generalized when nimvm:

[timotheecour]

/cc @xflywind you suggested:
VM and gen should use separate ConfigRef/options but I'm not sure how that'd work.
I'm opening this as a place holder to discuss this.

This is also very related to when nimvm construct in compiler; the current implementation for nimvm has many known limitations which make it more awkward to use than we'd like.

example 1

int.sizeof is 4 for js + rt and therefore also 4 for js+vm even on a 64bit platform

example 2

see all bugs/limitations of when nimvm

very rough idea

allow this (currently impossible):

when nimvm:
  type Foo = object  # only visible in VM
    x1: int
  proc fn(): int = # only visible in VM
    1
else:
  type Foo = object  # only visible in RT
    x2: float
  proc fn(): int = # only visible in RT
    2

static: doAssert fn() == 1
doAssert fn() == 2
static: doAssert Foo.default.x1 is int
doAssert Foo.default.x2 is float