empty or pattern message

compiler/reporting/src/error/parse.rs

@@ -198,6 +198,7 @@ fn to_when_report<'a>(
             }
             _ => to_syntax_report(alloc, filename, nested, row, col),
         },
+        When::Pattern(ref pat, row, col) => to_pattern_report(alloc, filename, pat, row, col),
         _ => todo!("unhandled parse error: {:?}", parse_problem),
     }
 }
@@ -206,12 +207,28 @@ fn to_pattern_report<'a>(
     alloc: &'a RocDocAllocator<'a>,
     filename: PathBuf,
     parse_problem: &roc_parse::parser::EPattern<'a>,
-    _start_row: Row,
+    start_row: Row,
-    _start_col: Col,
+    start_col: Col,
 ) -> Report<'a> {
     use roc_parse::parser::EPattern;
 
     match parse_problem {
+        EPattern::Start(row, col) => {
+            let surroundings = Region::from_rows_cols(start_row, start_col, *row, *col);
+            let region = Region::from_row_col(*row, *col);
+
+            let doc = alloc.stack(vec![
+                alloc.reflow(r"I just started parsing a pattern, but I got stuck here:"),
+                alloc.region_with_subregion(surroundings, region),
+                alloc.note("I may be confused by indentation"),
+            ]);
+
+            Report {
+                filename,
+                doc,
+                title: "UNFINISHED PATTERN".to_string(),
+            }
+        }
         EPattern::Record(record, row, col) => {
             to_precord_report(alloc, filename, &record, *row, *col)
         }

compiler/reporting/tests/test_reporting.rs

@@ -4774,6 +4774,7 @@ mod test_reporting {
             ),
         )
     }
+
     #[test]
     fn if_guard_without_condition() {
         // this should get better with time
@@ -4804,6 +4805,35 @@ mod test_reporting {
         )
     }
 
+    #[test]
+    fn empty_or_pattern() {
+        // this should get better with time
+        report_problem_as(
+            indoc!(
+                r#"
+                when Just 4 is
+                    Just 4 | ->
+                        4
+
+                    _ ->
+                        2
+                "#
+            ),
+            indoc!(
+                r#"
+                ── UNFINISHED PATTERN ──────────────────────────────────────────────────────────
+                
+                I just started parsing a pattern, but I got stuck here:
+                
+                2│      Just 4 | ->
+                                 ^
+                
+                Note: I may be confused by indentation
+            "#
+            ),
+        )
+    }
+
     #[test]
     #[ignore]
     fn pattern_binds_keyword() {