Add basic GATs reference information

src/items/associated-items.md

@@ -205,26 +205,46 @@ types cannot be defined in [inherent implementations] nor can they be given a
 default implementation in traits.
 
 An *associated type declaration* declares a signature for associated type
-definitions. It is written as `type`, then an [identifier], and
-finally an optional list of trait bounds.
+definitions. It is written in one of the following forms, where `Assoc` is the
+name of the associated type, `Params` is a comma-separated list of type,
+lifetime or const parameters, `Bounds` is a plus-separated list of trait bounds
+on the associated type, and `WhereBounds` is a comma-separated list of bounds on
+parameters:
+
+```rust,ignore
+type Assoc;
+type Assoc: Bounds;
+type Assoc<Params>;
+type Assoc<Params>: Bounds;
+type Assoc<Params> where WhereBounds;
+type Assoc<Params>: Bounds where WhereBounds;
+```
 
 The identifier is the name of the declared type alias. The optional trait bounds
 must be fulfilled by the implementations of the type alias.
 There is an implicit [`Sized`] bound on associated types that can be relaxed using the special `?Sized` bound.
 
-An *associated type definition* defines a type alias on another type. It is
-written as `type`, then an [identifier], then an `=`, and finally a [type].
+An *associated type definition* defines a type alias on for the implementation
+of a trait on a type. They are written similarly to an *associated type declaration*,
+but cannot contain `Bounds`, but instead must contain a `Type`:
+
+```rust,ignore
+type Assoc = Type;
+type Assoc<Params> = Type<Params>;
+type Assoc<Params> where WhereBounds = Type;
+type Assoc<Params> = Type where WhereBounds;
+```
 
 If a type `Item` has an associated type `Assoc` from a trait `Trait`, then
 `<Item as Trait>::Assoc` is a type that is an alias of the type specified in the
 associated type definition. Furthermore, if `Item` is a type parameter, then
 `Item::Assoc` can be used in type parameters.
 
-Associated types may include [generic parameters] or [where clauses]; these may
-be referred to as 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
+Associated types may include [generic parameters] and [where clauses]; these are
+often referred to as *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

src/items/traits.md

@@ -70,6 +70,7 @@ Object safe traits can be the base trait of a [trait object]. A trait is
 * All [supertraits] must also be object safe.
 * `Sized` must not be a [supertrait][supertraits]. In other words, it must not require `Self: Sized`.
 * It must not have any associated constants.
+* It must not have any associated types with generics.
 * All associated functions must either be dispatchable from a trait object or be explicitly non-dispatchable:
     * Dispatchable functions require:
         * Not have any type parameters (although lifetime parameters are allowed),

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> `;`
+>              [_WhereClause_]<sup>?</sup> ( `=` [_Type_] [_WhereClause_]<sup>?</sup>)<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
@@ -34,11 +34,15 @@ let _ = TypeAlias(5); // Doesn't work
 A type alias without the [_Type_] specification may only appear as an
 [associated type] in a [trait].
 
+A type alias with the [_Type_] specification may only appear as an
+[associated type] in a [trait impl].
+
 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].
+Where clauses before the equals sign on a type alias in a [trait impl] (like 
+`type TypeAlias<T> where T: Foo = Bar<T>`) are deprecated. Where clauses after
+the equals sign (like `type TypeAlias<T> where T: Foo = Bar<T>`) are preferred.
 
 [IDENTIFIER]: ../identifiers.md
 [_GenericParams_]: generics.md