do not compute line offsets after the last token

Previously when constructing the root Pairs instance line offsets were
computed for the full input string. If the parse stopped before EOI this
can require a substantial amount of unnecessary work.

pest/src/iterators/flat_pairs.rs

@@ -32,13 +32,14 @@ pub struct FlatPairs<'i, R> {
 pub fn new<'i, R: RuleType>(
     queue: Rc<Vec<QueueableToken<'i, R>>>,
     input: &'i str,
+    line_index: Rc<LineIndex>,
     start: usize,
     end: usize,
 ) -> FlatPairs<'i, R> {
     FlatPairs {
         queue,
         input,
-        line_index: Rc::new(LineIndex::new(input)),
+        line_index,
         start,
         end,
     }

pest/src/iterators/pairs.rs

@@ -51,7 +51,17 @@ pub fn new<'i, R: RuleType>(
 ) -> Pairs<'i, R> {
     let line_index = match line_index {
         Some(line_index) => line_index,
-        None => Rc::new(LineIndex::new(input)),
+        None => {
+            let last_input_pos = queue
+                .last()
+                .map(|token| match *token {
+                    QueueableToken::Start { input_pos, .. }
+                    | QueueableToken::End { input_pos, .. } => input_pos,
+                })
+                .unwrap_or(0);
+
+            Rc::new(LineIndex::new(&input[..last_input_pos]))
+        }
     };
 
     let mut pairs_count = 0;
@@ -205,7 +215,13 @@ impl<'i, R: RuleType> Pairs<'i, R> {
     /// ```
     #[inline]
     pub fn flatten(self) -> FlatPairs<'i, R> {
-        flat_pairs::new(self.queue, self.input, self.start, self.end)
+        flat_pairs::new(
+            self.queue,
+            self.input,
+            self.line_index,
+            self.start,
+            self.end,
+        )
     }
 
     /// Finds the first pair that has its node or branch tagged with the provided