add let chain feature

Author: matthiaskrgr

compiler/rustc_passes/src/lib.rs

@@ -8,6 +8,7 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(iter_intersperse)]
 #![feature(let_else)]
+#![feature(let_chains)]
 #![feature(map_try_insert)]
 #![feature(min_specialization)]
 #![feature(try_blocks)]