Re-introduce crate-wide lint that got disabled by default.

src/cargo/lib.rs

@@ -1,7 +1,7 @@
 // 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))]
+#![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)]