I believe the specific advice they're referring to has been stable for a while. You take your generic function & split it into a thin generic wrapper, and a non-generic worker.

As an example, say your function takes anything that can be turned into a String. You'd write a generic wrapper that does the ToString step, then change the existing function to just take a String. That way when your function is called, only the thin outer function is monomorphised, and the bulk of the work is a single implementation.

It's not _that_ commonly known, as it only becomes a problem for a library that becomes popular.

  fn foo<S: Into<String>>(s: S) {
      fn inner(s: String) { ... }
      inner(s.into())
  }