Set the source using the full range of affiliated keywords to trailing whitespace.

src/parser/source.rs

@@ -1,3 +1,6 @@
 pub trait Source<'s> {
     fn get_source(&'s self) -> &'s str;
 }
+pub trait SetSource<'s> {
+    fn set_source(&mut self, source: &'s str);
+}