diff --git a/src/libcore/convert.rs b/src/libcore/convert.rs index 774d648558b48ebd80f30dc519b4f253c7e619c1..cee4fc6f49a713cdfb8ae38350154d5628e226eb 100644 --- a/src/libcore/convert.rs +++ b/src/libcore/convert.rs @@ -1,26 +1,25 @@ //! Traits for conversions between types. //! -//! The traits in this module provide a general way to talk about conversions -//! from one type to another. They follow the standard Rust conventions of -//! `as`/`into`/`from`. +//! The traits in this module provide a way to convert from one type to another type. +//! Each trait serves a different purpose: //! -//! Like many traits, these are often used as bounds for generic functions, to -//! support arguments of multiple types. +//! - 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. //! -//! - Implement the `As*` traits for reference-to-reference conversions -//! - Implement the [`Into`] trait when you want to consume the value in the conversion -//! - The [`From`] trait is the most flexible, useful for value _and_ reference conversions -//! - The [`TryFrom`] and [`TryInto`] traits behave like [`From`] and [`Into`], but allow for the -//! conversion to 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 prefer implementing [`From`][`From`] or +//! As a library author, you should always prefer implementing [`From`][`From`] or //! [`TryFrom`][`TryFrom`] rather than [`Into`][`Into`] or [`TryInto`][`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. However, there are some cases -//! where this is not possible, such as creating conversions into a type defined -//! outside your library, so implementing [`Into`] instead of [`From`] is -//! sometimes necessary. +//! blanket implementation in the standard library. Only implement [`Into`] or [`TryInto`] +//! when a conversion to a type outside the current crate is required. //! //! # Generic Implementations //! @@ -99,20 +98,14 @@ #[inline] pub const fn identity(x: T) -> T { x } -/// A cheap reference-to-reference conversion. Used to convert a value to a -/// reference value within generic code. +/// Used to do a cheap reference-to-reference conversion. /// -/// `AsRef` is very similar to, but serves a slightly different purpose than, -/// [`Borrow`]. +/// This trait is similar to [`AsMut`] which is used for converting between mutable references. +/// If you need to do a costly conversion it is better to implement [`From`] with type +/// `&T` or write a custom function. /// -/// `AsRef` is to be used when wishing to convert to a reference of another -/// type. -/// `Borrow` is more related to the notion of taking the reference. It is -/// useful when wishing to abstract over the type of reference -/// (`&T`, `&mut T`) or allow both the referenced and owned type to be treated -/// in the same manner. /// -/// The key difference between the two traits is the intention: +/// `AsRef` is very similar to, but serves a slightly different purpose than [`Borrow`]: /// /// - Use `AsRef` when the goal is to simply convert into a reference /// - Use `Borrow` when the goal is related to writing code that is agnostic to @@ -120,7 +113,7 @@ pub const fn identity(x: T) -> T { x } /// /// [`Borrow`]: ../../std/borrow/trait.Borrow.html /// -/// **Note: this trait must not fail**. If the conversion can fail, use a +/// **Note: This trait must not fail**. If the conversion can fail, use a /// dedicated method which returns an [`Option`] or a [`Result`]. /// /// [`Option`]: ../../std/option/enum.Option.html @@ -134,7 +127,12 @@ pub const fn identity(x: T) -> T { x } /// /// # Examples /// -/// Both [`String`] and `&str` implement `AsRef`: +/// By using trait bounds we can accept arguments of different types as long as they can be +/// converted a the specified type `T`. +/// +/// For example: By creating a generic function that takes an `AsRef` 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` we can accept both as input argument. /// /// [`String`]: ../../std/string/struct.String.html /// @@ -157,12 +155,13 @@ pub trait AsRef { fn as_ref(&self) -> &T; } -/// A cheap, mutable reference-to-mutable reference conversion. +/// Used to do a cheap mutable-to-mutable reference conversion. /// -/// This trait is similar to `AsRef` but used for converting between mutable -/// references. +/// 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 +/// **Note: This trait must not fail**. If the conversion can fail, use a /// dedicated method which returns an [`Option`] or a [`Result`]. /// /// [`Option`]: ../../std/option/enum.Option.html @@ -176,10 +175,11 @@ pub trait AsRef { /// /// # Examples /// -/// [`Box`] implements `AsMut`: -/// -/// [`Box`]: ../../std/boxed/struct.Box.html -/// +/// Using `AsMut` as trait bound for a generic function we can accept all mutable references +/// that can be converted to type `&mut T`. Because [`Box`] implements `AsMut` we can +/// write a function `add_one`that takes all arguments that can be converted to `&mut u64`. +/// Because [`Box`] implements `AsMut` `add_one` accepts arguments of type +/// `&mut Box` as well: /// ``` /// fn add_one>(num: &mut T) { /// *num.as_mut() += 1; @@ -189,7 +189,7 @@ pub trait AsRef { /// add_one(&mut boxed_num); /// assert_eq!(*boxed_num, 1); /// ``` -/// +/// [`Box`]: ../../std/boxed/struct.Box.html /// #[stable(feature = "rust1", since = "1.0.0")] pub trait AsMut { @@ -198,29 +198,27 @@ pub trait AsMut { fn as_mut(&mut self) -> &mut T; } -/// A conversion that consumes `self`, which may or may not be expensive. The -/// reciprocal of [`From`][From]. +/// A value-to-value conversion that consumes the input value. The +/// opposite of [`From`]. /// -/// **Note: this trait must not fail**. If the conversion can fail, use -/// [`TryInto`] or a dedicated method which returns an [`Option`] or a -/// [`Result`]. +/// One should only implement [`Into`] if a conversion to a type outside the current crate is +/// required. Otherwise one should always prefer implementing [`From`] over [`Into`] because +/// implementing [`From`] automatically provides one with a implementation of [`Into`] thanks to +/// the blanket implementation in the standard library. [`From`] cannot do these type of +/// conversions because of Rust's orphaning rules. /// -/// Library authors should not directly implement this trait, but should prefer -/// implementing the [`From`][From] trait, which offers greater flexibility and -/// provides an equivalent `Into` implementation for free, thanks to a blanket -/// implementation in the standard library. +/// **Note: This trait must not fail**. If the conversion can fail, use [`TryInto`]. /// /// # Generic Implementations /// -/// - [`From`][From]` for U` implies `Into for T` -/// - [`into`] is reflexive, which means that `Into for T` is implemented +/// - [`From`]` for U` implies `Into for T` +/// - [`Into`]` is reflexive, which means that `Into for T` is implemented /// -/// # Implementing `Into` +/// # Implementing `Into` for conversions to external types /// -/// There is one exception to implementing `Into`, and it's kind of esoteric. -/// If the destination type is not part of the current crate, and it uses a -/// generic variable, then you can't implement `From` directly. For example, -/// take this crate: +/// If the destination type is not part of the current crate +/// then you can't implement [`From`] directly. +/// For example, take this code: /// /// ```compile_fail /// struct Wrapper(Vec); @@ -230,8 +228,9 @@ pub trait AsMut { /// } /// } /// ``` -/// -/// To fix this, you can implement `Into` directly: +/// This will fail to compile because we cannot implement a trait for a type +/// if both the trait and the type are not defined by the current crate. +/// This is due to Rust's orphaning rules. To bypass this, you can implement `Into` directly: /// /// ``` /// struct Wrapper(Vec); @@ -242,17 +241,22 @@ pub trait AsMut { /// } /// ``` /// -/// This won't always allow the conversion: for example, `try!` and `?` -/// always use `From`. However, in most cases, people use `Into` to do the -/// conversions, and this will allow that. +/// 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. /// -/// In almost all cases, you should try to implement `From`, then fall back -/// to `Into` if `From` can't be implemented. +/// Prefer using `Into` over [`From`] when specifying trait bounds on a generic function +/// to ensure that types that only implement `Into` can be used as well. /// /// # Examples /// /// [`String`] implements `Into>`: /// +/// 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`. +/// For example: The function `is_hello` takes all arguments that can be converted into a +/// `Vec`. +/// /// ``` /// fn is_hello>>(s: T) { /// let bytes = b"hello".to_vec(); @@ -276,36 +280,38 @@ pub trait Into: Sized { fn into(self) -> T; } -/// Simple and safe type conversions in to `Self`. It is the reciprocal of -/// `Into`. +/// Used to do value-to-value conversions while consuming the input value. It is the reciprocal of +/// [`Into`]. /// -/// This trait is useful when performing error handling as described by -/// [the book][book] and is closely related to the `?` operator. +/// One should always prefer implementing [`From`] over [`Into`] +/// because implementing [`From`] automatically provides one with a implementation of [`Into`] +/// thanks to the blanket implementation in the standard library. /// -/// When constructing a function that is capable of failing the return type -/// will generally be of the form `Result`. +/// Only implement [`Into`] if a conversion to a type outside the current crate is required. +/// [`From`] cannot do these type of conversions because of Rust's orphaning rules. +/// See [`Into`] for more details. /// -/// The `From` trait allows for simplification of error handling by providing a -/// means of returning a single error type that encapsulates numerous possible -/// erroneous situations. +/// 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. /// -/// This trait is not limited to error handling, rather the general case for -/// this trait would be in any type conversions to have an explicit definition -/// of how they are performed. +/// 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`. +/// The `From` trait simplifies error handling by allowing a function to return a single error type +/// that encapsulate multiple error types. See the "Examples" section and [the book][book] for more +/// details. /// -/// **Note: this trait must not fail**. If the conversion can fail, use -/// [`TryFrom`] or a dedicated method which returns an [`Option`] or a -/// [`Result`]. +/// **Note: This trait must not fail**. If the conversion can fail, use [`TryFrom`]. /// /// # Generic Implementations /// -/// - `From for U` implies [`Into`]` for T` -/// - [`from`] is reflexive, which means that `From for T` is implemented +/// - [`From`]` for U` implies [`Into`]` for T` +/// - [`From`] is reflexive, which means that `From for T` is implemented /// /// # Examples /// /// [`String`] implements `From<&str>`: /// +/// An explicit conversion from a &str to a String is done as follows: /// ``` /// let string = "hello".to_string(); /// let other_string = String::from("hello"); @@ -313,7 +319,12 @@ pub trait Into: Sized { /// assert_eq!(string, other_string); /// ``` /// -/// An example usage for error handling: +/// While performing error handling it is often useful to implement `From` for your own error type. +/// By converting underlying error types to our own custom error type that encapsulates the +/// underlying error type, we can return a single error type without losing information on the +/// underlying cause. The '?' operator automatically converts the underlying error type to our +/// custom error type by calling `Into::into` which is automatically provided when +/// implementing `From`. The compiler then infers which implementation of `Into` should be used. /// /// ``` /// use std::fs;