Implement rw handling for types in AttributeContainer role; also fact…

…or attribute composition into that role to take a little more bulk out of ClassHOW.

src/Perl6/Metamodel/AttributeContainer.pm

@@ -2,6 +2,9 @@ role Perl6::Metamodel::AttributeContainer {
     # Attributes list.
     has @!attributes;
     has %!attribute_lookup;
+
+    # Do we default them to rw?
+    has $!attr_rw_by_default;
 
     # Adds an attribute.
     method add_attribute($obj, $meta_attr) {
@@ -14,6 +17,25 @@ role Perl6::Metamodel::AttributeContainer {
         %!attribute_lookup{$name}   := $meta_attr;
     }
 
+    # Composes all attributes.
+    method compose_attributes($obj) {
+        for @!attributes {
+            if $!attr_rw_by_default { $_.default_to_rw() }
+            $_.compose($obj);
+        }
+    }
+
+    # Makes setting the type represented by the meta-object rw mean that its
+    # attributes are rw by default.
+    method set_rw($obj) {
+        $!attr_rw_by_default := 1;
+    }
+
+    # Is this type's attributes rw by default?
+    method rw($obj) {
+        $!attr_rw_by_default
+    }
+
     # Gets the attribute meta-object for an attribute if it exists.
     # This is called by the parser so it should only return attributes
     # that are visible inside the current package.

src/Perl6/Metamodel/ClassHOW.pm

@@ -52,9 +52,7 @@ class Perl6::Metamodel::ClassHOW
         self.incorporate_multi_candidates($obj);
 
         # Compose attributes.
-        for self.attributes($obj, :local) {
-            $_.compose($obj);
-        }
+        self.compose_attributes($obj);
 
         # Publish type and method caches.
         self.publish_type_cache($obj);