I hope I'm wrong, but after browsing the SLS, I don't believe you can apply the annotation in the right place without changing your code.
Since annotations "may apply to definitions or declarations, types, or expressions" (Chapter 11), you need one of those for your annotation application to be syntactically correct. The two most likely candidates here seem to be either a type or an expression. However, looking at Chapter 8 Pattern Matching, it seems
Fun(f)
i.e. the statement where you need to apply the annotation to, is neither, since it looks like it corresponds to:
StableId '(' varid ')'
none of which seem to fit the bill for either an expression or a type (or any other valid annotation target).