| //! 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}; |
| |
| 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")] |
| #[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`). |
| /// |
| /// 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")] |
| #[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 |
| /// |
| /// # 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", |
| ))] |
| 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`]. |
| /// |
| /// # 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`] 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 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")] |
| 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")] |
| impl<T: ?Sized, U: ?Sized> AsRef<U> for &T |
| where |
| T: 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")] |
| impl<T: ?Sized, U: ?Sized> AsRef<U> for &mut T |
| where |
| T: 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")] |
| impl<T: ?Sized, U: ?Sized> AsMut<U> for &mut T |
| where |
| T: 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")] |
| impl<T, U> Into<U> for T |
| where |
| U: From<T>, |
| { |
| /// Calls `U::from(self)`. |
| /// |
| /// That is, this conversion is whatever the implementation of |
| /// <code>[From]<T> 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")] |
| impl<T> 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")] |
| #[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"] |
| impl<T> From<!> for T { |
| fn from(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; |
| |
| #[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")] |
| impl<T, U> TryFrom<U> for T |
| where |
| U: 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")] |
| impl<T> AsRef<[T]> for [T] { |
| #[inline(always)] |
| fn as_ref(&self) -> &[T] { |
| self |
| } |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl<T> AsMut<[T]> for [T] { |
| #[inline(always)] |
| fn as_mut(&mut self) -> &mut [T] { |
| self |
| } |
| } |
| |
| #[stable(feature = "rust1", since = "1.0.0")] |
| impl AsRef<str> for str { |
| #[inline(always)] |
| fn as_ref(&self) -> &str { |
| self |
| } |
| } |
| |
| #[stable(feature = "as_mut_str_for_str", since = "1.51.0")] |
| impl 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")] |
| 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 = "str_parse_error2", since = "1.8.0")] |
| impl Error for Infallible { |
| fn description(&self) -> &str { |
| 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 { |
| #[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 {} |
| } |
| } |