Rollup merge of #46518 - partim:asref-borrow-doc, r=dtolnay

Improve documentation for Borrow

This is the first step in improving the documentation for all the reference conversion traits. It proposes new text for the trait documentation of `Borrow`. Since I feel it is a somewhat radical rewrite and includes a stricter contract for `Borrow` then the previous text—namely that *all* shared traits need to behave the same, not just a select few—, I wanted to get some feedback before continuing.

Apart from the ‘normative’ description, the new text also includes a fairly extensive explanation of how the trait is used in the examples section. I included it because every time I look at how `HashMap` uses the trait, I need to think for a while as the use is a bit twisted. So, I thought having this thinking written down as part of the trait itself might be useful. One could argue that this should go into The Book, and, while I really like having everything important in the docs, I can see the text moved there, too.

So, before I move on: is this new text any good? Do we feel it is correct, useful, comprehensive, and understandable?

(This PR is in response to #44868 and #24140.)

src/libcore/borrow.rs

@@ -14,24 +14,154 @@
 
 /// A trait for borrowing data.
 ///
-/// In general, there may be several ways to "borrow" a piece of data.  The
-/// typical ways of borrowing a type `T` are `&T` (a shared borrow) and `&mut T`
-/// (a mutable borrow). But types like `Vec<T>` provide additional kinds of
-/// borrows: the borrowed slices `&[T]` and `&mut [T]`.
+/// In Rust, it is common to provide different representations of a type for
+/// different use cases. For instance, storage location and management for a
+/// value can be specifically chosen as appropriate for a particular use via
+/// pointer types such as [`Box<T>`] or [`Rc<T>`]. Beyond these generic
+/// wrappers that can be used with any type, some types provide optional
+/// facets providing potentially costly functionality. An example for such a
+/// type is [`String`] which adds the ability to extend a string to the basic
+/// [`str`]. This requires keeping additional information unnecessary for a
+/// simple, immutable string.
 ///
-/// When writing generic code, it is often desirable to abstract over all ways
-/// of borrowing data from a given type. That is the role of the `Borrow`
-/// trait: if `T: Borrow<U>`, then `&U` can be borrowed from `&T`.  A given
-/// type can be borrowed as multiple different types. In particular, `Vec<T>:
-/// Borrow<Vec<T>>` and `Vec<T>: Borrow<[T]>`.
+/// These types provide access to the underlying data through references
+/// to the type of that data. They are said to be ‘borrowed as’ that type.
+/// For instance, a [`Box<T>`] can be borrowed as `T` while a [`String`]
+/// can be borrowed as `str`.
 ///
-/// If you are implementing `Borrow` and both `Self` and `Borrowed` implement
-/// `Hash`, `Eq`, and/or `Ord`, they must produce the same result.
+/// Types express that they can be borrowed as some type `T` by implementing
+/// `Borrow<T>`, providing a reference to a `T` in the trait’s
+/// [`borrow`] method. A type is free to borrow as several different types.
+/// If it wishes to mutably borrow as the type – allowing the underlying data
+/// to be modified, it can additionally implement [`BorrowMut<T>`].
 ///
-/// `Borrow` is very similar to, but different than, `AsRef`. See
-/// [the book][book] for more.
+/// Further, when providing implementations for additional traits, it needs
+/// to be considered whether they should behave identical to those of the
+/// underlying type as a consequence of acting as a representation of that
+/// underlying type. Generic code typically uses `Borrow<T>` when it relies
+/// on the identical behavior of these additional trait implementations.
+/// These traits will likely appear as additional trait bounds.
 ///
-/// [book]: ../../book/first-edition/borrow-and-asref.html
+/// If generic code merely needs to work for all types that can
+/// provide a reference to related type `T`, it is often better to use
+/// [`AsRef<T>`] as more types can safely implement it.
+///
+/// [`AsRef<T>`]: ../../std/convert/trait.AsRef.html
+/// [`BorrowMut<T>`]: trait.BorrowMut.html
+/// [`Box<T>`]: ../../std/boxed/struct.Box.html
+/// [`Mutex<T>`]: ../../std/sync/struct.Mutex.html
+/// [`Rc<T>`]: ../../std/rc/struct.Rc.html
+/// [`str`]: ../../std/primitive.str.html
+/// [`String`]: ../../std/string/struct.String.html
+/// [`borrow`]: #tymethod.borrow
+///
+/// # Examples
+///
+/// As a data collection, [`HashMap<K, V>`] owns both keys and values. If
+/// the key’s actual data is wrapped in a managing type of some kind, it
+/// should, however, still be possible to search for a value using a
+/// reference to the key’s data. For instance, if the key is a string, then
+/// it is likely stored with the hash map as a [`String`], while it should
+/// be possible to search using a [`&str`][`str`]. Thus, `insert` needs to
+/// operate on a `String` while `get` needs to be able to use a `&str`.
+///
+/// Slightly simplified, the relevant parts of `HashMap<K, V>` look like
+/// this:
+///
+/// ```
+/// use std::borrow::Borrow;
+/// use std::hash::Hash;
+///
+/// pub struct HashMap<K, V> {
+///     # marker: ::std::marker::PhantomData<(K, V)>,
+///     // fields omitted
+/// }
+///
+/// impl<K, V> HashMap<K, V> {
+///     pub fn insert(&self, key: K, value: V) -> Option<V>
+///     where K: Hash + Eq
+///     {
+///         # unimplemented!()
+///         // ...
+///     }
+///
+///     pub fn get<Q>(&self, k: &Q) -> Option<&V>
+///     where
+///         K: Borrow<Q>,
+///         Q: Hash + Eq + ?Sized
+///     {
+///         # unimplemented!()
+///         // ...
+///     }
+/// }
+/// ```
+///
+/// The entire hash map is generic over a key type `K`. Because these keys
+/// are stored with the hash map, this type has to own the key’s data.
+/// When inserting a key-value pair, the map is given such a `K` and needs
+/// to find the correct hash bucket and check if the key is already present
+/// based on that `K`. It therefore requires `K: Hash + Eq`.
+///
+/// When searching for a value in the map, however, having to provide a
+/// reference to a `K` as the key to search for would require to always
+/// create such an owned value. For string keys, this would mean a `String`
+/// value needs to be created just for the search for cases where only a
+/// `str` is available.
+///
+/// Instead, the `get` method is generic over the type of the underlying key
+/// data, called `Q` in the method signature above. It states that `K`
+/// borrows as a `Q` by requiring that `K: Borrow<Q>`. By additionally
+/// requiring `Q: Hash + Eq`, it signals the requirement that `K` and `Q`
+/// have implementations of the `Hash` and `Eq` traits that produce identical
+/// results.
+///
+/// The implementation of `get` relies in particular on identical
+/// implementations of `Hash` by determining the key’s hash bucket by calling
+/// `Hash::hash` on the `Q` value even though it inserted the key based on
+/// the hash value calculated from the `K` value.
+///
+/// As a consequence, the hash map breaks if a `K` wrapping a `Q` value
+/// produces a different hash than `Q`. For instance, imagine you have a
+/// type that wraps a string but compares ASCII letters ignoring their case:
+///
+/// ```
+/// pub struct CaseInsensitiveString(String);
+///
+/// impl PartialEq for CaseInsensitiveString {
+///     fn eq(&self, other: &Self) -> bool {
+///         self.0.eq_ignore_ascii_case(&other.0)
+///     }
+/// }
+///
+/// impl Eq for CaseInsensitiveString { }
+/// ```
+///
+/// Because two equal values need to produce the same hash value, the
+/// implementation of `Hash` needs to ignore ASCII case, too:
+///
+/// ```
+/// # use std::hash::{Hash, Hasher};
+/// # pub struct CaseInsensitiveString(String);
+/// impl Hash for CaseInsensitiveString {
+///     fn hash<H: Hasher>(&self, state: &mut H) {
+///         for c in self.0.as_bytes() {
+///             c.to_ascii_lowercase().hash(state)
+///         }
+///     }
+/// }
+/// ```
+///
+/// Can `CaseInsensitiveString` implement `Borrow<str>`? It certainly can
+/// provide a reference to a string slice via its contained owned string.
+/// But because its `Hash` implementation differs, it behaves differently
+/// from `str` and therefore must not, in fact, implement `Borrow<str>`.
+/// If it wants to allow others access to the underlying `str`, it can do
+/// that via `AsRef<str>` which doesn’t carry any extra requirements.
+///
+/// [`Hash`]: ../../std/hash/trait.Hash.html
+/// [`HashMap<K, V>`]: ../../std/collections/struct.HashMap.html
+/// [`String`]: ../../std/string/struct.String.html
+/// [`str`]: ../../std/primitive.str.html
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait Borrow<Borrowed: ?Sized> {
     /// Immutably borrows from an owned value.
@@ -59,7 +189,11 @@ pub trait Borrow<Borrowed: ?Sized> {
 
 /// A trait for mutably borrowing data.
 ///
-/// Similar to `Borrow`, but for mutable borrows.
+/// As a companion to [`Borrow<T>`] this trait allows a type to borrow as
+/// an underlying type by providing a mutable reference. See [`Borrow<T>`]
+/// for more information on borrowing as another type.
+///
+/// [`Borrow<T>`]: trait.Borrow.html
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait BorrowMut<Borrowed: ?Sized> : Borrow<Borrowed> {
     /// Mutably borrows from an owned value.