Array size self mutation or string concatenation issue

[codec-abc]

In the following code we create an array of 56 bytes and never add or delete element. Then, we call a behavior that will print the array size and call itself again. The first calls print the correct size (56) but at some point in time, it starts to print other (wrong) values. After a few iterations I got the following output:

in behavior - array size is 47 in behavior
in behavior - array size is 8241904171254706025

I tested it on Windows with the version 0.19.3-6a0dd3a [release] and compiled it with ponyc -d

use "collections"
use "debug"

struct ArrayWrapper
  var array : Array[U8]
  new create() =>
        array = Array[U8]()
        for i in Range(0, 56) do
            array.push(0)
        end
        Debug.out("constructor - array size is " + array.size().string())
  

actor Main
  var array_wrapper : ArrayWrapper
  
  new create(env: Env) =>
    array_wrapper = ArrayWrapper
    Debug.out("after assignment - array size is " + array_wrapper.array.size().string())
    a_call()
    
  be a_call() =>
    Debug.out("in behavior - array size is " + array_wrapper.array.size().string())
    a_call()

Since I reported it on the IRC channel first, @Praetonus has managed to reproduce it and started to investigate it and thinks it is a bug in the string concatenation.

[codec-abc]

I ran more tests on it on found that the bug is only triggered because using a struct. If using a class instead the bug is never triggered.

[jemc]

@Praetonus - if you're not actively working on this, can you dump what you learned in your investigation so far, and then unassign yourself here, so that someone else might be able to pick it up?

[Praetonus]

This is a GC issue with structs where trace functions aren't getting generated, which in this case causes a premature collection of the inner array. This should be an easy fix.