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Rollup merge of #59330 - DevQps:improve-std-convert-documentation, r=steveklabnik

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Improve the documentation for std::convert (From, Into, AsRef and AsMut)

# Description
In this PR I updated the documentation of From, Into, AsRef and AsMut, as well as the general std::convert module documentation. The discussion in #59163 provided information that was not yet present in the docs, or was not expressed clearly enough. I tried to clarify the examples that were already present in the docs as well as add more information about considered best-practices that came out of the discussion in #59163

@steveklabnik I hope I didn't change too much. This is an initial version! I will scan through everything tomorrow as well again to see if I made any typo's or errors, and maybe make some small changes here and there.

All suggestions are welcome!

closes #59163
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Guillaume Gomez 2019-03-26 22:26:42 +01:00 committed by GitHub
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167
src/libcore/convert.rs
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@ -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<T>`][`From`] or
//! 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. 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 @@ use fmt;
#[inline]
pub const fn identity<T>(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<T>(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<T>`] or a [`Result<T, E>`].
///
/// [`Option<T>`]: ../../std/option/enum.Option.html
@ -134,7 +127,12 @@ pub const fn identity<T>(x: T) -> T { x }
///
/// # Examples
///
/// Both [`String`] and `&str` implement `AsRef<str>`:
/// 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<str>` we express that we
/// want to accept all references that can be converted to &str as an argument.
/// Since both [`String`] and `&str` implement `AsRef<str>` we can accept both as input argument.
///
/// [`String`]: ../../std/string/struct.String.html
///
@ -157,12 +155,13 @@ pub trait AsRef<T: ?Sized> {
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<T>`] or a [`Result<T, E>`].
///
/// [`Option<T>`]: ../../std/option/enum.Option.html
@ -176,10 +175,11 @@ pub trait AsRef<T: ?Sized> {
///
/// # Examples
///
/// [`Box<T>`] implements `AsMut<T>`:
///
/// [`Box<T>`]: ../../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<T>`] implements `AsMut<T>` we can
/// write a function `add_one`that takes all arguments that can be converted to `&mut u64`.
/// Because [`Box<T>`] implements `AsMut<T>` `add_one` accepts arguments of type
/// `&mut Box<u64>` as well:
/// ```
/// fn add_one<T: AsMut<u64>>(num: &mut T) {
/// *num.as_mut() += 1;
@ -189,7 +189,7 @@ pub trait AsRef<T: ?Sized> {
/// add_one(&mut boxed_num);
/// assert_eq!(*boxed_num, 1);
/// ```
///
/// [`Box<T>`]: ../../std/boxed/struct.Box.html
///
#[stable(feature = "rust1", since = "1.0.0")]
pub trait AsMut<T: ?Sized> {
@ -198,29 +198,27 @@ pub trait AsMut<T: ?Sized> {
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<T>`] or a
/// [`Result<T, E>`].
/// 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<T>`][From]` for U` implies `Into<U> for T`
/// - [`into`] is reflexive, which means that `Into<T> for T` is implemented
/// - [`From<T>`]` for U` implies `Into<U> for T`
/// - [`Into`]` is reflexive, which means that `Into<T> 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<T>(Vec<T>);
@ -230,8 +228,9 @@ pub trait AsMut<T: ?Sized> {
/// }
/// }
/// ```
///
/// 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<T>(Vec<T>);
@ -242,17 +241,22 @@ pub trait AsMut<T: ?Sized> {
/// }
/// ```
///
/// 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<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();
@ -276,36 +280,38 @@ pub trait Into<T>: 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<T, E>`.
/// 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<T, E>`.
/// The `From` trait simplifies error handling by allowing a function to return a single error type
/// that encapsulate multiple error types. See the "Examples" section and [the book][book] for more
/// details.
///
/// **Note: this trait must not fail**. If the conversion can fail, use
/// [`TryFrom`] or a dedicated method which returns an [`Option<T>`] or a
/// [`Result<T, E>`].
/// **Note: This trait must not fail**. If the conversion can fail, use [`TryFrom`].
///
/// # Generic Implementations
///
/// - `From<T> for U` implies [`Into<U>`]` for T`
/// - [`from`] is reflexive, which means that `From<T> for T` is implemented
/// - [`From<T>`]` for U` implies [`Into<U>`]` for T`
/// - [`From`] is reflexive, which means that `From<T> for T` is implemented
///
/// # Examples
///
/// [`String`] implements `From<&str>`:
///
/// An explicit conversion from a &str to a String is done as follows:
/// ```
/// let string = "hello".to_string();
/// let other_string = String::from("hello");
@ -313,7 +319,12 @@ pub trait Into<T>: 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<CliError>::into` which is automatically provided when
/// implementing `From`. The compiler then infers which implementation of `Into` should be used.
///
/// ```
/// use std::fs;