Return None for too-large offset

Author: azdavis

lib/line-index/src/lib.rs

@@ -84,6 +84,8 @@ impl WideChar {
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct LineIndex {
     /// Offset the beginning of each line (except the first, which always has offset 0).
+    ///
+    /// Invariant: Always non-empty and the last element holds the length of the original text.
     newlines: Box<[TextSize]>,
     /// List of non-ASCII characters on each line.
     line_wide_chars: IntMap<u32, Box<[WideChar]>>,
@@ -125,6 +127,8 @@ impl LineIndex {
             cur_col += c_len;
         }
 
+        newlines.push(TextSize::of(text));
+
         // Save any wide characters seen in the last line
         if !wide_chars.is_empty() {
             line_wide_chars.insert(line, wide_chars.into_boxed_slice());
@@ -143,8 +147,12 @@ impl LineIndex {
     }
 
     /// Transforms the `TextSize` into a `LineCol`, or returns `None` if the `offset` was invalid,
-    /// e.g. if it points to the middle of a multi-byte character.
+    /// e.g. if it extends past the end of the text or points to the middle of a multi-byte
+    /// character.
     pub fn try_line_col(&self, offset: TextSize) -> Option<LineCol> {
+        if offset > *self.newlines.last().unwrap() {
+            return None;
+        }
         let line = self.newlines.partition_point(|&it| it <= offset);
         let start = self.start_offset(line)?;
         let col = offset - start;