docs: Improve AsRef / AsMut docs on blanket impls

- 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 rust-lang#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`.

library/core/src/convert/mod.rs

@@ -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> {