Add basic GATs info

Author: jackh726

src/items/associated-items.md

@@ -220,7 +220,12 @@ If a type `Item` has an associated type `Assoc` from a trait `Trait`, then
 associated type definition. Furthermore, if `Item` is a type parameter, then
 `Item::Assoc` can be used in type parameters.
 
-Associated types must not include [generic parameters] or [where clauses].
+Associated types may include [generic parameters] or [where clauses]; these may
+be referred to generic associated types, or GATs. If the type `Thing` has an
+associated type `Item` from a trait `Trait` with the generics `<'a>` , the type
+can be named like `<Thing as Trait>::Item<'x>`, where `'x` is some lifetime in
+scope. In this case, `'x` will be used wherever `'a` appears in the associated
+type definitions on impls.
 
 ```rust
 trait AssociatedType {
@@ -249,6 +254,37 @@ fn main() {
 }
 ```
 
+An example of associated types with generics and where clauses:
+
+```rust
+struct ArrayLender<'a, T>(&'a mut [T; 16]);
+
+trait Lend {
+    // Generic associated type declaration
+    type Lender<'a> where Self: 'a;
+    fn lend<'a>(&'a mut self) -> Self::Lender<'a>;
+}
+
+impl<T> Lend for [T; 16] {
+    // Generic associated type definition
+    type Lender<'a> = ArrayLender<'a, T> where Self: 'a;
+
+    fn lend<'a>(&'a mut self) -> Self::Lender<'a> {
+        ArrayLender(self)
+    }
+}
+
+fn borrow<'a, T: Lend>(array: &'a mut T) -> <T as Lend>::Lender<'a> {
+    array.lend()
+}
+
+
+fn main() {
+    let mut array = [0usize; 16];
+    let lender = borrow(&mut array);
+}
+```
+
 ### Associated Types Container Example
 
 Consider the following example of a `Container` trait. Notice that the type is

src/items/type-aliases.md

@@ -4,7 +4,7 @@
 > _TypeAlias_ :\
 > &nbsp;&nbsp; `type` [IDENTIFIER]&nbsp;[_GenericParams_]<sup>?</sup>
 >              ( `:` [_TypeParamBounds_] )<sup>?</sup>
->              [_WhereClause_]<sup>?</sup> ( `=` [_Type_] )<sup>?</sup> `;`
+>              [_WhereClause_]<sup>?</sup> ( `=` [_Type_] )<sup>?</sup> [_WhereClause_]<sup>?</sup> `;`
 
 A _type alias_ defines a new name for an existing [type]. Type aliases are
 declared with the keyword `type`. Every value has a single, specific type, but
@@ -37,6 +37,9 @@ A type alias without the [_Type_] specification may only appear as an
 A type alias with [_TypeParamBounds_] may only specified when used as
 an [associated type] in a [trait].
 
+A type alias with where clauses after the equals sign may only appear as an
+[associated type] in a [trait] or a [trait impl].
+
 [IDENTIFIER]: ../identifiers.md
 [_GenericParams_]: generics.md
 [_TypeParamBounds_]: ../trait-bounds.md
@@ -45,3 +48,4 @@ an [associated type] in a [trait].
 [associated type]: associated-items.md#associated-types
 [trait]: traits.md
 [type]: ../types.md
+[trait impl]: implementations.md#trait-implementations