Enable --gc:orc testing for nim-chronos (#161)

* enable --gc:orc testing on nim devel (1.5.x)

chronos.nimble

@@ -13,12 +13,15 @@ requires "nim > 1.2.0",
          "httputils"
 
 task test, "Run all tests":
-  var commands = [
+  var commands = @[
     "nim c -r -d:useSysAssert -d:useGcAssert tests/",
     "nim c -r -d:chronosStackTrace tests/",
     "nim c -r -d:release tests/",
     "nim c -r -d:release -d:chronosFutureTracking tests/"
   ]
+  when (NimMajor, NimMinor) >= (1, 5):
+    commands.add "nim c -r --gc:orc -d:chronosFutureTracking -d:release -d:chronosStackTrace tests/"
+
   for testname in ["testall"]:
     for cmd in commands:
       let curcmd = cmd & testname

chronos/transports/common.nim

@@ -516,10 +516,16 @@ type
     length, reserved: int
 
 proc isLiteral*(s: string): bool {.inline.} =
-  (cast[ptr SeqHeader](s).reserved and (1 shl (sizeof(int) * 8 - 2))) != 0
+  when defined(gcOrc) or defined(gcArc):
+    false
+  else:
+    (cast[ptr SeqHeader](s).reserved and (1 shl (sizeof(int) * 8 - 2))) != 0
 
 proc isLiteral*[T](s: seq[T]): bool {.inline.} =
-  (cast[ptr SeqHeader](s).reserved and (1 shl (sizeof(int) * 8 - 2))) != 0
+  when defined(gcOrc) or defined(gcArc):
+    false
+  else:
+    (cast[ptr SeqHeader](s).reserved and (1 shl (sizeof(int) * 8 - 2))) != 0
 
 when defined(windows):
   import winlean