message: Update 2018 idioms comment.
diff: -#![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)]
  