Report elision failures on the AST.

compiler/rustc_resolve/src/lib.rs

@@ -11,6 +11,7 @@
 #![feature(drain_filter)]
 #![feature(if_let_guard)]
 #![cfg_attr(bootstrap, feature(let_chains))]
+#![feature(iter_intersperse)]
 #![feature(let_else)]
 #![feature(never_type)]
 #![recursion_limit = "256"]