Document the union keyword

Author: poliorcetics

src/libstd/keyword_docs.rs

@@ -1624,8 +1624,71 @@ mod dyn_keyword {}
 //
 /// The [Rust equivalent of a C-style union][union].
 ///
-/// The documentation for this keyword is [not yet complete]. Pull requests welcome!
+/// A `union` looks like a [`struct`] in terms of declaration, but all of its
+/// fields exist simultaneously, superimposed over one another. For instance,
+/// if we wanted some bits in memory that we sometimes interpret as a `u32` and
+/// sometimes as an `f32`, we would write:
+///
+/// ```rust
+/// union IntOrFloat {
+///     i: u32,
+///     f: f32,
+/// }
+///
+/// let mut u = IntOrFloat { f: 1.0 };
+/// // Reading the fields of an union is always unsafe
+/// assert_eq!(unsafe { u.i }, 1065353216);
+/// // Updating through any of the field will modify all of them
+/// u.i = 1073741824;
+/// assert_eq!(unsafe { u.f }, 2.0);
+/// ```
+///
+/// # Matching on unions
+///
+/// It is possible to use pattern matching on `union`s. A single field name must
+/// be used and it must match the name of one of the `union`'s field.
+///
+/// ```rust
+/// union IntOrFloat {
+///     i: u32,
+///     f: f32,
+/// }
+///
+/// let u = IntOrFloat { f: 1.0 };
+///
+/// unsafe {
+///     match u {
+///         IntOrFloat { i: 10 } => println!("Found exactly ten !"),
+///         // The field name is used to deduce the type
+///         IntOrFloat { f } => println!("Found f = {} !", f),
+///     }
+/// }
+/// ```
+///
+/// # References to union fields
+///
+/// All fields in a union are all at the same place in memory which means
+/// borrowing one borrows all of them, for the same duration:
+///
+/// ```rust,compile_fail,E0502
+/// union IntOrFloat {
+///     i: u32,
+///     f: f32,
+/// }
 ///
+/// let mut u = IntOrFloat { f: 1.0 };
+///
+/// let f = unsafe { &u.f };
+/// // This will not compile because the field has already been borrowed, even
+/// // if only immutably
+/// let i = unsafe { &mut u.i };
+///
+/// *i = 10;
+/// println!("f = {} and i = {}", f, i);
+/// ```
+///
+/// See the [Reference][union] for more informations on `union`s.
+///
+/// [`struct`]: keyword.struct.html
 /// [union]: ../reference/items/unions.html
-/// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
 mod union_keyword {}