test for malformed header

compiler/reporting/src/error/parse.rs

@@ -3263,6 +3263,34 @@ fn to_requires_report<'a>(
             }
         }
 
+        ERequires::Rigid(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 am partway through parsing a header, but I got stuck here:"),
+                alloc.region_with_subregion(surroundings, region),
+                alloc.concat(vec![
+                    alloc.reflow("I am expecting a list of rigids like "),
+                    alloc.keyword("{}"),
+                    alloc.reflow(" or "),
+                    alloc.keyword("{model=>Model}"),
+                    alloc.reflow(" next. A full "),
+                    alloc.keyword("requires"),
+                    alloc.reflow(" definition looks like"),
+                ]),
+                alloc
+                    .parser_suggestion("requires {model=>Model, msg=>Msg} {main : Effect {}} ")
+                    .indent(4),
+            ]);
+
+            Report {
+                filename,
+                doc,
+                title: "BAD RIGIDS".to_string(),
+            }
+        }
+
         _ => todo!("unhandled parse error {:?}", parse_problem),
     }
 }

compiler/reporting/tests/test_reporting.rs

@@ -5929,6 +5929,43 @@ mod test_reporting {
         )
     }
 
+    #[test]
+    fn platform_requires_rigids() {
+        report_header_problem_as(
+            indoc!(
+                r#"
+                platform folkertdev/foo
+                    requires { main : Effect {} }
+                    exposes []
+                    packages {}
+                    imports [Task]
+                    provides [ mainForHost ]
+                    effects fx.Effect
+                        {
+                            putChar : I64 -> Effect {},
+                            putLine : Str -> Effect {},
+                            getLine : Effect Str
+                        }
+                "#
+            ),
+            indoc!(
+                r#"
+                ── WEIRD PROVIDES ──────────────────────────────────────────────────────────────
+
+                I am partway through parsing a provides list, but I got stuck here:
+
+                3│      imports [base.Task, Base64 ]
+                4│      provides [ main, @Foo ] to base
+                                         ^
+
+                I was expecting a type name, value name or function name next, like
+
+                    provides [ Animal, default, tame ]
+            "#
+            ),
+        )
+    }
+
     #[test]
     fn exposes_identifier() {
         report_header_problem_as(