[TODO] VM: allow const ref objects

[timotheecour]

this PR enables const ref objects.
It seems to work, and makes it usable to use more code at CT.
The pre-existing caveat (when the ref object points to a case object) is a separate, pre-existing issue (see #13081) that will get fixed eventually, but shouldn't block this; the problem is the case-object not the ref object

example use case1: with ref object

type Config = ref object
  paths: seq[string]
  name: string

const j = parseFile("cfgFile.json").to(Config)
# now you can use j.paths, j.name etc at runtime; no caveats

example use case2: even useful with ref case objects

import std/json
const j = parseFile("cfgFile.json")
const paths = j["paths"].to(seq[string]) # extract const field to avoid caveat mentioned in https://github.com/nim-lang/Nim/issues/13081
const name = j["name"].to(string)
# now you can use paths, name at runtime

example use case3: const TableRef

from here: #13430 (comment)

import tables
const t = {1:"foo", 2:"bar"}.newTable
doAssert $t == """{1: "foo", 2: "bar"}"""
doAssert t is TableRef
doAssert t[1] == "foo"
static: doAssert t[1] == "foo"

[Araq]

Er what? The C codegen does not support that and it bites the GCs.

[Araq]

No feedback, closing for now as I noticed Timothee won't mind.

[timotheecour]

/cc @Araq still working on this (had a bunch of other PRs in the meantime) I had written additional tests which I didn't push yet, it still seems to all just work except for the above mentioned case objects #13081 which is a separate issue; I can hide this behind an experimental flag if needed in the meantime until #13081 is addressed , but it does solve practical problems and makes nim less fragmented bw VM vs RT; I'm not sure what you mean by bites the GCs , do you have an example where this would fail (that isn't #13081) ?

[Araq]

For example:

const x = ConfigRef(...)
var y = x # GC's write barrier assumes this has a PCell header, but it doesn't!
#--> crash.

[timotheecour]

@Araq I'm not seeing this; if I use:

import compiler/options
const x = ConfigRef()

it simply gives a CT error invalid type for const: ConfigRef and after investigation that's because ConfigRef contains a object of RootObj field, which is out of scope for this PR; likewise with simply:

  type
    Bar = object of RootObj
  const j = Bar()

however if I don't use a object of RootObj object, I dont' get a crash:

  type Foo = ref object
    x1: int
    x2: string
    x3: seq[string]
  proc main() =
    let n = 1_000000
    for i in 0..<n:
      const j2 = Foo(x1: 12, x2: "asdf", x3: @["foo", "bar"])
      var j2b = j2
      echo j2b[]
  main()

=> no crash

(and even if it did crash, the PR could be updated to forbid var x = val where val is a const ref, if needed)