[Macros] Unique VM Id for ref objects and NimNode

[mratsim]

I'm quite often in need of an unique identifier in macros to use NimNodes and other ref types in Tables and HashSets.

Looking into the implementation of == for ref types (mEqRef magic), it seems like they have a rkNodeAddr that could serve this purpose.

The current alternative is to have a compile time Id with a scheme similar to this.

type
  MyRef = ref object
    value: int

    # Cannot use when nimvm for object fields
    id: int

# 1. First, show that at compile-time ref objects are still considered different
static:
  let x = MyRef(value: 0, id: 0)
  let y = MyRef(value: 0, id: 0)
  let z = y

  assert x != y
  assert x != z
  assert y == z

# 2. Now the workaround needed at compile-time to store them in HashSet/HashTables
import random, hashes

var myRefRng {.compileTime.} = initRand(0x42)

proc newRef(value: int): MyRef =
  new result
  result.value = value

  when nimvm:
    result.id = myRefRng.rand(high(int))
  else: # Necessary otherwise illegal context
    discard

proc hash(x: MyRef): Hash =
  when nimvm:
    Hash(x.id)
  else:
    cast[Hash](x)

[Araq]

I think reference semantics are pretty bad and the hash should capture the structure of the tree. Nodes do not have an identity, they can be copied.

[mratsim]

Currently I use = in macros when I want shallow copies and copyNimTree when I want deep copy.

In my case I want ref semantics because I want to use my type at both compile-time and runtime, having different semantics would be harder to maintain.

Furthermore I'm using this to implement my own AST and reference semantics are easier to work with for scoping:

block:
  let a{.compileTime.} = newRef(10)

block:
  let a{.compileTime.} = newRef(10)

^ The a are not the same.

[github-actions]

This RFC is stale because it has been open for 1095 days with no activity. Contribute a fix or comment on the issue, or it will be closed in 7 days.