Rework look-ahead machinery as lazy-evaluated (#1212)

- make `LookAheadValue::List` to contain new iterable `LookAheadList` type
- make `LookAheadValue::Object` to contain new iterable `LookAheadObject` type
- remove `LookAheadMethods` trait and redundant `ConcreteLookAheadSelection` struct making all APIs accessible as inherent methods on `LookAheadSelection` and `LookAheadChildren` decoupled types
- move `LookAheadMethods::child_names()` to `LookAheadChildren::names()`
- move `LookAheadMethods::has_children()` to `LookAheadChildren::is_empty()`
- move `LookAheadMethods::select_child()` to `LookAheadChildren::select()`
- move `LookAheadSelection::for_explicit_type()` to `LookAheadSelection::children_for_explicit_type()`
- make `LookAheadSelection::arguments()` returning iterator over `LookAheadArgument`
- make `LookAheadSelection::children()` returning `LookAheadChildren`

Co-authored-by: Kai Ren <tyranron@gmail.com>

book/src/types/interfaces.md

@@ -391,7 +391,7 @@ This requires to explicitly parametrize over [`ScalarValue`][3], as [`Executor`]
 
 ```rust
 # extern crate juniper;
-use juniper::{graphql_interface, graphql_object, Executor, LookAheadMethods as _, ScalarValue};
+use juniper::{graphql_interface, graphql_object, Executor, ScalarValue};
 
 #[graphql_interface(for = Human, Scalar = S)] // notice specifying `ScalarValue` as existing type parameter
 trait Character<S: ScalarValue> {             

juniper/CHANGELOG.md

@@ -52,12 +52,22 @@ All user visible changes to `juniper` crate will be documented in this file. Thi
     - Removed `scalar-naivetime` [Cargo feature].
 - Removed lifetime parameter from `ParseError`, `GraphlQLError`, `GraphQLBatchRequest` and `GraphQLRequest`. ([#1081], [#528])
 - Upgraded [GraphiQL] to 3.1.0 version (requires new [`graphql-transport-ws` GraphQL over WebSocket Protocol] integration on server, see `juniper_warp/examples/subscription.rs`). ([#1188], [#1193], [#1235])
-- Made `LookAheadMethods::children()` method to return slice instead of `Vec`. ([#1200])
 - Abstracted `Spanning::start` and `Spanning::end` fields into separate struct `Span`. ([#1207], [#1208])
-- Added `Span` to `Arguments` and `LookAheadArguments`. ([#1206], [#1209])
 - Removed `graphql-parser-integration` and `graphql-parser` [Cargo feature]s by merging them into `schema-language` [Cargo feature]. ([#1237])
 - Renamed `RootNode::as_schema_language()` method as `RootNode::as_sdl()`. ([#1237]) 
-- Renamed `RootNode::as_parser_document()` method as `RootNode::as_document()`. ([#1237]) 
+- Renamed `RootNode::as_parser_document()` method as `RootNode::as_document()`. ([#1237])
+- Reworked look-ahead machinery: ([#1212])
+    - Turned from eagerly-evaluated into lazy-evaluated:
+        - Made `LookAheadValue::List` to contain new iterable `LookAheadList` type.
+        - Made `LookAheadValue::Object` to contain new iterable `LookAheadObject` type.
+    - Removed `LookAheadMethods` trait and redundant `ConcreteLookAheadSelection` type, making all APIs accessible as inherent methods on `LookAheadSelection` and `LookAheadChildren` decoupled types:
+        - Moved `LookAheadMethods::child_names()` to `LookAheadChildren::names()`.
+        - Moved `LookAheadMethods::has_children()` to `LookAheadChildren::is_empty()`.
+        - Moved `LookAheadMethods::select_child()` to `LookAheadChildren::select()`.
+        - Moved `LookAheadSelection::for_explicit_type()` to `LookAheadSelection::children_for_explicit_type()`.
+        - Made `LookAheadSelection::arguments()` returning iterator over `LookAheadArgument`.
+        - Made `LookAheadSelection::children()` returning `LookAheadChildren`.
+    - Added `Span` to `Arguments` and `LookAheadArguments`. ([#1206], [#1209])
 
 ### Added
 
@@ -145,11 +155,11 @@ All user visible changes to `juniper` crate will be documented in this file. Thi
 [#1190]: /../../pull/1190
 [#1193]: /../../pull/1193
 [#1199]: /../../pull/1199
-[#1200]: /../../pull/1200
 [#1206]: /../../pull/1206
 [#1207]: /../../pull/1207
 [#1208]: /../../pull/1208
 [#1209]: /../../pull/1209
+[#1212]: /../../pull/1212
 [#1215]: /../../pull/1215
 [#1227]: /../../pull/1227
 [#1228]: /../../pull/1228