Fix ref check in VM

[nitely]

Remove unittest2EvalOnceIter(var T): var T to work around nim-lang/Nim#25210 .

Alternatively the template could be defined when not in nimvm, but is it needed?

[arnetheduck]

it's needed to forward the "varness" of something through evalonce which ut2 needs to work with functions that take var params - try removing it from here and you'll see:

iterator unittest2EvalOnceIter[T](x: T): auto =
  yield x
iterator unittest2EvalOnceIter[T](x: var T): var T =
  yield x

template unittest2EvalOnce(name: untyped, param: typed, blk: untyped) =
  for name in unittest2EvalOnceIter(param):
    blk

proc x(v: var int) = echo v
template testit(xxx: typed) =
  unittest2EvalOnce(ff, xxx):
    x(ff)

var v: int
testit(v)

In general, I'm pretty sure that if you try this change with nimbus-eth2 or similar which has a large test suite, it'll show a few breaking examples.

[nitely]

The way it's used in ut2, it would require for the operator proc to only accept var params which seems iffy, but valid I guess:

import ./unittest2

suite "test foo":
  test "test bar":
    type DisInt = distinct int

    proc `==`(a, b: var DisInt): bool =
      a.int == b.int

    proc x(v: var DisInt): var DisInt = v

    var a = 1.DisInt
    var b = a
    check x(a) == x(b)

The check generates:

unittest2EvalOnce(:c2, x(b), unittest2EvalOnce(:c1, x(a)) do:
  block:
    if not(:c1 == :c2):
      checkpoint("/home/esteban/AtomProjects/nim-unittest2/test2.nim(14, 15)" &
          ": Check failed: " &
          "x(a) == x(b)")
      when compiles(string($:c1)):
        checkpoint("x(a)" & " was " & $:c1)
      when compiles(string($:c2)):
        checkpoint("x(b)" & " was " & $:c2)
      fail())

The nimbus-eth2 tests do pass. I'll just define it when not in nimvm to be safe it's not that trivial.