Suppose you absentmindedly type try in a function declaration when you meant to type throws. Swift's diagnostic in this situation is not very good:
A message along the lines of "expected throwing specifier; did you mean throws?" with a fix-it would provide a much nicer experience.
To implement this, you would find the place(s) in the parser where we parse the "throws" keyword and attempt to parse the "try" keyword there if "throws" fails. If you parse it, you should emit the diagnostic and then have the compiler pretend that it parsed "throws" to recover from the error. We're already doing this for "throw", so you might just be able to add "try" to the same code path.