First sketch of mixin support.

src/Perl6/Metamodel/ClassHOW.pm

@@ -8,6 +8,7 @@ class Perl6::Metamodel::ClassHOW
     does Perl6::Metamodel::RoleContainer
     does Perl6::Metamodel::MultipleInheritance
     does Perl6::Metamodel::C3MRO
+    does Perl6::Metamodel::Mixins
     does Perl6::Metamodel::NonGeneric
     does Perl6::Metamodel::ParrotInterop
 {

src/Perl6/Metamodel/Mixins.pm

@@ -0,0 +1,21 @@
+role Perl6::Metamodel::Mixins {
+    method mixin($obj, *@roles) {
+        # Work out a type name for the post-mixed-in role.
+        my @role_names;
+        for @roles { @role_names.push($_.HOW.name($_)) }
+        my $new_name := self.name($obj) ~ '+{' ~
+            pir::join(',', @role_names) ~ '}';
+
+        # Create new type, derive it from ourself and then add
+        # all the roles we're mixing it.
+        my $new_type := self.new_type(:name($new_name));
+        $new_type.HOW.add_parent($new_type, $obj.WHAT);
+        for @roles {
+            $new_type.HOW.add_role($new_type, $_);
+        }
+        $new_type.HOW.compose($new_type);
+
+        # Call low-level op to do the actual mixing in.
+        pir::repr_change_type__0PP($obj, $new_type)
+    }
+}

tools/build/Makefile.in

@@ -93,6 +93,7 @@ METAMODEL_SOURCES = \
   src/Perl6/Metamodel/RoleContainer.pm \
   src/Perl6/Metamodel/MultipleInheritance.pm \
   src/Perl6/Metamodel/C3MRO.pm \
+  src/Perl6/Metamodel/Mixins.pm \
   src/Perl6/Metamodel/ParrotInterop.pm \
   src/Perl6/Metamodel/TypePretence.pm \
   src/Perl6/Metamodel/RolePunning.pm \