Update 2018 idioms comment.

src/cargo/lib.rs

@@ -1,6 +1,7 @@
-#![cfg_attr(test, deny(warnings))]
-// While we're getting used to 2018:
+// For various reasons, some idioms are still allow'ed, but we would like to
+// test and enforce them.
 #![warn(rust_2018_idioms)]
+#![cfg_attr(test, deny(warnings))]
 // Due to some of the default clippy lints being somewhat subjective and not
 // necessarily an improvement, we prefer to not use them at this time.
 #![allow(clippy::all)]