Create attribute accessors. Mostly stolen from nqpclr

src/metamodel/how/NQPAttribute.pm

@@ -26,4 +26,44 @@ knowhow NQPAttribute {
     method box_target() {
         $!box_target ?? 1 !! 0
     }
+
+    method compose($obj) {
+        my $long_name := ~$!name;
+        if self.has_mutator {
+            my $method := pir::substr($long_name, 1);
+            unless has_method($obj, $method, 0) {
+                $obj.HOW.add_method($obj.WHAT, $method, method ($value?) {
+                        pir::setattribute__vppsp(self, $obj.WHAT, $long_name, $value)
+                            if pir::defined($value);
+
+                        pir::getattribute__ppps(self, $obj.WHAT, $long_name);
+                    }
+                );
+            }
+        }
+        else {
+            my $method := pir::substr($long_name, 2);
+            unless has_method($obj, $method, 0) {
+                $obj.HOW.add_method($obj, $method,
+                    method () {
+                        pir::getattribute__PPPs(self, $obj.WHAT, $long_name);
+                    }
+                );
+            }
+        }
+    }
+
+    # Hack to check twigil.
+    method has_mutator() {
+        pir::substr(~$!name, 1, 1) ne '!';
+    }
+
+    sub has_method($target, $name, $local) {
+        my @methods := $target.HOW.methods($target, :local($local));
+        for @methods {
+            if $_ eq $name { return 1; }
+        }
+        return 0;
+    }
+
 }

src/metamodel/how/NQPClassHOW.pm

@@ -146,6 +146,9 @@ knowhow NQPClassHOW {
         # Incorporate any new multi candidates (needs MRO built).
         self.incorporate_multi_candidates($obj);
 
+        # Compose attributes.
+        for self.attributes($obj, :local<0> ) { $_.compose($obj) }
+
         # Publish type and method caches.
         self.publish_type_cache($obj);
         self.publish_method_cache($obj);

t/nqp/58-attrs.t

@@ -0,0 +1,18 @@
+#! nqp
+
+plan(2);
+
+class Foo {
+    has $bar;
+
+    has $!answer;
+    method question() { $!answer := 42 };
+};
+
+my $foo := Foo.new;
+
+$foo.question();
+ok($foo.answer == 42, "Read-only accessors works");
+
+$foo.bar(42);
+ok($foo.bar == 42, 'Read-write accessors works');