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docs: Improve AsRef / AsMut docs on blanket impls

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- Explicitly mention that `AsRef` and `AsMut` do not auto-dereference
  generally for all dereferencable types (but only if inner type is a
  shared and/or mutable reference)
- Give advice to not use `AsRef` or `AsMut` for the sole purpose of
  dereferencing
- Suggest providing a transitive `AsRef` or `AsMut` implementation for
  types which implement `Deref`
- Add new section "Reflexivity" in documentation comments for `AsRef`
  and `AsMut`
- Provide better example for `AsMut`
- Added heading "Relation to `Borrow`" in `AsRef`'s docs to improve
  structure

Issue #45742 and a corresponding FIXME in the libcore suggest that
`AsRef` and `AsMut` should provide a blanket implementation over
`Deref`. As that is difficult to realize at the moment, this commit
updates the documentation to better describe the status-quo and to give
advice on how to use `AsRef` and `AsMut`.
This commit is contained in:
Jan Behrens 2022-07-19 11:40:40 +02:00
parent c3f35504d1
commit 551d921de0
1 changed files with 159 additions and 18 deletions
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177
library/core/src/convert/mod.rs
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@ -25,6 +25,7 @@
//! # Generic Implementations
//!
//! - [`AsRef`] and [`AsMut`] auto-dereference if the inner type is a reference
//! (but not generally for all [dereferenceable types][core::ops::Deref])
//! - [`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
@ -108,10 +109,12 @@ pub const fn identity<T>(x: T) -> T {
/// If you need to do a costly conversion it is better to implement [`From`] with type
/// `&T` or write a custom function.
///
/// # Relation to `Borrow`
///
/// `AsRef` has the same signature as [`Borrow`], but [`Borrow`] is different in a few aspects:
///
/// - Unlike `AsRef`, [`Borrow`] has a blanket impl for any `T`, and can be used to accept either
/// a reference or a value.
/// a reference or a value. (See also note on `AsRef`'s reflexibility below.)
/// - [`Borrow`] also requires that [`Hash`], [`Eq`] and [`Ord`] for a borrowed value are
/// equivalent to those of the owned value. For this reason, if you want to
/// borrow only a single field of a struct you can implement `AsRef`, but not [`Borrow`].
@ -121,9 +124,55 @@ pub const fn identity<T>(x: T) -> T {
///
/// # 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`)
/// `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`).
///
/// Note that due to historic reasons, the above currently does not hold generally for all
/// [dereferenceable types], e.g. `foo.as_ref()` will *not* work the same as
/// `Box::new(foo).as_ref()`. Instead, many smart pointers provide an `as_ref` implementation which
/// simply returns a reference to the [pointed-to value] (but do not perform a cheap
/// reference-to-reference conversion for that value). However, [`AsRef::as_ref`] should not be
/// used for the sole purpose of dereferencing; instead ['`Deref` coercion'] can be used:
///
/// [dereferenceable types]: core::ops::Deref
/// [pointed-to value]: core::ops::Deref::Target
/// ['`Deref` coercion']: core::ops::Deref#more-on-deref-coercion
///
/// ```
/// let x = Box::new(5i32);
/// // Avoid this:
/// // let y: &i32 = x.as_ref();
/// // Better just write:
/// let y: &i32 = &x;
/// ```
///
/// Types which implement [`Deref`][core::ops::Deref] should consider implementing `AsRef` as
/// follows:
///
/// ```
/// impl<T> AsRef<T> for SomeType
/// where
/// T: ?Sized,
/// <SomeType as Deref>::Target: AsRef<T>,
/// {
/// fn as_ref(&self) -> &T {
/// self.deref().as_ref()
/// }
/// }
/// ```
///
/// # Reflexivity
///
/// Ideally, `AsRef` would be reflexive, that is there is an `impl<T: ?Sized> AsRef<T> for T`, with
/// [`as_ref`][AsRef::as_ref] simply returning its argument unchanged.
/// Such a blanket implementation is currently *not* provided due to technical restrictions of
/// Rust's type system (it would be overlapping with another existing blanket implementation for
/// `&T where T: AsRef<U>` which allows `AsRef` to auto-dereference, see "Generic Implementations"
/// above).
///
/// A trivial implementation of `AsRef<T> for T` must be added explicitly for a particular type `T`
/// where needed or desired. Note, however, that not all types from `std` contain such an
/// implementation, and those cannot be added by external code due to orphan rules.
///
/// # Examples
///
@ -170,29 +219,121 @@ pub trait AsRef<T: ?Sized> {
///
/// # Generic Implementations
///
/// - `AsMut` auto-dereferences if the inner type is a mutable reference
/// (e.g.: `foo.as_mut()` will work the same if `foo` has type `&mut Foo`
/// or `&mut &mut Foo`)
/// `AsMut` auto-dereferences if the inner type is a mutable reference
/// (e.g.: `foo.as_mut()` will work the same if `foo` has type `&mut Foo` or `&mut &mut Foo`).
///
/// Note that due to historic reasons, the above currently does not hold generally for all
/// [mutably dereferenceable types], e.g. `foo.as_mut()` will *not* work the same as
/// `Box::new(foo).as_mut()`. Instead, many smart pointers provide an `as_mut` implementation which
/// simply returns a reference to the [pointed-to value] (but do not perform a cheap
/// reference-to-reference conversion for that value). However, [`AsMut::as_mut`] should not be
/// used for the sole purpose of mutable dereferencing; instead ['`Deref` coercion'] can be used:
///
/// [mutably dereferenceable types]: core::ops::DerefMut
/// [pointed-to value]: core::ops::Deref::Target
/// ['`Deref` coercion']: core::ops::DerefMut#more-on-deref-coercion
///
/// ```
/// let mut x = Box::new(5i32);
/// // Avoid this:
/// // let y: &mut i32 = x.as_mut();
/// // Better just write:
/// let y: &mut i32 = &mut x;
/// ```
///
/// Types which implement [`DerefMut`](core::ops::DerefMut) should consider to add an
/// implementation of `AsMut` as follows:
///
/// ```
/// impl<T> AsMut<T> for SomeType
/// where
/// <SomeType as Deref>::Target: AsMut<T>,
/// {
/// fn as_mut(&mut self) -> &mut T {
/// self.deref_mut().as_mut()
/// }
/// }
/// ```
///
/// # Reflexivity
///
/// Ideally, `AsMut` would be reflexive, that is there is an `impl<T: ?Sized> AsMut<T> for T`, with
/// [`as_mut`][AsMut::as_mut] simply returning its argument unchanged.
/// Such a blanket implementation is currently *not* provided due to technical restrictions of
/// Rust's type system (it would be overlapping with another existing blanket implementation for
/// `&mut T where T: AsMut<U>` which allows `AsMut` to auto-dereference, see "Generic
/// Implementations" above).
///
/// A trivial implementation of `AsMut<T> for T` must be added explicitly for a particular type `T`
/// where needed or desired. Note, however, that not all types from `std` contain such an
/// implementation, and those cannot be added by external code due to orphan rules.
///
/// # Examples
///
/// 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:
/// Using `AsMut` as trait bound for a generic function, we can accept all mutable references that
/// can be converted to type `&mut T`. Unlike [dereference], which has a single [target type],
/// there can be multiple implementations of `AsMut` for a type. In particular, `Vec<T>` implements
/// both `AsMut<Vec<T>>` and `AsMut<[T]>`.
///
/// In the following, the example functions `caesar` and `null_terminate` provide a generic
/// interface which work with any type that can be converted by cheap mutable-to-mutable conversion
/// into a byte slice or byte `Vec`, respectively.
///
/// [dereference]: core::ops::DerefMut
/// [target type]: core::ops::Deref::Target
///
/// ```
/// fn add_one<T: AsMut<u64>>(num: &mut T) {
/// *num.as_mut() += 1;
/// struct Document {
/// info: String,
/// content: Vec<u8>,
/// }
///
/// let mut boxed_num = Box::new(0);
/// add_one(&mut boxed_num);
/// assert_eq!(*boxed_num, 1);
/// impl<T: ?Sized> AsMut<T> for Document
/// where
/// Vec<u8>: AsMut<T>,
/// {
/// fn as_mut(&mut self) -> &mut T {
/// self.content.as_mut()
/// }
/// }
///
/// fn caesar<T: AsMut<[u8]>>(data: &mut T, key: u8) {
/// for byte in data.as_mut() {
/// *byte = byte.wrapping_add(key);
/// }
/// }
///
/// fn null_terminate<T: AsMut<Vec<u8>>>(data: &mut T) {
/// // Using a non-generic inner function, which contains most of the
/// // functionality, helps to minimize monomorphization overhead.
/// fn doit(data: &mut Vec<u8>) {
/// let len = data.len();
/// if len == 0 || data[len-1] != 0 {
/// data.push(0);
/// }
/// }
/// doit(data.as_mut());
/// }
///
/// fn main() {
/// let mut v: Vec<u8> = vec![1, 2, 3];
/// caesar(&mut v, 5);
/// assert_eq!(v, [6, 7, 8]);
/// null_terminate(&mut v);
/// assert_eq!(v, [6, 7, 8, 0]);
/// let mut doc = Document {
/// info: String::from("Example"),
/// content: vec![17, 19, 8],
/// };
/// caesar(&mut doc, 1);
/// assert_eq!(doc.content, [18, 20, 9]);
/// null_terminate(&mut doc);
/// assert_eq!(doc.content, [18, 20, 9, 0]);
/// }
/// ```
///
/// [`Box<T>`]: ../../std/boxed/struct.Box.html
/// Note, however, that APIs don't need to be generic. In many cases taking a `&mut [u8]` or
/// `&mut Vec<u8>`, for example, is the better choice (callers need to pass the correct type then).
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(not(test), rustc_diagnostic_item = "AsMut")]
pub trait AsMut<T: ?Sized> {