[rust] add support for accesing the list of newline offsets

Author: froydnj

rust/ruby-prism/src/lib.rs

@@ -600,6 +600,17 @@ impl<'pr> ParseResult<'pr> {
         &self.source[start..end]
     }
 
+    /// Returns a slice containing the offsets of the newlines in the source string that
+    /// was parsed.  The slice is guaranteed to be non-empty as Prism considers the source
+    /// string to start with a newline; thus the first offset will always be 0.
+    #[must_use]
+    pub fn newlines(&self) -> &'pr [usize] {
+        unsafe {
+            let list = &(*self.parser.as_ptr()).newline_list;
+            std::slice::from_raw_parts(list.offsets, list.size)
+        }
+    }
+
     /// Returns an iterator that can be used to iterate over the errors in the
     /// parse result.
     #[must_use]
@@ -723,6 +734,27 @@ mod tests {
         }
     }
 
+    #[test]
+    fn newlines_test() {
+        let source = "";
+        let result = parse(source.as_ref());
+
+        let expected: [usize; 1] = [0];
+        assert_eq!(expected, result.newlines());
+
+        let source = "1 + 1";
+        let result = parse(source.as_ref());
+
+        let expected: [usize; 1] = [0];
+        assert_eq!(expected, result.newlines());
+
+        let source = "begin\n1 + 1\n2 + 2\nend";
+        let result = parse(source.as_ref());
+
+        let expected: [usize; 4] = [0, 6, 12, 18];
+        assert_eq!(expected, result.newlines());
+    }
+
     #[test]
     fn magic_comments_test() {
         use crate::MagicComment;