Add capturing and precise capturing rules

The Reference didn't include any description of capturing or capturing
behavior for `impl Trait` opaque types.  Let's describe briefly what
capturing is and what the currently-stable automatic capturing rules
are.  Then let's describe the syntax and behavior of RFC 3617 precise
capturing.

src/trait-bounds.md

@@ -5,7 +5,7 @@
 > &nbsp;&nbsp; _TypeParamBound_ ( `+` _TypeParamBound_ )<sup>\*</sup> `+`<sup>?</sup>
 >
 > _TypeParamBound_ :\
-> &nbsp;&nbsp; &nbsp;&nbsp; _Lifetime_ | _TraitBound_
+> &nbsp;&nbsp; &nbsp;&nbsp; _Lifetime_ | _TraitBound_ | _UseBound_
 >
 > _TraitBound_ :\
 > &nbsp;&nbsp; &nbsp;&nbsp; `?`<sup>?</sup>
@@ -19,6 +19,20 @@
 > _Lifetime_ :\
 > &nbsp;&nbsp; &nbsp;&nbsp; [LIFETIME_OR_LABEL]\
 > &nbsp;&nbsp; | `'static`
+>
+> _UseBound_ :\
+> &nbsp;&nbsp; `use` _UseBoundGenericArgs_
+>
+> _UseBoundGenericArgs_ :\
+> &nbsp;&nbsp; &nbsp;&nbsp; `<` `>` \
+> &nbsp;&nbsp; | `<` \
+> &nbsp;&nbsp; &nbsp;&nbsp; ( _UseBoundGenericArg_ `,`)<sup>\*</sup> \
+> &nbsp;&nbsp; &nbsp;&nbsp; _UseBoundGenericArg_ `,`<sup>?</sup> \
+> &nbsp;&nbsp; &nbsp;&nbsp; `>`
+>
+> _UseBoundGenericArg_ :\
+> &nbsp;&nbsp; &nbsp;&nbsp; [LIFETIME_OR_LABEL][] \
+> &nbsp;&nbsp; | [IDENTIFIER][]
 
 [Trait] and lifetime bounds provide a way for [generic items][generic] to
 restrict which types and lifetimes are used as their parameters. Bounds can be
@@ -227,20 +241,26 @@ trait Trait<'a, T: 'a> {}
 impl<'a, T> Trait<'a, T> for &'a T {}
 ```
 
+## Use bounds
+
+Certain bounds lists may include a `use<..>` bound to control which generic parameters are captured by the `impl Trait` [abstract return type].  See [precise capturing] for more details.
 
+[IDENTIFIER]: identifiers.html
 [LIFETIME_OR_LABEL]: tokens.md#lifetimes-and-loop-labels
 [_GenericParams_]: items/generics.md
 [_TypePath_]: paths.md#paths-in-types
 [`Clone`]: special-types-and-traits.md#clone
 [`Copy`]: special-types-and-traits.md#copy
 [`Sized`]: special-types-and-traits.md#sized
 
+[abstract return type]: types/impl-trait.md#abstract-return-types
 [arrays]: types/array.md
 [associated types]: items/associated-items.md#associated-types
 [hrtb-scopes]: names/scopes.md#higher-ranked-trait-bound-scopes
 [supertraits]: items/traits.md#supertraits
 [generic]: items/generics.md
 [higher-ranked lifetimes]: #higher-ranked-trait-bounds
+[precise capturing]: types/impl-trait.md#precise-capturing
 [slice]: types/slice.md
 [Trait]: items/traits.md#trait-bounds
 [trait object]: types/trait-object.md

src/types/impl-trait.md

@@ -91,7 +91,33 @@ Functions in traits may also use `impl Trait` as a syntax for an anonymous assoc
 
 Every `impl Trait` in the return type of an associated function in a trait is desugared to an anonymous associated type. The return type that appears in the implementation's function signature is used to determine the value of the associated type.
 
-### Differences between generics and `impl Trait` in return position
+## Capturing
+
+Behind each return-position `impl Trait` abstract return type is some hidden concrete type.  For this concrete type to use a generic parameter, that generic parameter must be *captured* by the abstract type.
+
+## Automatic capturing
+
+Return-position `impl Trait` abstract return types automatically capture certain of the in-scope generic parameters.  Everywhere, these automatically capture all in-scope type and const generic parameters.
+
+On items of trait impls and trait definitions, these types additionally automatically capture all in-scope generic lifetime parameters, including higher-ranked ones.  On free functions and on associated functions and methods of inherent impls, only the generic lifetime parameters that appear in the bounds of abstract return type are captured.
+
+## Precise capturing
+
+The set of generic parameters captured by a return-position `impl Trait` abstract type may be explicitly controlled with a [`use<..>` bound].  If present, only the generic parameters listed in the bound will be captured.  E.g.:
+
+```rust
+fn capture<'a, 'b, T>(x: &'a (), y: T) -> impl Sized + use<'a, T> {
+  //                                      ~~~~~~~~~~~~~~~~~~~~~~~
+  //                                     Captures `'a` and `T` only.
+  (x, y)
+}
+```
+
+Currently, only one `use<..>` bound may be present in a bounds list, such bounds are not allowed in the signature of items of a trait definition, and all in-scope type and const generic parameters must be included.  If the elided lifetime (`'_`) is otherwise allowed to appear within the `impl Trait` return type, it may be named within the `use<..>` bound.
+
+Because all in-scope type parameters must be included by name, a `use<..>` bound may not be used in the signature of items that use argument-position `impl Trait`, as those items have anonymous type parameters in scope.
+
+## Differences between generics and `impl Trait` in return position
 
 In argument position, `impl Trait` is very similar in semantics to a generic type parameter.
 However, there are significant differences between the two in return position.
@@ -127,9 +153,10 @@ Instead, the function chooses the return type, but only promises that it will im
 `impl Trait` can only appear as a parameter or return type of a non-`extern` function.
 It cannot be the type of a `let` binding, field type, or appear inside a type alias.
 
-[closures]: closure.md
 [_GenericArgs_]: ../paths.md#paths-in-expressions
 [_GenericParams_]: ../items/generics.md
 [_TraitBound_]: ../trait-bounds.md
-[trait object]: trait-object.md
 [_TypeParamBounds_]: ../trait-bounds.md
+[`use<..>` bound]: ../trait-bounds.md#use-bounds
+[closures]: closure.md
+[trait object]: trait-object.md