make more things compile without isNil

lib/pure/collections/sets.nim

@@ -654,7 +654,7 @@ proc isValid*[A](s: OrderedSet[A]): bool =
   ##   proc saveTarotCards(cards: OrderedSet[int]) =
   ##     assert cards.isValid, "Pass an initialized set!"
   ##     # Do stuff here, may crash in release builds!
-  result = not s.data.isNil
+  result = s.data.len > 0
 
 proc len*[A](s: OrderedSet[A]): int {.inline.} =
   ## Returns the number of keys in `s`.

tests/gc/gctest.nim

@@ -66,8 +66,7 @@ proc caseTree(lvl: int = 0): PCaseNode =
 proc finalizeNode(n: PNode) =
   assert(n != nil)
   write(stdout, "finalizing: ")
-  if isNil(n.data): writeLine(stdout, "nil!")
-  else: writeLine(stdout, "not nil")
+  writeLine(stdout, "not nil")
 
 var
   id: int = 1

tools/nimgrep.nim

@@ -315,7 +315,7 @@ checkOptions({optFilenames, optReplace}, "filenames", "replace")
 
 if optStdin in options:
   pattern = ask("pattern [ENTER to exit]: ")
-  if isNil(pattern) or pattern.len == 0: quit(0)
+  if pattern.len == 0: quit(0)
   if optReplace in options:
     replacement = ask("replacement [supports $1, $# notations]: ")