Hello to all,
this PR allows to support the @defer directive. Although the implementation is mostly complete, the PR is in draft to discuss it together and evaluate its integration.

In the PR I also created tests and I performed the integration tests with Relay, express-graphql & fetch-multipart-graphql with positive results.

In summary how I implemented defer:

• @defer has two arguments:
  • if (optional, default true) the fragment is defer when true.
  • label (required) used to distinguish deferred fragments
• @defer currently provides only FRAGMENT_SPREAD & INLINE_FRAGMENT (Relay) as location, but support for FIELD (Apollo) is commented in the PR

  graphql-js/src/execution/execute.js

  Lines 663 to 668 in 4226f23

  	/*
  	in order to support defer on field
  	const isFieldDefer = shouldDeferNode(exeContext, selection);
  	if (deferDirective && !isFieldDefer) {
  	*/
  	if (deferDirective) {

• I added resolverResult: ResultResolver new property in ExecutionContext which allows the management of the results to be returned at the end of the execution
• modified execute function now its return type is AsyncIterable<Promise<ExecutionResult> | PromiseOrValue<ExecutionResult>; . Using isAsyncIterable (result) it is possible to distinguish if there are deferred results.
• fields that are marked as deferred are resolved after the resolution of the previous result. I also implemented an optimization to avoid solving the same field multiple times.

this is a slow motion gif of how it works.

Let me know if more information is needed.

Thank you
Lorenzo