lang: ast: Add scope feedback for classes

We did this elsewhere for functions, let's add classes too.

lang/ast/structs.go

@@ -6828,6 +6828,9 @@ func (obj *StmtInclude) SetScope(scope *interfaces.Scope) error {
 
 	stmt, exists := scope.Classes[obj.Name]
 	if !exists {
+		if obj.data.Debug || true { // TODO: leave this on permanently?
+			classScopeFeedback(scope, obj.data.Logf)
+		}
 		err := fmt.Errorf("class `%s` does not exist in this scope", obj.Name)
 		return highlightHelper(obj, obj.data.Logf, err)
 	}

lang/ast/util.go

@@ -562,6 +562,21 @@ func lambdaScopeFeedback(scope *interfaces.Scope, logf func(format string, v ...
 	}
 }
 
+// classScopeFeedback logs some messages about what is actually in scope so that
+// the user gets a hint about what's going on. This is useful for catching bugs
+// in our programming or in user code!
+func classScopeFeedback(scope *interfaces.Scope, logf func(format string, v ...interface{})) {
+	logf("classes in scope:")
+	names := []string{}
+	for name := range scope.Classes {
+		names = append(names, name)
+	}
+	sort.Strings(names)
+	for _, name := range names {
+		logf("class %s", name)
+	}
+}
+
 // highlightHelper give the user better file/line number feedback.
 func highlightHelper(node interfaces.Node, logf func(format string, v ...interface{}), err error) error {
 	displayer, ok := node.(interfaces.TextDisplayer)