How can I preserve the original newline character(whether Newline or Carriage Return) when no rules are applied?

[sangwoo-joh]

For example, for the below code where ^M as carriage return,

def foo(a: str,^M
            b: str):^M
   ...

And I have a rule for rewriting the wrongly typed string to str as follow:

match="def :[fun](:[args])"
rewrite="def :[fun](:[args])"
rule='where rewrite :[exp] { ":[var]: string" -> ":[var]: str" }'

Since there is no violation in the above code, I didn't expect any modifications to be made. However, after I ran the rule, I found that the original ^Ms were disappeared:

def foo(a: str,
            b: str):
   ...

How can I preserve the original newline character?