line-index method to allow clamping column to line length

Part of #18240

lib/line-index/src/tests.rs

@@ -195,3 +195,26 @@ fn test_every_chars() {
         }
     }
 }
+
+#[test]
+fn test_line() {
+    use text_size::TextRange;
+
+    macro_rules! validate {
+        ($text:expr, $line:expr, $expected_start:literal .. $expected_end:literal) => {
+            let line_index = LineIndex::new($text);
+            assert_eq!(
+                line_index.line($line),
+                Some(TextRange::new(
+                    TextSize::from($expected_start),
+                    TextSize::from($expected_end)
+                ))
+            );
+        };
+    }
+
+    validate!("", 0, 0..0);
+    validate!("\n", 1, 1..1);
+    validate!("\nabc", 1, 1..4);
+    validate!("\nabc\ndef", 1, 1..5);
+}