Nim/tests/pragmas/mpushexperimental.nim

import macros

macro enumerate(x: ForLoopStmt): untyped =
  expectKind x, nnkForStmt
  # we strip off the first for loop variable and use
  # it as an integer counter:
  result = newStmtList()
  result.add newVarStmt(x[0], newLit(0))
  var body = x[^1]
  if body.kind != nnkStmtList:
    body = newTree(nnkStmtList, body)
  body.add newCall(bindSym"inc", x[0])
  var newFor = newTree(nnkForStmt)
  for i in 1..x.len-3:
    newFor.add x[i]
  # transform enumerate(X) to 'X'
  newFor.add x[^2][1]
  newFor.add body
  result.add newFor

proc main*[T](x: T) =
  {.push experimental: "forLoopMacros".}

  for a, b in enumerate(items([1, 2, 3])):
    echo a, " ", b

  for a2, b2 in enumerate([1, 2, 3, 5]):
    echo a2, " ", b2
  {.pop.}