317 lines
9.9 KiB
Rust
317 lines
9.9 KiB
Rust
// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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//! Traits for conversions between types.
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//!
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//! The traits in this module provide a general way to talk about conversions
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//! from one type to another. They follow the standard Rust conventions of
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//! `as`/`into`/`from`.
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//!
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//! Like many traits, these are often used as bounds for generic functions, to
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//! support arguments of multiple types.
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//!
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//! - Impl the `As*` traits for reference-to-reference conversions
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//! - Impl the `Into` trait when you want to consume the value in the conversion
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//! - The `From` trait is the most flexible, useful for value _and_ reference conversions
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//! - The `TryFrom` and `TryInto` traits behave like `From` and `Into`, but allow for the
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//! conversion to fail
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//!
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//! As a library author, you should prefer implementing `From<T>` or `TryFrom<T>` rather than
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//! `Into<U>` or `TryInto<U>`, as `From` and `TryFrom` provide greater flexibility and offer
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//! equivalent `Into` or `TryInto` implementations for free, thanks to a blanket implementation
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//! in the standard library.
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//!
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//! # Generic impl
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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 `into()`
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//! 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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/// A cheap, reference-to-reference conversion.
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///
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/// `AsRef` is very similar to, but different than, [`Borrow`]. See
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/// [the book][book] for more.
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///
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/// [book]: ../../book/borrow-and-asref.html
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/// [`Borrow`]: ../../std/borrow/trait.Borrow.html
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///
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/// **Note: this trait must not fail**. If the conversion can fail, use a dedicated method which
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/// returns an [`Option<T>`] or a [`Result<T, E>`].
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///
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/// [`Option<T>`]: ../../std/option/enum.Option.html
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/// [`Result<T, E>`]: ../../std/result/enum.Result.html
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///
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/// # Examples
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///
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/// Both [`String`] and `&str` implement `AsRef<str>`:
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///
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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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///
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/// # Generic Impls
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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 `&mut Foo` or `&&mut Foo`)
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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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/// A cheap, mutable reference-to-mutable reference conversion.
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///
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/// **Note: this trait must not fail**. If the conversion can fail, use a dedicated method which
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/// returns an [`Option<T>`] or a [`Result<T, E>`].
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///
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/// [`Option<T>`]: ../../std/option/enum.Option.html
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/// [`Result<T, E>`]: ../../std/result/enum.Result.html
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///
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/// # Examples
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///
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/// [`Box<T>`] implements `AsMut<T>`:
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///
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/// [`Box<T>`]: ../../std/boxed/struct.Box.html
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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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/// # Generic Impls
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///
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/// - `AsMut` 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 `&mut Foo` or `&&mut Foo`)
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///
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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 conversion that consumes `self`, which may or may not be expensive.
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///
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/// **Note: this trait must not fail**. If the conversion can fail, use [`TryInto`] or a dedicated
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/// method which returns an [`Option<T>`] or a [`Result<T, E>`].
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///
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/// Library authors should not directly implement this trait, but should prefer implementing
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/// the [`From`][From] trait, which offers greater flexibility and provides an equivalent `Into`
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/// implementation for free, thanks to a blanket implementation in the standard library.
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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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/// ```
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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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/// # Generic Impls
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///
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/// - `[From<T>][From] 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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/// [`TryInto`]: trait.TryInto.html
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/// [`Option<T>`]: ../../std/option/enum.Option.html
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/// [`Result<T, E>`]: ../../std/result/enum.Result.html
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/// [`String`]: ../../std/string/struct.String.html
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/// [From]: trait.From.html
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/// [`into()`]: trait.Into.html#tymethod.into
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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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/// Construct `Self` via a conversion.
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///
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/// **Note: this trait must not fail**. If the conversion can fail, use [`TryFrom`] or a dedicated
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/// method which returns an [`Option<T>`] or a [`Result<T, E>`].
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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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/// ```
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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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/// # Generic impls
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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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/// [`TryFrom`]: trait.TryFrom.html
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/// [`Option<T>`]: ../../std/option/enum.Option.html
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/// [`Result<T, E>`]: ../../std/result/enum.Result.html
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/// [`String`]: ../../std/string/struct.String.html
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/// [Into<U>]: trait.Into.html
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/// [`from()`]: trait.From.html#tymethod.from
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#[stable(feature = "rust1", since = "1.0.0")]
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pub trait From<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 from(T) -> Self;
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}
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/// An attempted conversion that consumes `self`, which may or may not be expensive.
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///
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/// Library authors should not directly implement this trait, but should prefer implementing
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/// the [`TryFrom`] trait, which offers greater flexibility and provides an equivalent `TryInto`
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/// implementation for free, thanks to a blanket implementation in the standard library.
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///
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/// [`TryFrom`]: trait.TryFrom.html
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#[unstable(feature = "try_from", issue = "33417")]
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pub trait TryInto<T>: Sized {
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/// The type returned in the event of a conversion error.
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type Err;
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/// Performs the conversion.
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fn try_into(self) -> Result<T, Self::Err>;
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}
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/// Attempt to construct `Self` via a conversion.
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#[unstable(feature = "try_from", issue = "33417")]
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pub trait TryFrom<T>: Sized {
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/// The type returned in the event of a conversion error.
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type Err;
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/// Performs the conversion.
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fn try_from(T) -> Result<Self, Self::Err>;
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}
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////////////////////////////////////////////////////////////////////////////////
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// GENERIC IMPLS
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////////////////////////////////////////////////////////////////////////////////
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// As lifts over &
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#[stable(feature = "rust1", since = "1.0.0")]
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impl<'a, T: ?Sized, U: ?Sized> AsRef<U> for &'a T where T: AsRef<U> {
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fn as_ref(&self) -> &U {
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<T as AsRef<U>>::as_ref(*self)
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}
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}
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// As lifts over &mut
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#[stable(feature = "rust1", since = "1.0.0")]
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impl<'a, T: ?Sized, U: ?Sized> AsRef<U> for &'a mut T where T: AsRef<U> {
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fn as_ref(&self) -> &U {
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<T as AsRef<U>>::as_ref(*self)
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}
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}
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// FIXME (#23442): replace the above impls for &/&mut with the following more general one:
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// // As lifts over Deref
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// impl<D: ?Sized + Deref, U: ?Sized> AsRef<U> for D where D::Target: AsRef<U> {
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// fn as_ref(&self) -> &U {
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// self.deref().as_ref()
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// }
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// }
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// AsMut lifts over &mut
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#[stable(feature = "rust1", since = "1.0.0")]
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impl<'a, T: ?Sized, U: ?Sized> AsMut<U> for &'a mut T where T: AsMut<U> {
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fn as_mut(&mut self) -> &mut U {
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(*self).as_mut()
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}
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}
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// FIXME (#23442): replace the above impl for &mut with the following more general one:
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// // AsMut lifts over DerefMut
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// impl<D: ?Sized + Deref, U: ?Sized> AsMut<U> for D where D::Target: AsMut<U> {
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// fn as_mut(&mut self) -> &mut U {
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// self.deref_mut().as_mut()
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// }
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// }
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// From implies Into
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#[stable(feature = "rust1", since = "1.0.0")]
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impl<T, U> Into<U> for T where U: From<T> {
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fn into(self) -> U {
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U::from(self)
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}
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}
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// From (and thus Into) is reflexive
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#[stable(feature = "rust1", since = "1.0.0")]
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impl<T> From<T> for T {
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fn from(t: T) -> T { t }
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}
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// TryFrom implies TryInto
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#[unstable(feature = "try_from", issue = "33417")]
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impl<T, U> TryInto<U> for T where U: TryFrom<T> {
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type Err = U::Err;
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fn try_into(self) -> Result<U, U::Err> {
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U::try_from(self)
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}
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}
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////////////////////////////////////////////////////////////////////////////////
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// CONCRETE IMPLS
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////////////////////////////////////////////////////////////////////////////////
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#[stable(feature = "rust1", since = "1.0.0")]
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impl<T> AsRef<[T]> for [T] {
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fn as_ref(&self) -> &[T] {
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self
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}
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}
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#[stable(feature = "rust1", since = "1.0.0")]
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impl<T> AsMut<[T]> for [T] {
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fn as_mut(&mut self) -> &mut [T] {
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self
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}
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}
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#[stable(feature = "rust1", since = "1.0.0")]
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impl AsRef<str> for str {
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#[inline]
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fn as_ref(&self) -> &str {
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self
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}
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}
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