fix off-by-one error

compiler/reporting/src/report.rs

@@ -294,7 +294,9 @@ impl ReportText {
             Region(region) => {
                 buf.push('\n');
                 buf.push('\n');
-                let max_line_number_length = region.end_line.to_string().len();
+
+                // widest displayed line number
+                let max_line_number_length = (region.end_line + 1).to_string().len();
 
                 if region.start_line == region.end_line {
                     let i = region.start_line;

compiler/reporting/tests/test_reporting.rs

@@ -621,6 +621,43 @@ mod test_reporting {
         );
     }
 
+    #[test]
+    fn report_region_line_number_length_edge_case() {
+        // the highest line number is 9, but it's rendered as 10.
+        // Make sure that we render the line number as 2-wide
+        report_renders_as_from_src(
+            indoc!(
+                r#"
+
+
+
+
+                    x = 1
+                    y = 2
+                    f = \a -> a + 4
+
+                    f x
+                "#
+            ),
+            to_simple_report(Region(roc_region::all::Region {
+                start_line: 7,
+                end_line: 9,
+                start_col: 0,
+                end_col: 0,
+            })),
+            indoc!(
+                r#"
+
+
+                 8 ┆>
+                 9 ┆>  f x
+                10 ┆>
+
+                "#
+            ),
+        );
+    }
+
     #[test]
     fn report_region_different_line_number_lengths() {
         report_renders_as_from_src(