WIP: an API for VM replay global state support

lib/core/macrocache.nim

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+#
+#
+#            Nim's Runtime Library
+#        (c) Copyright 2018 Andreas Rumpf
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+## This module provides an API for macros that need to collect compile
+## time information across module boundaries in global variables.
+## Starting with version 0.19 of Nim this is not directly supported anymore
+## as it breaks incremental compilations.
+## Instead the API here needs to be used. See XXX (wikipedia page) for a
+## theoretical foundation behind this.
+
+type
+  CacheSeq* = distinct string
+  CacheTable* = distinct string
+  CacheCounter* = distinct string
+
+proc value*(c: CacheCounter): int {.magic: "NccValue".}
+proc inc*(c: CacheCounter; by = 1) {.magic: "NccInc".}
+
+proc add*(s: CacheSeq; value: NimNode) {.magic: "NcsAdd".}
+proc incl*(s: CacheSeq; value: NimNode) {.magic: "NcsIncl".}
+proc len*(s: CacheSeq): int {.magic: "NcsLen".}
+proc `[]`*(s: CacheSeq; i: int): NimNode {.magic: "NcsAt".}
+
+iterator items*(s: CacheSeq): NimNode =
+  for i in 0 ..< len(s): yield s[i]
+
+proc `[]=`*(t: CacheTable; key: string, value: NimNode) {.magic: "NctPut".}
+  ## 'key' has to be unique!
+
+proc len*(t: CacheTable): int {.magic: "NctLen".}
+proc `[]`*(t: CacheTable; key: string): NimNode {.magic: "NctGet".}
+
+proc hasNext(t: CacheTable; iter: int): bool {.magic: "NctHasNext".}
+proc next(t: CacheTable; iter: int): (string, NimNode, int) {.magic: "NctNext".}
+
+iterator pairs*(t: CacheTable): (string, NimNode) =
+  var h = 0
+  while hasNext(t, h):
+    let (a, b, h2) = next(t, h)
+    yield (a, b)
+    h = h2