Update unions for safe ManuallyDrop assignment.

src/items/unions.md

@@ -66,14 +66,18 @@ unsafe {
 }
 ```
 
-Writes to `Copy` union fields do not require reads for running destructors, so
-these writes don't have to be placed in `unsafe` blocks
+Writes to [`Copy`] or [`ManuallyDrop`][ManuallyDrop] union fields do not
+require reads for running destructors, so these writes don't have to be placed
+in `unsafe` blocks
 
 ```rust
-# union MyUnion { f1: u32, f2: f32 }
-# let mut u = MyUnion { f1: 1 };
-#
+# use std::mem::ManuallyDrop;
+union MyUnion { f1: u32, f2: ManuallyDrop<String> }
+let mut u = MyUnion { f1: 1 };
+
+// These do not require `unsafe`.
 u.f1 = 2;
+u.f2 = ManuallyDrop::new(String::from("example"));
 ```
 
 Commonly, code using unions will provide safe wrappers around unsafe union
@@ -82,9 +86,9 @@ field accesses.
 ## Unions and `Drop`
 
 When a union is dropped, it cannot know which of its fields needs to be dropped.
-For this reason, all union fields must either be of a `Copy` type or of the
-shape [`ManuallyDrop<_>`].  This ensures that a union does not need to drop
-anything when it goes out of scope.
+For this reason, all union fields must either be of a [`Copy`] type or of the
+shape [`ManuallyDrop<_>`][ManuallyDrop].  This ensures that a union does not
+need to drop anything when it goes out of scope.
 
 Like for structs and enums, it is possible to `impl Drop` for a union to
 manually define what happens when it gets dropped.
@@ -177,4 +181,5 @@ checking, etc etc etc).
 [_WhereClause_]: generics.md#where-clauses
 [_StructFields_]: structs.md
 [`transmute`]: ../../std/mem/fn.transmute.html
-[`ManuallyDrop<_>`]: ../../std/mem/struct.ManuallyDrop.html
+[`Copy`]: ../../std/marker/trait.Copy.html
+[ManuallyDrop]: ../../std/mem/struct.ManuallyDrop.html

src/types/union.md

@@ -5,12 +5,14 @@ a [`union` item][item].
 
 Unions have no notion of an "active field". Instead, every union access
 transmutes parts of the content of the union to the type of the accessed
-field. Since transmutes can cause unexpected or undefined behaviour, `unsafe` is
-required to read from a union field, or to write to a field that doesn't
-implement [`Copy`]. See the [item] documentation for further details.
+field. Since transmutes can cause unexpected or undefined behaviour, `unsafe`
+is required to read from a union field, or to write to a field that doesn't
+implement [`Copy`] or is a [`ManuallyDrop`] type. See the [item] documentation
+for further details.
 
 The memory layout of a `union` is undefined by default, but the `#[repr(...)]`
 attribute can be used to fix a layout.
 
 [`Copy`]: ../special-types-and-traits.md#copy
+[`ManuallyDrop`]: ../../std/mem/struct.ManuallyDrop.html
 [item]: ../items/unions.md