Only allow feeding a value to newly created definitions.

compiler/rustc_middle/src/lib.rs

@@ -30,8 +30,10 @@
 #![feature(core_intrinsics)]
 #![feature(discriminant_kind)]
 #![feature(exhaustive_patterns)]
+#![feature(generators)]
 #![feature(get_mut_unchecked)]
 #![feature(if_let_guard)]
+#![feature(iter_from_generator)]
 #![feature(negative_impls)]
 #![feature(never_type)]
 #![feature(extern_types)]