Padding

src/type-layout.md

@@ -1,11 +1,30 @@
 # Type Layout
 
-The layout of a type is its size, alignment, and the relative offsets of its
-fields. For enums, how the discriminant is laid out and interpreted is also part
-of type layout.
+The *layout* of a type is the combination of its:
 
-Type layout can be changed with each compilation. Instead of trying to document
-exactly what is done, we only document what is guaranteed today.
+* size
+* alignment
+* relative offsets of its fields or elements
+* relative offsets of padding
+* for enums: how the discriminant is laid out and interpreted
+
+> **Note**: Type layout is allowed to change with each compilation. Instead of
+> trying to document exactly how each type is laid out, we only document the
+> properties that are guaranteed.
+
+## Padding
+
+Types may contain *padding*: bytes that have no semantic meaning that may take
+on any bit pattern. When a type is [moved or copied], the value of padding bytes
+may be changed.
+
+If a field can be accessed, its padding is also the padding of the containing
+type.
+
+> **Note:**: Padding is usually added to make sure that fields and the type
+> itself are properly aligned. As such, padding is usally only added to types
+> with multiple fields. As an exception, [manually setting the
+> alignment][alignment modifiers] can create padding for a type with one field.
 
 ## Size and Alignment
 
@@ -32,7 +51,7 @@ the alignment of the type respectively.
 
 ## Primitive Data Layout
 
-The size of most primitives is given in this table.
+The size of most primitives is guaranteed and given in this table.
 
 | Type              | `size_of::<Type>()`|
 |--                 |--                  |
@@ -51,8 +70,8 @@ target platform. For example, on a 32 bit target, this is 4 bytes and on a 64
 bit target, this is 8 bytes.
 
 Most primitives are generally aligned to their size, although this is
-platform-specific behavior. In particular, on x86 u64 and f64 are only
-aligned to 32 bits.
+platform-specific behavior. In particular, on `x86` targets, `u64` and `f64` are
+only aligned to 32 bits.
 
 ## Pointers and References Layout
 
@@ -82,7 +101,9 @@ Slices have the same layout as the section of the array they slice.
 > etc.) to slices.
 
 ## `str` Layout
-String slices are a UTF-8 representation of characters that have the same layout as slices of type `[u8]`.
+
+String slices are a UTF-8 representation of characters that have the same layout
+as slices of type `[u8]`.
 
 ## Tuple Layout
 
@@ -557,9 +578,11 @@ used with any other representation.
 [`size_of`]: ../std/mem/fn.size_of.html
 [`Sized`]: ../std/marker/trait.Sized.html
 [`Copy`]: ../std/marker/trait.Copy.html
+[alignment modifiers]: #the-alignment-modifiers
 [dynamically sized types]: dynamically-sized-types.md
 [field-less enums]: items/enumerations.md#custom-discriminant-values-for-fieldless-enumerations
 [enumerations]: items/enumerations.md
+[moved or copied]: expressions.md#moved-and-copied-types
 [zero-variant enums]: items/enumerations.md#zero-variant-enums
 [undefined behavior]: behavior-considered-undefined.md
 [27060]: https://github.com/rust-lang/rust/issues/27060