Fix review comment

compiler/rustc_passes/src/intrinsicck.rs

@@ -411,7 +411,11 @@ impl<'tcx> ExprVisitor<'tcx> {
                             let msg = format!(
                                 "register class `{}` requires at least one of the following target features: {}",
                                 reg_class.name(),
-                                features.iter().map(|f| f.as_str()).collect::<Vec<_>>().join(", ")
+                                features
+                                    .iter()
+                                    .map(|f| f.as_str())
+                                    .intersperse(", ")
+                                    .collect::<String>(),
                             );
                             self.tcx.sess.struct_span_err(*op_sp, &msg).emit();
                             // register isn't enabled, don't do more checks

compiler/rustc_passes/src/lib.rs

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