Add generator lang-item

Author: compiler-errors

core/src/ops/generator.rs

@@ -82,6 +82,7 @@ pub trait Generator<R = ()> {
     /// `return` statement or implicitly as the last expression of a generator
     /// literal. For example futures would use this as `Result<T, E>` as it
     /// represents a completed future.
+    #[cfg_attr(not(bootstrap), lang = "generator_return")]
     type Return;
 
     /// Resumes the execution of this generator.