Use map_while instead of take_while + map

compiler/rustc_data_structures/src/lib.rs

@@ -27,6 +27,8 @@
 #![feature(min_type_alias_impl_trait)]
 #![allow(rustc::default_hash_types)]
 #![deny(unaligned_references)]
+#![feature(iter_map_while)]
+#![feature(bool_to_option)]
 
 #[macro_use]
 extern crate tracing;