I believe instead we should require the label never be dropped. I dislike trailing closures in general because they serve no practical purpose except to not have to write a ) at the end of the closure, and it encourages a pattern that diverges from the declaration site, but a happy medium for me would that the label MUST be used and never elided.
array.drop while: { criteria }
The use of labels is a core feature and guideline for Swift. The very fact that we ever allowed them to be dropped is contradictory to a tenets of the language.