Issue Description:

As a user of swift language, a don't feel comfortable with using recursive functions, because I cannot reason about when tail recursion optimization would be used.

To solve this problem an annotation could be used that would give a hint that I would like to use a recursive function. If the tail recursive optimization could not be guaranteed, a warning with the appropriate message could be displayed.

currently:
func fibonacci(n: Int) {
if (n < 2) {
return 1;
} else {
return fibonacci(n-2) + fibonacci(n-1)
}
}

with annotation:
@tail
func fibonacci(n: Int)

if the optimization could not be guaranteed, a warning could be displayed:

@tail < warning: could not guarantee
func fibonacci(n: Int)