report a Foo.Bar malformed identifier

compiler/can/src/expr.rs

@@ -725,10 +725,11 @@ pub fn canonicalize_expr<'a>(
         }
         ast::Expr::MalformedIdent(name) => {
             use roc_problem::can::RuntimeError::*;
-            (
-                RuntimeError(MalformedIdentifier((*name).into(), region)),
-                Output::default(),
-            )
+
+            let problem = MalformedIdentifier((*name).into(), region);
+            env.problem(Problem::RuntimeError(problem.clone()));
+
+            (RuntimeError(problem), Output::default())
         }
         ast::Expr::Nested(sub_expr) => {
             let (answer, output) = canonicalize_expr(env, var_store, scope, region, sub_expr);

compiler/reporting/src/error/canonicalize.rs

@@ -427,8 +427,15 @@ fn pretty_runtime_error<'b>(
             // do nothing, reported with PrecedenceProblem
             unreachable!()
         }
-        RuntimeError::MalformedIdentifier(_, _) => {
-            todo!("malformed identifier, currently gives a parse error and thus is unreachable")
+        RuntimeError::MalformedIdentifier(box_str, region) => {
+            alloc.stack(vec![
+                alloc.concat(vec![
+                    alloc.reflow("The ")
+                    .append(format!("`{}`", box_str))
+                    .append(alloc.reflow(" identifier is malformed:")),
+                ]),
+                alloc.region(region),
+            ])
         }
         RuntimeError::MalformedClosure(_) => todo!(""),
         RuntimeError::InvalidFloat(sign @ FloatErrorKind::PositiveInfinity, region, _raw_str)

compiler/reporting/tests/test_reporting.rs

@@ -3930,4 +3930,25 @@ mod test_reporting {
             ),
         )
     }
+
+    #[test]
+    fn qualified_global_tag() {
+        report_problem_as(
+            indoc!(
+                r#"
+                Foo.Bar
+                "#
+            ),
+            indoc!(
+                r#"
+                ── SYNTAX PROBLEM ──────────────────────────────────────────────────────────────
+
+                The `Foo.Bar` identifier is malformed:
+
+                1│  Foo.Bar
+                    ^^^^^^^
+            "#
+            ),
+        )
+    }
 }