Auto merge of rust-lang#73645 - poliorcetics:ref-keyword, r=jyn514

Document the ref keyword

Partial fix for rust-lang#34601.

This documents the `ref` keyword with two examples, one failing to compile because the `ref` keyword is missing, and the same example fixed with the keyword inserted in the correct place.

It also explains (very *very* rapidly) the differences between `&` and `ref`.

I put a link to the best place I could find in the Reference but there may be something better that I didn't find.

Author: bors

src/libstd/keyword_docs.rs

@@ -1060,9 +1060,50 @@ mod pub_keyword {}
 //
 /// Bind by reference during pattern matching.
 ///
-/// The documentation for this keyword is [not yet complete]. Pull requests welcome!
+/// `ref` annotates pattern bindings to make them borrow rather than move.
+/// It is **not** a part of the pattern as far as matching is concerned: it does
+/// not affect *whether* a value is matched, only *how* it is matched.
 ///
-/// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
+/// By default, [`match`] statements consume all they can, which can sometimes
+/// be a problem, when you don't really need the value to be moved and owned:
+///
+/// ```compile_fail,E0382
+/// let maybe_name = Some(String::from("Alice"));
+/// // The variable 'maybe_name' is consumed here ...
+/// match maybe_name {
+///     Some(n) => println!("Hello, {}", n),
+///     _ => println!("Hello, world"),
+/// }
+/// // ... and is now unavailable.
+/// println!("Hello again, {}", maybe_name.unwrap_or("world".into()));
+/// ```
+///
+/// Using the `ref` keyword, the value is only borrowed, not moved, making it
+/// available for use after the [`match`] statement:
+///
+/// ```
+/// let maybe_name = Some(String::from("Alice"));
+/// // Using `ref`, the value is borrowed, not moved ...
+/// match maybe_name {
+///     Some(ref n) => println!("Hello, {}", n),
+///     _ => println!("Hello, world"),
+/// }
+/// // ... so it's available here!
+/// println!("Hello again, {}", maybe_name.unwrap_or("world".into()));
+/// ```
+///
+/// # `&` vs `ref`
+///
+/// - `&` denotes that your pattern expects a reference to an object. Hence `&`
+/// is a part of said pattern: `&Foo` matches different objects than `Foo` does.
+///
+/// - `ref` indicates that you want a reference to an unpacked value. It is not
+/// matched against: `Foo(ref foo)` matches the same objects as `Foo(foo)`.
+///
+/// See also the [Reference] for more information.
+///
+/// [`match`]: keyword.match.html
+/// [Reference]: ../reference/patterns.html#identifier-patterns
 mod ref_keyword {}
 
 #[doc(keyword = "return")]