Replace some once(x).chain(once(y)) with [x, y] IntoIter

Now that we have by-value array iterators...

compiler/rustc_trait_selection/src/lib.rs

@@ -11,6 +11,7 @@
 //! This API is completely unstable and subject to change.
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
+#![feature(array_value_iter)]
 #![feature(bool_to_option)]
 #![feature(box_patterns)]
 #![feature(drain_filter)]