[backport 1.0] add backend support for js bigint (#16606)

* add backend support for js bigint

* cleanup

* add tests

* add -d:nimHasJsBigIntBackend

* cleanup

* more tests

(cherry picked from commit 025ca66)

compiler/condsyms.nim

@@ -123,3 +123,4 @@ proc initDefines*(symbols: StringTableRef) =
   defineSymbol("nimHasStacktracesModule")
   defineSymbol("nimHasEffectTraitsModule")
   defineSymbol("nimHasCastPragmaBlocks")
+  defineSymbol("nimHasJsBigIntBackend")

compiler/jsgen.nim

@@ -1672,7 +1672,10 @@ proc createVar(p: PProc, typ: PType, indirect: bool): Rope =
   var t = skipTypes(typ, abstractInst)
   case t.kind
   of tyInt..tyInt64, tyUInt..tyUInt64, tyEnum, tyChar:
-    result = putToSeq("0", indirect)
+    if $t.sym.loc.r == "bigint":
+      result = putToSeq("0n", indirect)
+    else:
+      result = putToSeq("0", indirect)
   of tyFloat..tyFloat128:
     result = putToSeq("0.0", indirect)
   of tyRange, tyGenericInst, tyAlias, tySink, tyOwned:

tests/js/tbigint_backend.nim

@@ -0,0 +1,58 @@
+proc jsTypeOf*[T](x: T): cstring {.importjs: "typeof(#)".}
+  ## Returns the name of the JsObject's JavaScript type as a cstring.
+  # xxx replace jsffi.jsTypeOf with this definition and add tests
+
+type JsBigIntImpl {.importc: "bigint".} = int
+type JsBigInt = distinct JsBigIntImpl
+
+doAssert JsBigInt isnot int
+func big*(integer: SomeInteger): JsBigInt {.importjs: "BigInt(#)".}
+func big*(integer: cstring): JsBigInt {.importjs: "BigInt(#)".}
+func `<=`*(x, y: JsBigInt): bool {.importjs: "(# $1 #)".}
+func `==`*(x, y: JsBigInt): bool {.importjs: "(# === #)".}
+func inc*(x: var JsBigInt) {.importjs: "[#][0][0]++".}
+func inc2*(x: var JsBigInt) {.importjs: "#++".}
+func toCstring*(this: JsBigInt): cstring {.importjs: "#.toString()".}
+func `$`*(this: JsBigInt): string =
+  $toCstring(this)
+
+block:
+  doAssert defined(nimHasJsBigIntBackend)
+  let z1 = big"10"
+  let z2 = big"15"
+  doAssert z1 == big"10"
+  doAssert z1 == z1
+  doAssert z1 != z2
+  var s: seq[cstring]
+  for i in z1 .. z2:
+    s.add $i
+  doAssert s == @["10".cstring, "11", "12", "13", "14", "15"]
+  block:
+    var a=big"3"
+    a.inc
+    doAssert a == big"4"
+  block:
+    var z: JsBigInt
+    doAssert $z == "0"
+    doAssert z.jsTypeOf == "bigint" # would fail without codegen change
+    doAssert z != big(1)
+    doAssert z == big"0" # ditto
+
+  # ditto below
+  block:
+    let z: JsBigInt = big"1"
+    doAssert $z == "1"
+    doAssert z.jsTypeOf == "bigint"
+    doAssert z == big"1"
+
+  block:
+    let z = JsBigInt.default
+    doAssert $z == "0"
+    doAssert z.jsTypeOf == "bigint"
+    doAssert z == big"0"
+
+  block:
+    var a: seq[JsBigInt]
+    a.setLen 3
+    doAssert a[^1].jsTypeOf == "bigint"
+    doAssert a[^1] == big"0"