> for T where U: From<T> |
| { |
| fn into(self) -> U { |
| U::from(self) |
| } |
| } |
| |
| // From (and thus Into) is reflexive |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T> From<T> for T { |
| fn from(t: T) -> T { t } |
| } |
| |
| |
| // TryFrom implies TryInto |
| #[stable(feature = "try_from", since = "1.34.0")] |
| impl<T, U> TryInto<U> for T where U: TryFrom<T> |
| { |
| type Error = U::Error; |
| |
| fn try_into(self) -> Result<U, U::Error> { |
| U::try_from(self) |
| } |
| } |
| |
| // Infallible conversions are semantically equivalent to fallible conversions |
| // with an uninhabited error type. |
| #[stable(feature = "try_from", since = "1.34.0")] |
| impl<T, U> TryFrom<U> for T where U: Into<T> { |
| type Error = Infallible; |
| |
| fn try_from(value: U) -> Result<Self, Self::Error> { |
| Ok(U::into(value)) |
| } |
| } |
| |
| //////////////////////////////////////////////////////////////////////////////// |
| // CONCRETE IMPLS |
| //////////////////////////////////////////////////////////////////////////////// |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T> AsRef<[T]> for [T] { |
| fn as_ref(&self) -> &[T] { |
| self |
| } |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T> AsMut<[T]> for [T] { |
| fn as_mut(&mut self) -> &mut [T] { |
| self |
| } |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl AsRef<str> for str { |
| #[inline] |
| fn as_ref(&self) -> &str { |
| self |
| } |
| } |
| |
| //////////////////////////////////////////////////////////////////////////////// |
| // THE NO-ERROR ERROR TYPE |
| //////////////////////////////////////////////////////////////////////////////// |
| |
| /// The error type for errors that can never happen. |
| /// |
| /// Since this enum has no variant, a value of this type can never actually exist. |
| /// This can be useful for generic APIs that use [`Result`] and parameterize the error type, |
| /// to indicate that the result is always [`Ok`]. |
| /// |
| /// For example, the [`TryFrom`] trait (conversion that returns a [`Result`]) |
| /// has a blanket implementation for all types where a reverse [`Into`] implementation exists. |
| /// |
| /// ```ignore (illustrates std code, duplicating the impl in a doctest would be an error) |
| /// impl<T, U> TryFrom<U> for T where U: Into<T> { |
| /// type Error = Infallible; |
| /// |
| /// fn try_from(value: U) -> Result<Self, Infallible> { |
| /// Ok(U::into(value)) // Never returns `Err` |
| /// } |
| /// } |
| /// ``` |
| /// |
| /// # Future compatibility |
| /// |
| /// This enum has the same role as [the `!` “never” type][never], |
| /// which is unstable in this version of Rust. |
| /// When `!` is stabilized, we plan to make `Infallible` a type alias to it: |
| /// |
| /// ```ignore (illustrates future std change) |
| /// pub type Infallible = !; |
| /// ``` |
| /// |
| /// … and eventually deprecate `Infallible`. |
| /// |
| /// |
| /// However there is one case where `!` syntax can be used |
| /// before `!` is stabilized as a full-fleged type: in the position of a function’s return type. |
| /// Specifically, it is possible implementations for two different function pointer types: |
| /// |
| /// ``` |
| /// trait MyTrait {} |
| /// impl MyTrait for fn() -> ! {} |
| /// impl MyTrait for fn() -> std::convert::Infallible {} |
| /// ``` |
| /// |
| /// With `Infallible` being an enum, this code is valid. |
| /// However when `Infallible` becomes an alias for the never type, |
| /// the two `impl`s will start to overlap |
| /// and therefore will be disallowed by the language’s trait coherence rules. |
| /// |
| /// [`Ok`]: ../result/enum.Result.html#variant.Ok |
| /// [`Result`]: ../result/enum.Result.html |
| /// [`TryFrom`]: trait.TryFrom.html |
| /// [`Into`]: trait.Into.html |
| /// [never]: ../../std/primitive.never.html |
| #[stable(feature = "convert_infallible", since = "1.34.0")] |
| #[derive(Copy)] |
| pub enum Infallible {} |
| |
| #[stable(feature = "convert_infallible", since = "1.34.0")] |
| impl Clone for Infallible { |
| fn clone(&self) -> Infallible { |
| match *self {} |
| } |
| } |
| |
| #[stable(feature = "convert_infallible", since = "1.34.0")] |
| impl fmt::Debug for Infallible { |
| fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result { |
| match *self {} |
| } |
| } |
| |
| #[stable(feature = "convert_infallible", since = "1.34.0")] |
| impl fmt::Display for Infallible { |
| fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result { |
| match *self {} |
| } |
| } |
| |
| #[stable(feature = "convert_infallible", since = "1.34.0")] |
| impl PartialEq for Infallible { |
| fn eq(&self, _: &Infallible) -> bool { |
| match *self {} |
| } |
| } |
| |
| #[stable(feature = "convert_infallible", since = "1.34.0")] |
| impl Eq for Infallible {} |
| |
| #[stable(feature = "convert_infallible", since = "1.34.0")] |
| impl PartialOrd for Infallible { |
| fn partial_cmp(&self, _other: &Self) -> Option<crate::cmp::Ordering> { |
| match *self {} |
| } |
| } |
| |
| #[stable(feature = "convert_infallible", since = "1.34.0")] |
| impl Ord for Infallible { |
| fn cmp(&self, _other: &Self) -> crate::cmp::Ordering { |
| match *self {} |
| } |
| } |
| |
| #[stable(feature = "convert_infallible", since = "1.34.0")] |
| impl From<!> for Infallible { |
| fn from(x: !) -> Self { |
| x |
| } |
| } |