739 lines
24 KiB
Rust
739 lines
24 KiB
Rust
//! Traits for conversions between types.
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//!
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//! The traits in this module provide a way to convert from one type to another type.
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//! Each trait serves a different purpose:
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//!
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//! - Implement the [`AsRef`] trait for cheap reference-to-reference conversions
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//! - Implement the [`AsMut`] trait for cheap mutable-to-mutable conversions
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//! - Implement the [`From`] trait for consuming value-to-value conversions
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//! - Implement the [`Into`] trait for consuming value-to-value conversions to types
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//! outside the current crate
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//! - The [`TryFrom`] and [`TryInto`] traits behave like [`From`] and [`Into`],
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//! but should be implemented when the conversion can fail.
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//!
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//! The traits in this module are often used as trait bounds for generic functions such that to
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//! arguments of multiple types are supported. See the documentation of each trait for examples.
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//!
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//! As a library author, you should always prefer implementing [`From<T>`][`From`] or
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//! [`TryFrom<T>`][`TryFrom`] rather than [`Into<U>`][`Into`] or [`TryInto<U>`][`TryInto`],
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//! as [`From`] and [`TryFrom`] provide greater flexibility and offer
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//! equivalent [`Into`] or [`TryInto`] implementations for free, thanks to a
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//! blanket implementation in the standard library. When targeting a version prior to Rust 1.41, it
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//! may be necessary to implement [`Into`] or [`TryInto`] directly when converting to a type
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//! outside the current crate.
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//!
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//! # Generic Implementations
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//!
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//! - [`AsRef`] and [`AsMut`] auto-dereference if the inner type is a reference
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//! - [`From`]`<U> for T` implies [`Into`]`<T> for U`
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//! - [`TryFrom`]`<U> for T` implies [`TryInto`]`<T> for U`
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//! - [`From`] and [`Into`] are reflexive, which means that all types can
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//! `into` themselves and `from` themselves
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//!
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//! See each trait for usage examples.
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#![stable(feature = "rust1", since = "1.0.0")]
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use crate::fmt;
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use crate::hash::{Hash, Hasher};
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mod num;
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#[unstable(feature = "convert_float_to_int", issue = "67057")]
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pub use num::FloatToInt;
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/// The identity function.
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///
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/// Two things are important to note about this function:
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///
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/// - It is not always equivalent to a closure like `|x| x`, since the
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/// closure may coerce `x` into a different type.
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///
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/// - It moves the input `x` passed to the function.
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///
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/// While it might seem strange to have a function that just returns back the
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/// input, there are some interesting uses.
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///
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/// # Examples
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///
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/// Using `identity` to do nothing in a sequence of other, interesting,
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/// functions:
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///
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/// ```rust
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/// use std::convert::identity;
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///
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/// fn manipulation(x: u32) -> u32 {
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/// // Let's pretend that adding one is an interesting function.
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/// x + 1
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/// }
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///
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/// let _arr = &[identity, manipulation];
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/// ```
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///
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/// Using `identity` as a "do nothing" base case in a conditional:
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///
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/// ```rust
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/// use std::convert::identity;
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///
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/// # let condition = true;
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/// #
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/// # fn manipulation(x: u32) -> u32 { x + 1 }
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/// #
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/// let do_stuff = if condition { manipulation } else { identity };
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///
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/// // Do more interesting stuff...
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///
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/// let _results = do_stuff(42);
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/// ```
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///
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/// Using `identity` to keep the `Some` variants of an iterator of `Option<T>`:
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///
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/// ```rust
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/// use std::convert::identity;
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///
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/// let iter = vec![Some(1), None, Some(3)].into_iter();
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/// let filtered = iter.filter_map(identity).collect::<Vec<_>>();
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/// assert_eq!(vec![1, 3], filtered);
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/// ```
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#[stable(feature = "convert_id", since = "1.33.0")]
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#[rustc_const_stable(feature = "const_identity", since = "1.33.0")]
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#[inline]
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pub const fn identity<T>(x: T) -> T {
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x
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}
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/// Used to do a cheap reference-to-reference conversion.
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///
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/// This trait is similar to [`AsMut`] which is used for converting between mutable references.
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/// If you need to do a costly conversion it is better to implement [`From`] with type
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/// `&T` or write a custom function.
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///
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/// `AsRef` has the same signature as [`Borrow`], but [`Borrow`] is different in few aspects:
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///
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/// - Unlike `AsRef`, [`Borrow`] has a blanket impl for any `T`, and can be used to accept either
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/// a reference or a value.
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/// - [`Borrow`] also requires that [`Hash`], [`Eq`] and [`Ord`] for borrowed value are
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/// equivalent to those of the owned value. For this reason, if you want to
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/// borrow only a single field of a struct you can implement `AsRef`, but not [`Borrow`].
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///
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/// **Note: This trait must not fail**. If the conversion can fail, use a
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/// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`].
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///
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/// # Generic Implementations
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///
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/// - `AsRef` auto-dereferences if the inner type is a reference or a mutable
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/// reference (e.g.: `foo.as_ref()` will work the same if `foo` has type
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/// `&mut Foo` or `&&mut Foo`)
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///
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/// # Examples
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///
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/// By using trait bounds we can accept arguments of different types as long as they can be
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/// converted to the specified type `T`.
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///
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/// For example: By creating a generic function that takes an `AsRef<str>` we express that we
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/// want to accept all references that can be converted to [`&str`] as an argument.
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/// Since both [`String`] and [`&str`] implement `AsRef<str>` we can accept both as input argument.
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///
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/// [`&str`]: primitive@str
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/// [`Option<T>`]: Option
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/// [`Result<T, E>`]: Result
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/// [`Borrow`]: crate::borrow::Borrow
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/// [`Eq`]: crate::cmp::Eq
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/// [`Ord`]: crate::cmp::Ord
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/// [`String`]: ../../std/string/struct.String.html
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///
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/// ```
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/// fn is_hello<T: AsRef<str>>(s: T) {
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/// assert_eq!("hello", s.as_ref());
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/// }
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///
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/// let s = "hello";
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/// is_hello(s);
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///
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/// let s = "hello".to_string();
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/// is_hello(s);
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/// ```
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#[stable(feature = "rust1", since = "1.0.0")]
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pub trait AsRef<T: ?Sized> {
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/// Performs the conversion.
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#[stable(feature = "rust1", since = "1.0.0")]
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fn as_ref(&self) -> &T;
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}
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/// Used to do a cheap mutable-to-mutable reference conversion.
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///
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/// This trait is similar to [`AsRef`] but used for converting between mutable
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/// references. If you need to do a costly conversion it is better to
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/// implement [`From`] with type `&mut T` or write a custom function.
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///
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/// **Note: This trait must not fail**. If the conversion can fail, use a
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/// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`].
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///
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/// [`Option<T>`]: Option
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/// [`Result<T, E>`]: Result
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///
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/// # Generic Implementations
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///
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/// - `AsMut` auto-dereferences if the inner type is a mutable reference
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/// (e.g.: `foo.as_mut()` will work the same if `foo` has type `&mut Foo`
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/// or `&mut &mut Foo`)
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///
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/// # Examples
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///
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/// Using `AsMut` as trait bound for a generic function we can accept all mutable references
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/// that can be converted to type `&mut T`. Because [`Box<T>`] implements `AsMut<T>` we can
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/// write a function `add_one` that takes all arguments that can be converted to `&mut u64`.
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/// Because [`Box<T>`] implements `AsMut<T>`, `add_one` accepts arguments of type
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/// `&mut Box<u64>` as well:
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///
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/// ```
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/// fn add_one<T: AsMut<u64>>(num: &mut T) {
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/// *num.as_mut() += 1;
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/// }
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///
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/// let mut boxed_num = Box::new(0);
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/// add_one(&mut boxed_num);
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/// assert_eq!(*boxed_num, 1);
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/// ```
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///
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/// [`Box<T>`]: ../../std/boxed/struct.Box.html
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#[stable(feature = "rust1", since = "1.0.0")]
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pub trait AsMut<T: ?Sized> {
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/// Performs the conversion.
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#[stable(feature = "rust1", since = "1.0.0")]
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fn as_mut(&mut self) -> &mut T;
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}
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/// A value-to-value conversion that consumes the input value. The
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/// opposite of [`From`].
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///
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/// One should avoid implementing [`Into`] and implement [`From`] instead.
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/// Implementing [`From`] automatically provides one with an implementation of [`Into`]
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/// thanks to the blanket implementation in the standard library.
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///
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/// Prefer using [`Into`] over [`From`] when specifying trait bounds on a generic function
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/// to ensure that types that only implement [`Into`] can be used as well.
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///
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/// **Note: This trait must not fail**. If the conversion can fail, use [`TryInto`].
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///
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/// # Generic Implementations
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///
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/// - [`From`]`<T> for U` implies `Into<U> for T`
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/// - [`Into`] is reflexive, which means that `Into<T> for T` is implemented
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///
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/// # Implementing [`Into`] for conversions to external types in old versions of Rust
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///
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/// Prior to Rust 1.41, if the destination type was not part of the current crate
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/// then you couldn't implement [`From`] directly.
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/// For example, take this code:
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///
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/// ```
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/// struct Wrapper<T>(Vec<T>);
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/// impl<T> From<Wrapper<T>> for Vec<T> {
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/// fn from(w: Wrapper<T>) -> Vec<T> {
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/// w.0
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/// }
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/// }
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/// ```
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/// This will fail to compile in older versions of the language because Rust's orphaning rules
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/// used to be a little bit more strict. To bypass this, you could implement [`Into`] directly:
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///
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/// ```
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/// struct Wrapper<T>(Vec<T>);
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/// impl<T> Into<Vec<T>> for Wrapper<T> {
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/// fn into(self) -> Vec<T> {
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/// self.0
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/// }
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/// }
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/// ```
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///
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/// It is important to understand that [`Into`] does not provide a [`From`] implementation
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/// (as [`From`] does with [`Into`]). Therefore, you should always try to implement [`From`]
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/// and then fall back to [`Into`] if [`From`] can't be implemented.
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///
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/// # Examples
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///
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/// [`String`] implements [`Into`]`<`[`Vec`]`<`[`u8`]`>>`:
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///
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/// In order to express that we want a generic function to take all arguments that can be
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/// converted to a specified type `T`, we can use a trait bound of [`Into`]`<T>`.
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/// For example: The function `is_hello` takes all arguments that can be converted into a
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/// [`Vec`]`<`[`u8`]`>`.
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///
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/// ```
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/// fn is_hello<T: Into<Vec<u8>>>(s: T) {
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/// let bytes = b"hello".to_vec();
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/// assert_eq!(bytes, s.into());
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/// }
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///
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/// let s = "hello".to_string();
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/// is_hello(s);
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/// ```
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///
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/// [`Option<T>`]: Option
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/// [`Result<T, E>`]: Result
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/// [`String`]: ../../std/string/struct.String.html
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/// [`Vec`]: ../../std/vec/struct.Vec.html
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#[stable(feature = "rust1", since = "1.0.0")]
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pub trait Into<T>: Sized {
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/// Performs the conversion.
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#[stable(feature = "rust1", since = "1.0.0")]
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fn into(self) -> T;
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}
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/// Used to do value-to-value conversions while consuming the input value. It is the reciprocal of
|
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/// [`Into`].
|
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///
|
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/// One should always prefer implementing `From` over [`Into`]
|
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/// because implementing `From` automatically provides one with an implementation of [`Into`]
|
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/// thanks to the blanket implementation in the standard library.
|
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///
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/// Only implement [`Into`] when targeting a version prior to Rust 1.41 and converting to a type
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/// outside the current crate.
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/// `From` was not able to do these types of conversions in earlier versions because of Rust's
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/// orphaning rules.
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/// See [`Into`] for more details.
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///
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/// Prefer using [`Into`] over using `From` when specifying trait bounds on a generic function.
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/// This way, types that directly implement [`Into`] can be used as arguments as well.
|
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///
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/// The `From` is also very useful when performing error handling. When constructing a function
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/// that is capable of failing, the return type will generally be of the form `Result<T, E>`.
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/// The `From` trait simplifies error handling by allowing a function to return a single error type
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/// that encapsulate multiple error types. See the "Examples" section and [the book][book] for more
|
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/// details.
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///
|
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/// **Note: This trait must not fail**. If the conversion can fail, use [`TryFrom`].
|
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///
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/// # Generic Implementations
|
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///
|
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/// - `From<T> for U` implies [`Into`]`<U> for T`
|
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/// - `From` is reflexive, which means that `From<T> for T` is implemented
|
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///
|
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/// # Examples
|
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///
|
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/// [`String`] implements `From<&str>`:
|
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///
|
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/// An explicit conversion from a `&str` to a String is done as follows:
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///
|
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/// ```
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/// let string = "hello".to_string();
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/// let other_string = String::from("hello");
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///
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/// assert_eq!(string, other_string);
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/// ```
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///
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/// While performing error handling it is often useful to implement `From` for your own error type.
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/// By converting underlying error types to our own custom error type that encapsulates the
|
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/// underlying error type, we can return a single error type without losing information on the
|
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/// 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
|
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/// implementing `From`. The compiler then infers which implementation of `Into` should be used.
|
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///
|
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/// ```
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/// use std::fs;
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/// use std::io;
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/// use std::num;
|
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///
|
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/// enum CliError {
|
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/// IoError(io::Error),
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/// ParseError(num::ParseIntError),
|
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/// }
|
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///
|
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/// impl From<io::Error> for CliError {
|
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/// fn from(error: io::Error) -> Self {
|
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/// CliError::IoError(error)
|
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/// }
|
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/// }
|
||
///
|
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/// impl From<num::ParseIntError> for CliError {
|
||
/// fn from(error: num::ParseIntError) -> Self {
|
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/// CliError::ParseError(error)
|
||
/// }
|
||
/// }
|
||
///
|
||
/// fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> {
|
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/// let mut contents = fs::read_to_string(&file_name)?;
|
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/// let num: i32 = contents.trim().parse()?;
|
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/// Ok(num)
|
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/// }
|
||
/// ```
|
||
///
|
||
/// [`Option<T>`]: Option
|
||
/// [`Result<T, E>`]: Result
|
||
/// [`String`]: ../../std/string/struct.String.html
|
||
/// [`from`]: From::from
|
||
/// [book]: ../../book/ch09-00-error-handling.html
|
||
#[rustc_diagnostic_item = "from_trait"]
|
||
#[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.
|
||
#[lang = "from"]
|
||
#[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.
|
||
#[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:
|
||
///
|
||
/// ```
|
||
/// use std::convert::TryFrom;
|
||
///
|
||
/// 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`]`>`:
|
||
///
|
||
/// ```
|
||
/// 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());
|
||
/// ```
|
||
///
|
||
/// [`i32::MAX`]: crate::i32::MAX
|
||
/// [`try_from`]: TryFrom::try_from
|
||
/// [`!`]: ../../std/primitive.never.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<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>,
|
||
{
|
||
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>,
|
||
{
|
||
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
|
||
}
|
||
}
|
||
|
||
/// **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;
|
||
|
||
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-fledged 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.
|
||
///
|
||
/// [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
|
||
}
|
||
}
|
||
|
||
#[stable(feature = "convert_infallible_hash", since = "1.44.0")]
|
||
impl Hash for Infallible {
|
||
fn hash<H: Hasher>(&self, _: &mut H) {
|
||
match *self {}
|
||
}
|
||
}
|