CStructs do not support Str is rw Attributes

[Xliff]

The Problem

This should create a CStruct backed object with a string attribute that allows changing.

class A is repr('CStruct') { 
   has Str $.s is rw; 
}
my $a = A.new( s => 'Hello' );

But instead, this occurs:

rakudo-moar 6d58e0b: OUTPUT: «Cannot assign to an immutable value␤ in block at line 1␤␤»

Expected Behavior

my $a = A.new( s => 'Hello' );

Should create a new object $a with $a.s containing the string 'Hello';

For easy golfing, the entire string used to illustrate the problem is:

m: class A is repr('CStruct') { has Str $.s is rw; }; my $a = A.new( s => 'Hello' );

Oddly enough, the solution involves using nqp;

class A is repr('CStruct') { 
  has Str $!s; 

  method s is rw { 
    use nqp; 
    Proxy.new: 
      FETCH => -> $ {$!s}, 
      STORE => -> $, $new { nqp::bindattr( nqp::decont(self), A, '$!s', nqp::unbox_s(~$new) ) } 
  }
}
my $a = A.new; $a.s = Hello'; 
$a.s.say

Again, this can be replicated in IRC using:

m: class A is repr('CStruct') { has Str $!s; method s is rw { use nqp; Proxy.new: FETCH => -> $ {$!s}, STORE => -> $, $new { nqp::bindattr( nqp::decont(self), A, '$!s', nqp::unbox_s(~$new) ); }; }; }; my $a = A.new; $a.s = 'Hello'; $a.s.say

This trick was discovered by looking through old code from FROGGS in p6-XML-LibXML.

Environment

This was found on the #perl6 IRC bot camelia.
rakudo (2018.12.165.g.6.d.58.e.0.b.0.b)

[Xliff]

After talking to tadzik++ in IRC, I have learned about the "str" native variant of Str, which works as expected.

There is still an issue of assigning CPointer members using "is rw", but that is beyond the scope of this error.

Closing.