[RFC] Proposed change to directive location introspection

This proposes a change to how we represent the ability to validate the locations of directives via introspection.

Specifically, this *removes* `onField`, `onFragment`, and `onOperation` in favor of `locations` which is a list of `__DirectiveLocation`.

In addition, this changes the specification of the `@skip` and `@include` directives to no longer allow their use on fragment directives (but are still allowed on fragment spreads and inline fragments). Because of this, a section on "merging fragment directives" is no longer useful.

**Rationale:**

This allows for a more fine-grained validation of directive placement, now you can assert that a directive is allowed on queries but not mutations, or allowed on fragment definitions but not on fragment spreads.

Also, this makes expanding the locations a directive is allowed to be placed easier to do, as future expansions will not affect the introspection API. This should be considered a prereq to supporting directives in more locations.

Finally, this is a prereq to a forthcoming RFC to add directives to the type schema language, one of the last missing pieces to represent a full schema using this language.

**Drawbacks:**

Any change to the introspection API is a challenge. Tools like Graph*i*QL should continue to support the older representation if possible.

spec/Section 2 -- Language.md

@@ -909,44 +909,3 @@ and operations.
 
 As future versions of GraphQL adopts new configurable execution capabilities,
 they may be exposed via directives.
-
-#### Fragment Directives
-
-Fragments may include directives to alter their behavior. At runtime, the directives provided on a fragment spread override those described on the
-definition.
-
-For example, the following query:
-
-```graphql
-query hasConditionalFragment($condition: Boolean) {
-  ...maybeFragment @include(if: $condition)
-}
-
-fragment maybeFragment on Query {
-  me {
-    name
-  }
-}
-```
-
-Will have identical runtime behavior as
-
-```graphql
-query hasConditionalFragment($condition: Boolean) {
-  ...maybeFragment
-}
-
-fragment maybeFragment on Query @include(if: $condition) {
-  me {
-    name
-  }
-}
-```
-
-FragmentSpreadDirectives(fragmentSpread) :
-  * Let {directives} be the set of directives on {fragmentSpread}
-  * Let {fragmentDefinition} be the FragmentDefinition in the document named {fragmentSpread} refers to.
-  * For each {directive} in directives on {fragmentDefinition}
-    * If {directives} does not contain a directive named {directive}
-      * Add {directive} into {directives}
-  * Return {directives}

spec/Section 3 -- Type System.md

@@ -767,8 +767,9 @@ GraphQL implementations should provide the `@skip` and `@include` directives.
 
 ### @skip
 
-The `@skip` directive may be provided for fields or fragments, and allows
-for conditional exclusion during execution as described by the if argument.
+The `@skip` directive may be provided for fields, fragment spreads, and
+inline fragments, and allows for conditional exclusion during execution as
+described by the if argument.
 
 In this example `experimentalField` will be queried only if the `$someTest` is
 provided a `false` value.
@@ -781,8 +782,9 @@ query myQuery($someTest: Boolean) {
 
 ### @include
 
-The `@include` directive may be provided for fields or fragments, and allows
-for conditional inclusion during execution as described by the if argument.
+The `@include` directive may be provided for fields, fragment spreads, and
+inline fragments, and allows for conditional inclusion during execution as
+described by the if argument.
 
 In this example `experimentalField` will be queried only if the `$someTest` is
 provided a `true` value.

spec/Section 4 -- Introspection.md

@@ -181,10 +181,18 @@ enum __TypeKind {
 type __Directive {
   name: String!
   description: String
+  locations: [__DirectiveLocation!]!
   args: [__InputValue!]!
-  onOperation: Boolean!
-  onFragment: Boolean!
-  onField: Boolean!
+}
+
+enum __DirectiveLocation {
+  QUERY
+  MUTATION
+  SUBSCRIPTION
+  FIELD
+  FRAGMENT_DEFINITION
+  FRAGMENT_SPREAD
+  INLINE_FRAGMENT
 }
 ```
 
@@ -375,3 +383,17 @@ Fields
 * `defaultValue` may return a String encoding (using the GraphQL language) the
   default value used by this input value in the condition a value is not
   provided at runtime. If this input value has no default value, returns {null}.
+
+
+### The __Directive Type
+
+The `__Directive` type represents a Directive that a server supports.
+
+Fields
+
+* `name` must return a String
+* `description` may return a String or {null}
+* `locations` returns a List of `__DirectiveLocation` representing the valid
+  locations this directive may be placed.
+* `args` returns a List of `__InputValue` representing the arguments this
+  directive accepts.

spec/Section 5 -- Validation.md

@@ -1100,6 +1100,34 @@ For example the following query will not pass validation.
 GraphQL servers define what directives they support. For each
 usage of a directive, the directive must be available on that server.
 
+
+### Directives Are In Valid Locations
+
+** Formal Specification **
+
+  * For every {directive} in a document.
+  * Let {directiveName} be the name of {directive}.
+  * Let {directiveDefinition} be the directive named {directiveName}.
+  * Let {locations} be the valid locations for {directiveDefinition}.
+  * Let {adjacent} be the AST node the directive effects.
+  * {adjacent} must be represented by an item within {locations}.
+
+** Explanatory Text **
+
+GraphQL servers define what directives they support and where they support them.
+For each usage of a directive, the directive must be used in a location that the
+server has declared support for.
+
+For example the following query will not pass validation because `@skip` does
+not provide `QUERY` as a valid location.
+
+```!graphql
+query @skip(if: $foo) {
+  field
+}
+```
+
+
 ## Variables
 
 ### Variable Uniqueness