Documented the #[repr(align(x))] attribute.

src/attributes.md

@@ -13,6 +13,7 @@
 > _MetaItem_ :  
 >       IDENTIFIER  
 >    | IDENTIFIER `=` LITERAL  
+>    | IDENTIFIER `(` LITERAL `)`  
 >    | IDENTIFIER `(` _MetaSeq_ `)`  
 >    | IDENTIFIER `(` _MetaSeq_ `,` `)`  
 >   
@@ -30,6 +31,8 @@ may appear as any of:
 * A single identifier, the attribute name
 * An identifier followed by the equals sign '=' and a literal, providing a
   key/value pair
+* An identifier followed by a parenthesized literal, providing a
+  key/value pair
 * An identifier followed by a parenthesized list of sub-attribute arguments
 
 Attributes with a bang ("!") after the hash ("#") apply to the item that the
@@ -130,26 +133,8 @@ interpreted:
 - `linkage` - on a static, this specifies the [linkage
   type](http://llvm.org/docs/LangRef.html#linkage-types).
 
-On `enum`s:
-
-- `repr` - on C-like enums, this sets the underlying type used for
-  representation. Takes one argument, which is the primitive
-  type this enum should be represented for, or `C`, which specifies that it
-  should be the default `enum` size of the C ABI for that platform. Note that
-  enum representation in C is implementation-defined, and may not be compatible
-  when the C code is compiled with certain flags.
-
-On `struct`s:
-
-- `repr` - specifies the representation to use for this struct. Takes a list
-  of options. The currently accepted ones are `C` and `packed`, which may be
-  combined. `C` will use a C ABI compatible struct layout, and `packed` will
-  remove any padding between fields (note that this is very fragile and may
-  break platforms which require aligned access).
-
-On `union`s:
-
-- `repr` - Same as per `struct`.
+See [type layout](type-layout.html) for documentation on the `repr` attribute
+which can be used to control type layout.
 
 ## Macro-related attributes
 

src/type-layout.md

@@ -264,13 +264,29 @@ For all other enumerations, the layout is unspecified.
 
 Likewise, combining two primitive representations together is unspecified.
 
+### The `align` Representation
+
+The `align` representation can be used on `struct`s and `union`s to raise the
+alignment of the type to a given value.
+
+Alignment is specified as a parameter in the form of `#[repr(align(x))]`. The
+alignment value must be a power of two of type `u32`. The `align` representation
+can raise the alignment of a type to be greater than it's primitive alignment,
+it cannot lower the alignment of a type.
+
+The `align` and `packed` representations cannot be applied on the same type and
+a `packed` type cannot transitively contain another `align`ed type.
+
 ### The `packed` Representation
 
 The `packed` representation can only be used on `struct`s and `union`s.
 
 It modifies the representation (either the default or `C`) by removing any
 padding bytes and forcing the alignment of the type to `1`.
 
+The `align` and `packed` representations cannot be applied on the same type and
+a `packed` type cannot transitively contain another `align`ed type.
+
 > Warning: Dereferencing an unaligned pointer is [undefined behaviour] and it is
 > possible to [safely create unaligned pointers to `packed` fields][27060].
 > Like all ways to create undefined behavior in safe Rust, this is a bug.
@@ -284,4 +300,4 @@ padding bytes and forcing the alignment of the type to `1`.
 [C-like enumerations]:  items/enumerations.html#custom-discriminant-values-for-field-less-enumerations
 [zero-variant enumerations]: items/enumerations.html#zero-variant-enums
 [undefined behavior]: behavior-considered-undefined.html
-[27060]: https://github.com/rust-lang/rust/issues/27060
+[27060]: https://github.com/rust-lang/rust/issues/27060