define GENERATOR_REF

Summary:
Version of `GENERATOR` that we're already using in a few places where
it doesn't leave its original scope.

Reviewed By: ddrcoder, luciang

Differential Revision: D26909115

fbshipit-source-id: 5fbae8bd44c62d4d96427a4fd3ef60a8d9cb40e3

folly/gen/Base.h

@@ -488,9 +488,14 @@ Yield generator(Source&& source) {
  * Create inline generator, used like:
  *
  *  auto gen = GENERATOR(int) { yield(1); yield(2); };
+ *
+ * GENERATOR_REF can be useful for creating a generator that doesn't
+ * leave its original scope.
  */
 #define GENERATOR(TYPE) \
   ::folly::gen::detail::GeneratorBuilder<TYPE>() + [=](auto&& yield)
+#define GENERATOR_REF(TYPE) \
+  ::folly::gen::detail::GeneratorBuilder<TYPE>() + [&](auto&& yield)
 
 /*
  * empty() - for producing empty sequences.