LibWeb: Add code generation for reflecting IDL attributes

You can now tag reflecting attributes with [Reflect] to generate code
that does basic DOM element attribute get/set.

(This patch also makes it easy to add more extended attributes like
that going forward.)

From the HTML spec:

"Some IDL attributes are defined to reflect a particular content
attribute. This means that on getting, the IDL attribute returns
the current value of the content attribute, and on setting,
the IDL attribute changes the value of the content attribute
to the given value."

Libraries/LibWeb/CodeGenerators/WrapperGenerator.cpp

@@ -25,6 +25,7 @@
  */
 
 #include <AK/ByteBuffer.h>
+#include <AK/HashMap.h>
 #include <AK/StringBuilder.h>
 #include <LibCore/ArgsParser.h>
 #include <LibCore/File.h>
@@ -65,6 +66,7 @@ struct Function {
     Type return_type;
     String name;
     Vector<Parameter> parameters;
+    HashMap<String, String> extended_attributes;
 
     size_t length() const
     {
@@ -78,6 +80,7 @@ struct Attribute {
     bool unsigned_ { false };
     Type type;
     String name;
+    HashMap<String, String> extended_attributes;
 
     // Added for convenience after parsing
     String getter_callback_name;
@@ -174,7 +177,7 @@ OwnPtr<Interface> parse_interface(const StringView& input)
         return Type { name, nullable };
     };
 
-    auto parse_attribute = [&] {
+    auto parse_attribute = [&](HashMap<String, String>& extended_attributes) {
         bool readonly = false;
         bool unsigned_ = false;
         if (next_is("readonly")) {
@@ -202,10 +205,11 @@ OwnPtr<Interface> parse_interface(const StringView& input)
         attribute.name = name;
         attribute.getter_callback_name = String::format("%s_getter", snake_name(attribute.name).characters());
         attribute.setter_callback_name = String::format("%s_setter", snake_name(attribute.name).characters());
+        attribute.extended_attributes = move(extended_attributes);
         interface->attributes.append(move(attribute));
     };
 
-    auto parse_function = [&] {
+    auto parse_function = [&](HashMap<String, String>& extended_attributes) {
         auto return_type = parse_type();
         consume_whitespace();
         auto name = consume_while([](auto ch) { return !isspace(ch) && ch != '('; });
@@ -228,22 +232,45 @@ OwnPtr<Interface> parse_interface(const StringView& input)
 
         consume_specific(';');
 
-        interface->functions.append(Function { return_type, name, move(parameters) });
+        interface->functions.append(Function { return_type, name, move(parameters), move(extended_attributes) });
+    };
+
+    auto parse_extended_attributes = [&] {
+        HashMap<String, String> extended_attributes;
+        for (;;) {
+            consume_whitespace();
+            if (consume_if(']'))
+                break;
+            auto name = consume_while([](auto ch) { return ch != ']' && ch != '=' && ch != ','; });
+            if (consume_if('=')) {
+                auto value = consume_while([](auto ch) { return ch != ']' && ch != ','; });
+                extended_attributes.set(name, value);
+            } else {
+                extended_attributes.set(name, {});
+            }
+        }
+        consume_whitespace();
+        return extended_attributes;
     };
 
     for (;;) {
+        HashMap<String, String> extended_attributes;
 
         consume_whitespace();
 
         if (consume_if('}'))
             break;
 
+        if (consume_if('[')) {
+            extended_attributes = parse_extended_attributes();
+        }
+
         if (next_is("readonly") || next_is("attribute")) {
-            parse_attribute();
+            parse_attribute(extended_attributes);
             continue;
         }
 
-        parse_function();
+        parse_function(extended_attributes);
     }
 
     interface->wrapper_class = String::format("%sWrapper", interface->name.characters());
@@ -569,7 +596,13 @@ void generate_implementation(const IDL::Interface& interface)
         out() << "    auto* impl = impl_from(interpreter, global_object);";
         out() << "    if (!impl)";
         out() << "        return {};";
-        out() << "    auto retval = impl->" << snake_name(attribute.name) << "();";
+
+        if (attribute.extended_attributes.contains("Reflect")) {
+            out() << "    auto retval = impl->attribute(HTML::AttributeNames::" << attribute.name << ");";
+        } else {
+            out() << "    auto retval = impl->" << snake_name(attribute.name) << "();";
+        }
+
         generate_return_statement(attribute.type);
         out() << "}";
 
@@ -582,7 +615,11 @@ void generate_implementation(const IDL::Interface& interface)
 
             generate_to_cpp(attribute, "value", "", "cpp_value", true);
 
-            out() << "    impl->set_" << snake_name(attribute.name) << "(cpp_value);";
+            if (attribute.extended_attributes.contains("Reflect")) {
+                out() << "    impl->set_attribute(HTML::AttributeNames::" << attribute.name << ", cpp_value);";
+            } else {
+                out() << "    impl->set_" << snake_name(attribute.name) << "(cpp_value);";
+            }
             out() << "}";
         }
     }