Merge pull request #1265 from jackh726/gats

Add basic GATs reference information

src/items/associated-items.md

@@ -205,22 +205,49 @@ 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
+that the associated type must meet, and `WhereBounds` is a comma-separated list
+of bounds that the parameters must meet:
+
+<!-- ignore: illustrative example forms -->
+```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 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`:
+
+<!-- ignore: illustrative example forms -->
+```rust,ignore
+type Assoc = Type;
+type Assoc<Params> = Type; // the type `Type` here may reference `Params`
+type Assoc<Params> = Type where WhereBounds;
+type Assoc<Params> where WhereBounds = Type; // deprecated, prefer the form above
+```
 
 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 must not include [generic parameters] or [where clauses].
+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
 trait AssociatedType {
@@ -249,6 +276,37 @@ 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
@@ -279,6 +337,83 @@ impl<T> Container for Vec<T> {
 }
 ```
 
+### Relationship between `Bounds` and `WhereBounds`
+
+In this example:
+
+```rust
+# use std::fmt::Debug;
+trait Example {
+    type Output<T>: Ord where T: Debug;
+}
+```
+
+Given a reference to the associated type like `<X as Example>::Output<Y>`, the associated type itself must be `Ord`, and the type `Y` must be `Debug`.
+
+### Required where clauses on generic associated types
+
+Generic associated type declarations on traits currently may require a list of
+where clauses, dependent on functions in the trait and how the GAT is used. These
+rules may be loosened in the future; updates can be found [on the generic
+associated types initiative repository](https://rust-lang.github.io/generic-associated-types-initiative/explainer/required_bounds.html).
+
+In a few words, these where clauses are required in order to maximize the allowed
+definitions of the associated type in impls. To do this, any clauses that *can be
+proven to hold* on functions (using the parameters of the function or trait)
+where a GAT appears as an input or output must also be written on the GAT itself.
+
+```rust
+trait LendingIterator {
+    type Item<'x> where Self: 'x;
+    fn next<'a>(&'a mut self) -> Self::Item<'a>;
+}
+```
+
+In the above, on the `next` function, we can prove that `Self: 'a`, because of
+the implied bounds from `&'a mut self`; therefore, we must write the equivalent
+bound on the GAT itself: `where Self: 'x`.
+
+When there are multiple functions in a trait that use the GAT, then the
+*intersection* of the bounds from the different functions are used, rather than
+the union.
+
+```rust
+trait Check<T> {
+    type Checker<'x>;
+    fn create_checker<'a>(item: &'a T) -> Self::Checker<'a>;
+    fn do_check(checker: Self::Checker<'_>);
+}
+```
+
+In this example, no bounds are required on the `type Checker<'a>;`. While we
+know that `T: 'a` on `create_checker`, we do not know that on `do_check`. However,
+if `do_check` was commented out, then the `where T: 'x` bound would be required
+on `Checker`.
+
+The bounds on associated types also propagate required where clauses.
+
+```rust
+trait Iterable {
+    type Item<'a> where Self: 'a;
+    type Iterator<'a>: Iterator<Item = Self::Item<'a>> where Self: 'a;
+    fn iter<'a>(&'a self) -> Self::Iterator<'a>;
+}
+```
+
+Here, `where Self: 'a` is required on `Item` because of `iter`. However, `Item`
+is used in the bounds of `Iterator`, the `where Self: 'a` clause is also required
+there.
+
+Finally, any explicit uses of `'static` on GATs in the trait do not count towards
+the required bounds.
+
+```rust
+trait StaticReturn {
+    type Y<'a>;
+    fn foo(&self) -> Self::Y<'static>;
+}
+```
+
 ## Associated Constants
 
 *Associated constants* are [constants] associated with a type.

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> ( `=` [_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
@@ -31,11 +31,18 @@ let _ = UseAlias(5); // OK
 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, when not used as an associated type, must include a [_Type_] and
+may not include [_TypeParamBounds_].
 
-A type alias with [_TypeParamBounds_] may only specified when used as
-an [associated type] in a [trait].
+A type alias, when used as an [associated type] in a [trait], must not include a
+[_Type_] specification but may include [_TypeParamBounds_].
+
+A type alias, when used as an [associated type] in a [trait impl], must include
+a [_Type_] specification and may not include [_TypeParamBounds_].
+
+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> = Bar<T> where T: Foo`) are preferred.
 
 [IDENTIFIER]: ../identifiers.md
 [_GenericParams_]: generics.md
@@ -45,3 +52,4 @@ 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