Support for -Wcompletion-handler in the Swift compiler
Rationale
While async exists in the Swift language, a lot of code still uses the old completion handler style to implement asynchronous concerns.
When coding in Objective-C, you can use -Wcompletion-handler to have Clang analyse your code at compile time and show a warning when a completion handler is not called / used on all paths. This is invaluable because completions tend to be error prone, and you can (should, IMO) turn this warning into a compile error with -Werror=completion-handler.
Issue
Unfortunately, this only protects your Objective-C code. The Swift compiler does not have a corresponding warning. What could be done to have a similar feature?