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Rollup merge of #40987 - maccoda:convert-docs, r=steveklabnik
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API docs: Convert

Clean up of the convert module documentation following points in #29349
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@frewsxcv
frewsxcv authored Apr 20, 2017
2 parents 83e22b8 + 2877a01 commit 9d91822
Showing 1 changed file with 133 additions and 50 deletions.
183 changes: 133 additions & 50 deletions src/libcore/convert.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -17,25 +17,25 @@
//! Like many traits, these are often used as bounds for generic functions, to
//! support arguments of multiple types.
//!
//! - Impl the `As*` traits for reference-to-reference conversions
//! - Impl the [`Into`] trait when you want to consume the value in the conversion
//! - 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
//!
//! As a library author, you should 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.
//! equivalent [`Into`] or [`TryInto`] implementations for free, thanks to a
//! blanket implementation in the standard library.
//!
//! # Generic impl
//! # Generic Implementations
//!
//! - [`AsRef`] and [`AsMut`] auto-dereference if the inner type is a reference
//! - [`From`]`<U> for T` implies [`Into`]`<T> for U`
//! - [`TryFrom`]`<U> for T` implies [`TryInto`]`<T> for U`
//! - [`From`] and [`Into`] are reflexive, which means that all types can `into()`
//! themselves and `from()` themselves
//! - [`From`] and [`Into`] are reflexive, which means that all types can
//! `into` themselves and `from` themselves
//!
//! See each trait for usage examples.
//!
Expand All @@ -50,20 +50,42 @@

use str::FromStr;

/// A cheap, reference-to-reference conversion.
/// A cheap reference-to-reference conversion. Used to convert a value to a
/// reference value within generic code.
///
/// `AsRef` is very similar to, but different than, [`Borrow`]. See
/// [the book][book] for more.
/// `AsRef` is very similar to, but serves a slightly different purpose than,
/// [`Borrow`].
///
/// `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:
///
/// - Use `AsRef` when goal is to simply convert into a reference
/// - Use `Borrow` when goal is related to writing code that is agnostic to the
/// type of borrow and if is reference or value
///
/// See [the book][book] for a more detailed comparison.
///
/// [book]: ../../book/borrow-and-asref.html
/// [`Borrow`]: ../../std/borrow/trait.Borrow.html
///
/// **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>`].
/// **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
/// [`Result<T, E>`]: ../../std/result/enum.Result.html
///
/// # Generic Implementations
///
/// - `AsRef` auto-dereferences if the inner type is a reference or a mutable
/// reference (e.g.: `foo.as_ref()` will work the same if `foo` has type
/// `&mut Foo` or `&&mut Foo`)
///
/// # Examples
///
/// Both [`String`] and `&str` implement `AsRef<str>`:
Expand All @@ -82,11 +104,6 @@ use str::FromStr;
/// is_hello(s);
/// ```
///
/// # Generic Impls
///
/// - `AsRef` auto-dereferences if the inner type is a reference or a mutable
/// reference (e.g.: `foo.as_ref()` will work the same if `foo` has type `&mut Foo` or `&&mut Foo`)
///
#[stable(feature = "rust1", since = "1.0.0")]
pub trait AsRef<T: ?Sized> {
/// Performs the conversion.
Expand All @@ -96,12 +113,21 @@ pub trait AsRef<T: ?Sized> {

/// A cheap, mutable reference-to-mutable reference conversion.
///
/// **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>`].
/// This trait is similar to `AsRef` but used for converting between mutable
/// references.
///
/// **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
/// [`Result<T, E>`]: ../../std/result/enum.Result.html
///
/// # Generic Implementations
///
/// - `AsMut` auto-dereferences if the inner type is a reference or a mutable
/// reference (e.g.: `foo.as_ref()` will work the same if `foo` has type
/// `&mut Foo` or `&&mut Foo`)
///
/// # Examples
///
/// [`Box<T>`] implements `AsMut<T>`:
Expand All @@ -118,10 +144,6 @@ pub trait AsRef<T: ?Sized> {
/// assert_eq!(*boxed_num, 1);
/// ```
///
/// # Generic Impls
///
/// - `AsMut` auto-dereferences if the inner type is a reference or a mutable
/// reference (e.g.: `foo.as_ref()` will work the same if `foo` has type `&mut Foo` or `&&mut Foo`)
///
#[stable(feature = "rust1", since = "1.0.0")]
pub trait AsMut<T: ?Sized> {
Expand All @@ -130,14 +152,22 @@ pub trait AsMut<T: ?Sized> {
fn as_mut(&mut self) -> &mut T;
}

/// A conversion that consumes `self`, which may or may not be expensive.
/// A conversion that consumes `self`, which may or may not be expensive. The
/// reciprocal of [`From`][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>`].
///
/// **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>`].
/// 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.
///
/// 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.
/// # Generic Implementations
///
/// - [`From<T>`][From]` for U` implies `Into<U> for T`
/// - [`into`] is reflexive, which means that `Into<T> for T` is implemented
///
/// # Examples
///
Expand All @@ -153,11 +183,6 @@ pub trait AsMut<T: ?Sized> {
/// is_hello(s);
/// ```
///
/// # Generic Impls
///
/// - [`From<T>`][From]` for U` implies `Into<U> for T`
/// - [`into`] is reflexive, which means that `Into<T> for T` is implemented
///
/// [`TryInto`]: trait.TryInto.html
/// [`Option<T>`]: ../../std/option/enum.Option.html
/// [`Result<T, E>`]: ../../std/result/enum.Result.html
Expand All @@ -171,10 +196,31 @@ pub trait Into<T>: Sized {
fn into(self) -> T;
}

/// Construct `Self` via a conversion.
/// Simple and safe type conversions in to `Self`. 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.
///
/// When constructing a function that is capable of failing the return type
/// will generally be of the form `Result<T, E>`.
///
/// 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.
///
/// 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.
///
/// **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`] or a dedicated
/// method which returns an [`Option<T>`] or a [`Result<T, E>`].
/// # Generic Implementations
///
/// - `From<T> for U` implies [`Into<U>`]` for T`
/// - [`from`] is reflexive, which means that `From<T> for T` is implemented
///
/// # Examples
///
Expand All @@ -186,29 +232,60 @@ pub trait Into<T>: Sized {
///
/// assert_eq!(string, other_string);
/// ```
/// # Generic impls
///
/// - `From<T> for U` implies [`Into<U>`]` for T`
/// - [`from`] is reflexive, which means that `From<T> for T` is implemented
/// An example usage for error handling:
///
/// ```
/// use std::io::{self, Read};
/// use std::num;
///
/// enum CliError {
/// IoError(io::Error),
/// ParseError(num::ParseIntError),
/// }
///
/// impl From<io::Error> for CliError {
/// fn from(error: io::Error) -> Self {
/// CliError::IoError(error)
/// }
/// }
///
/// impl From<num::ParseIntError> for CliError {
/// fn from(error: num::ParseIntError) -> Self {
/// CliError::ParseError(error)
/// }
/// }
///
/// fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> {
/// let mut file = std::fs::File::open("test")?;
/// let mut contents = String::new();
/// file.read_to_string(&mut contents)?;
/// let num: i32 = contents.trim().parse()?;
/// Ok(num)
/// }
/// ```
///
/// [`TryFrom`]: trait.TryFrom.html
/// [`Option<T>`]: ../../std/option/enum.Option.html
/// [`Result<T, E>`]: ../../std/result/enum.Result.html
/// [`String`]: ../../std/string/struct.String.html
/// [`Into<U>`]: trait.Into.html
/// [`from`]: trait.From.html#tymethod.from
/// [book]: ../../book/error-handling.html
#[stable(feature = "rust1", since = "1.0.0")]
pub trait From<T>: Sized {
/// Performs the conversion.
#[stable(feature = "rust1", since = "1.0.0")]
fn from(T) -> Self;
}

/// An attempted conversion that consumes `self`, which may or may not be expensive.
/// An attempted conversion that consumes `self`, which may or may not be
/// expensive.
///
/// Library authors should 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.
/// Library authors should 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.
///
/// [`TryFrom`]: trait.TryFrom.html
#[unstable(feature = "try_from", issue = "33417")]
Expand Down Expand Up @@ -236,15 +313,17 @@ pub trait TryFrom<T>: Sized {

// As lifts over &
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T: ?Sized, U: ?Sized> AsRef<U> for &'a T where T: AsRef<U> {
impl<'a, T: ?Sized, U: ?Sized> AsRef<U> for &'a 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<'a, T: ?Sized, U: ?Sized> AsRef<U> for &'a mut T where T: AsRef<U> {
impl<'a, T: ?Sized, U: ?Sized> AsRef<U> for &'a mut T where T: AsRef<U>
{
fn as_ref(&self) -> &U {
<T as AsRef<U>>::as_ref(*self)
}
Expand All @@ -260,7 +339,8 @@ impl<'a, T: ?Sized, U: ?Sized> AsRef<U> for &'a mut T where T: AsRef<U> {

// AsMut lifts over &mut
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T: ?Sized, U: ?Sized> AsMut<U> for &'a mut T where T: AsMut<U> {
impl<'a, T: ?Sized, U: ?Sized> AsMut<U> for &'a mut T where T: AsMut<U>
{
fn as_mut(&mut self) -> &mut U {
(*self).as_mut()
}
Expand All @@ -276,7 +356,8 @@ impl<'a, T: ?Sized, U: ?Sized> AsMut<U> for &'a mut T where T: AsMut<U> {

// From implies Into
#[stable(feature = "rust1", since = "1.0.0")]
impl<T, U> Into<U> for T where U: From<T> {
impl<T, U> Into<U> for T where U: From<T>
{
fn into(self) -> U {
U::from(self)
}
Expand All @@ -291,7 +372,8 @@ impl<T> From<T> for T {

// TryFrom implies TryInto
#[unstable(feature = "try_from", issue = "33417")]
impl<T, U> TryInto<U> for T where U: TryFrom<T> {
impl<T, U> TryInto<U> for T where U: TryFrom<T>
{
type Error = U::Error;

fn try_into(self) -> Result<U, U::Error> {
Expand Down Expand Up @@ -327,7 +409,8 @@ impl AsRef<str> for str {

// FromStr implies TryFrom<&str>
#[unstable(feature = "try_from", issue = "33417")]
impl<'a, T> TryFrom<&'a str> for T where T: FromStr {
impl<'a, T> TryFrom<&'a str> for T where T: FromStr
{
type Error = <T as FromStr>::Err;

fn try_from(s: &'a str) -> Result<T, Self::Error> {
Expand Down

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