gistfile1.p6

role SomeTrait {
  method apply_handles($attr: Mu $pkg) {
    say "apply_handles";
    my $name = $attr.name;
    my $accessor = $name.subst(/^../, '');
    my $r = sub ($obj, |args) is rw {
      say "$name accessor";
      nextsame;
    };
    $pkg.HOW.find_method($pkg, $accessor).wrap($r);
  }

  method foo { say "fooing" }
}

multi trait_mod:<is>(Attribute $var, :$wtf!) {
  die "Must have accessor" unless $var.has-accessor;
  $var.set_rw;
  say "456";
  say $var;
  $var does SomeTrait;
  $var.foo;
}

class foo {
  has $.x is wtf;
}

say "begin";

my $foo = foo.new;#(x => 32);
my $bar = foo.new(x => 32);
say $foo.x;
say $bar.x;
say "---";
$foo.x = 64;
say "...";

dies with:

456
Mu $!x
fooing
apply_handles
begin
$!x accessor
(Any)
$!x accessor
32

---
$!x accessor
Cannot assign to a readonly variable or a value
  in block  at foo.6:36