Render code frame with context (#3901)

crates/ruff_python_ast/src/source_code/line_index.rs

@@ -100,6 +100,20 @@ impl LineIndex {
         }
     }
 
+    /// Returns the [byte offset](TextSize) of the `line`'s end.
+    /// The offset is the end of the line, up to and including the newline character ending the line (if any).
+    pub(crate) fn line_end(&self, line: OneIndexed, contents: &str) -> TextSize {
+        let row_index = line.to_zero_indexed();
+        let starts = self.line_starts();
+
+        // If start-of-line position after last line
+        if row_index.saturating_add(1) >= starts.len() {
+            contents.text_len()
+        } else {
+            starts[row_index + 1]
+        }
+    }
+
     /// Returns the [`TextRange`] of the `line` with the given index.
     /// The start points to the first character's [byte offset](TextSize), the end up to, and including
     /// the newline character ending the line (if any).