diff --git a/compiler/ast.nim b/compiler/ast.nim
index a722f63f6f..832463bd57 100644
--- a/compiler/ast.nim
+++ b/compiler/ast.nim
@@ -1020,13 +1020,16 @@ proc isCallExpr*(n: PNode): bool =
 proc discardSons*(father: PNode)
 
 proc len*(n: PNode): int {.inline.} =
-  if isNil(n.sons): result = 0
-  else: result = len(n.sons)
+  when defined(nimNoNilSeqs):
+    result = len(n.sons)
+  else:
+    if isNil(n.sons): result = 0
+    else: result = len(n.sons)
 
 proc safeLen*(n: PNode): int {.inline.} =
   ## works even for leaves.
-  if n.kind in {nkNone..nkNilLit} or isNil(n.sons): result = 0
-  else: result = len(n.sons)
+  if n.kind in {nkNone..nkNilLit}: result = 0
+  else: result = len(n)
 
 proc safeArrLen*(n: PNode): int {.inline.} =
   ## works for array-like objects (strings passed as openArray in VM).
@@ -1036,7 +1039,8 @@ proc safeArrLen*(n: PNode): int {.inline.} =
 
 proc add*(father, son: PNode) =
   assert son != nil
-  if isNil(father.sons): father.sons = @[]
+  when not defined(nimNoNilSeqs):
+    if isNil(father.sons): father.sons = @[]
   add(father.sons, son)
 
 type Indexable = PNode | PType
@@ -1134,19 +1138,16 @@ const                         # for all kind of hash tables:
 
 proc copyStrTable*(dest: var TStrTable, src: TStrTable) =
   dest.counter = src.counter
-  if isNil(src.data): return
   setLen(dest.data, len(src.data))
   for i in countup(0, high(src.data)): dest.data[i] = src.data[i]
 
 proc copyIdTable*(dest: var TIdTable, src: TIdTable) =
   dest.counter = src.counter
-  if isNil(src.data): return
   newSeq(dest.data, len(src.data))
   for i in countup(0, high(src.data)): dest.data[i] = src.data[i]
 
 proc copyObjectSet*(dest: var TObjectSet, src: TObjectSet) =
   dest.counter = src.counter
-  if isNil(src.data): return
   setLen(dest.data, len(src.data))
   for i in countup(0, high(src.data)): dest.data[i] = src.data[i]
 
@@ -1243,7 +1244,8 @@ proc newStrNode*(strVal: string; info: TLineInfo): PNode =
 
 proc addSon*(father, son: PNode) =
   assert son != nil
-  if isNil(father.sons): father.sons = @[]
+  when not defined(nimNoNilSeqs):
+    if isNil(father.sons): father.sons = @[]
   add(father.sons, son)
 
 proc newProcNode*(kind: TNodeKind, info: TLineInfo, body: PNode,
@@ -1287,16 +1289,22 @@ proc mergeLoc(a: var TLoc, b: TLoc) =
   if a.r == nil: a.r = b.r
 
 proc newSons*(father: PNode, length: int) =
-  if isNil(father.sons):
-    newSeq(father.sons, length)
-  else:
+  when defined(nimNoNilSeqs):
     setLen(father.sons, length)
+  else:
+    if isNil(father.sons):
+      newSeq(father.sons, length)
+    else:
+      setLen(father.sons, length)
 
 proc newSons*(father: PType, length: int) =
-  if isNil(father.sons):
-    newSeq(father.sons, length)
-  else:
+  when defined(nimNoNilSeqs):
     setLen(father.sons, length)
+  else:
+    if isNil(father.sons):
+      newSeq(father.sons, length)
+    else:
+      setLen(father.sons, length)
 
 proc sonsLen*(n: PType): int = n.sons.len
 proc len*(n: PType): int = n.sons.len
@@ -1464,20 +1472,26 @@ proc propagateToOwner*(owner, elem: PType) =
       owner.flags.incl tfHasGCedMem
 
 proc rawAddSon*(father, son: PType) =
-  if isNil(father.sons): father.sons = @[]
+  when not defined(nimNoNilSeqs):
+    if isNil(father.sons): father.sons = @[]
   add(father.sons, son)
   if not son.isNil: propagateToOwner(father, son)
 
 proc rawAddSonNoPropagationOfTypeFlags*(father, son: PType) =
-  if isNil(father.sons): father.sons = @[]
+  when not defined(nimNoNilSeqs):
+    if isNil(father.sons): father.sons = @[]
   add(father.sons, son)
 
 proc addSonNilAllowed*(father, son: PNode) =
-  if isNil(father.sons): father.sons = @[]
+  when not defined(nimNoNilSeqs):
+    if isNil(father.sons): father.sons = @[]
   add(father.sons, son)
 
 proc delSon*(father: PNode, idx: int) =
-  if isNil(father.sons): return
+  when defined(nimNoNilSeqs):
+    if father.len == 0: return
+  else:
+    if isNil(father.sons): return
   var length = sonsLen(father)
   for i in countup(idx, length - 2): father.sons[i] = father.sons[i + 1]
   setLen(father.sons, length - 1)
diff --git a/compiler/astalgo.nim b/compiler/astalgo.nim
index 152802ba16..34963ee838 100644
--- a/compiler/astalgo.nim
+++ b/compiler/astalgo.nim
@@ -206,7 +206,7 @@ proc makeYamlString*(s: string): Rope =
   const MaxLineLength = 64
   result = nil
   var res = "\""
-  for i in countup(0, if s.isNil: -1 else: (len(s)-1)):
+  for i in 0 ..< s.len:
     if (i + 1) mod MaxLineLength == 0:
       add(res, '\"')
       add(res, "\n")
@@ -314,10 +314,7 @@ proc treeToYamlAux(conf: ConfigRef; n: PNode, marker: var IntSet, indent: int,
         addf(result, ",$N$1\"floatVal\": $2",
             [istr, rope(n.floatVal.toStrMaxPrecision)])
       of nkStrLit..nkTripleStrLit:
-        if n.strVal.isNil:
-          addf(result, ",$N$1\"strVal\": null", [istr])
-        else:
-          addf(result, ",$N$1\"strVal\": $2", [istr, makeYamlString(n.strVal)])
+        addf(result, ",$N$1\"strVal\": $2", [istr, makeYamlString(n.strVal)])
       of nkSym:
         addf(result, ",$N$1\"sym\": $2",
              [istr, symToYamlAux(conf, n.sym, marker, indent + 2, maxRecDepth)])
@@ -395,10 +392,7 @@ proc debugTree(conf: ConfigRef; n: PNode, indent: int, maxRecDepth: int;
         addf(result, ",$N$1\"floatVal\": $2",
             [istr, rope(n.floatVal.toStrMaxPrecision)])
       of nkStrLit..nkTripleStrLit:
-        if n.strVal.isNil:
-          addf(result, ",$N$1\"strVal\": null", [istr])
-        else:
-          addf(result, ",$N$1\"strVal\": $2", [istr, makeYamlString(n.strVal)])
+        addf(result, ",$N$1\"strVal\": $2", [istr, makeYamlString(n.strVal)])
       of nkSym:
         addf(result, ",$N$1\"sym\": $2_$3",
             [istr, rope(n.sym.name.s), rope(n.sym.id)])
@@ -759,10 +753,6 @@ proc idNodeTableGet(t: TIdNodeTable, key: PIdObj): PNode =
   if index >= 0: result = t.data[index].val
   else: result = nil
 
-proc idNodeTableGetLazy*(t: TIdNodeTable, key: PIdObj): PNode =
-  if not isNil(t.data):
-    result = idNodeTableGet(t, key)
-
 proc idNodeTableRawInsert(data: var TIdNodePairSeq, key: PIdObj, val: PNode) =
   var h: Hash
   h = key.id and high(data)
@@ -789,10 +779,6 @@ proc idNodeTablePut(t: var TIdNodeTable, key: PIdObj, val: PNode) =
     idNodeTableRawInsert(t.data, key, val)
     inc(t.counter)
 
-proc idNodeTablePutLazy*(t: var TIdNodeTable, key: PIdObj, val: PNode) =
-  if isNil(t.data): initIdNodeTable(t)
-  idNodeTablePut(t, key, val)
-
 iterator pairs*(t: TIdNodeTable): tuple[key: PIdObj, val: PNode] =
   for i in 0 .. high(t.data):
     if not isNil(t.data[i].key): yield (t.data[i].key, t.data[i].val)
diff --git a/compiler/ccgexprs.nim b/compiler/ccgexprs.nim
index 1d8c156576..56ecf5ba3b 100644
--- a/compiler/ccgexprs.nim
+++ b/compiler/ccgexprs.nim
@@ -71,8 +71,7 @@ proc genLiteral(p: BProc, n: PNode, ty: PType): Rope =
       else:
         result = genStringLiteral(p.module, n)
     else:
-      if n.strVal.isNil: result = rope("NIM_NIL")
-      else: result = makeCString(n.strVal)
+      result = makeCString(n.strVal)
   of nkFloatLit, nkFloat64Lit:
     result = rope(n.floatVal.toStrMaxPrecision)
   of nkFloat32Lit:
@@ -2548,7 +2547,7 @@ proc genConstExpr(p: BProc, n: PNode): Rope =
     var t = skipTypes(n.typ, abstractInst)
     if t.kind == tySequence:
       result = genConstSeq(p, n, n.typ)
-    elif t.kind == tyProc and t.callConv == ccClosure and not n.sons.isNil and
+    elif t.kind == tyProc and t.callConv == ccClosure and n.len > 0 and
          n.sons[0].kind == nkNilLit and n.sons[1].kind == nkNilLit:
       # this hack fixes issue that nkNilLit is expanded to {NIM_NIL,NIM_NIL}
       # this behaviour is needed since closure_var = nil must be
diff --git a/compiler/jsgen.nim b/compiler/jsgen.nim
index 462c622aaa..b9b22d825e 100644
--- a/compiler/jsgen.nim
+++ b/compiler/jsgen.nim
@@ -271,9 +271,7 @@ proc escapeJSString(s: string): string =
   result.add("\"")
 
 proc makeJSString(s: string, escapeNonAscii = true): Rope =
-  if s.isNil:
-    result = "null".rope
-  elif escapeNonAscii:
+  if escapeNonAscii:
     result = strutils.escape(s).rope
   else:
     result = escapeJSString(s).rope
@@ -1369,7 +1367,7 @@ proc createVar(p: PProc, typ: PType, indirect: bool): Rope =
     let length = int(lengthOrd(p.config, t))
     let e = elemType(t)
     let jsTyp = arrayTypeForElemType(e)
-    if not jsTyp.isNil:
+    if jsTyp.len > 0:
       result = "new $1($2)" % [rope(jsTyp), rope(length)]
     elif length > 32:
       useMagic(p, "arrayConstr")
diff --git a/compiler/magicsys.nim b/compiler/magicsys.nim
index d40b9d732f..aeeb489c08 100644
--- a/compiler/magicsys.nim
+++ b/compiler/magicsys.nim
@@ -120,7 +120,8 @@ proc skipIntLit*(t: PType): PType {.inline.} =
     result = t
 
 proc addSonSkipIntLit*(father, son: PType) =
-  if isNil(father.sons): father.sons = @[]
+  when not defined(nimNoNilSeqs):
+    if isNil(father.sons): father.sons = @[]
   let s = son.skipIntLit
   add(father.sons, s)
   propagateToOwner(father, s)
diff --git a/compiler/msgs.nim b/compiler/msgs.nim
index 1d79391420..47ab878f1a 100644
--- a/compiler/msgs.nim
+++ b/compiler/msgs.nim
@@ -176,7 +176,7 @@ proc getHash*(conf: ConfigRef; fileIdx: FileIndex): string =
 proc toFullPathConsiderDirty*(conf: ConfigRef; fileIdx: FileIndex): string =
   if fileIdx.int32 < 0:
     result = "???"
-  elif not conf.m.fileInfos[fileIdx.int32].dirtyFile.isNil:
+  elif conf.m.fileInfos[fileIdx.int32].dirtyFile.len > 0:
     result = conf.m.fileInfos[fileIdx.int32].dirtyFile
   else:
     result = conf.m.fileInfos[fileIdx.int32].fullPath
diff --git a/compiler/packagehandling.nim b/compiler/packagehandling.nim
index 2efab58b06..7414aeb71d 100644
--- a/compiler/packagehandling.nim
+++ b/compiler/packagehandling.nim
@@ -33,7 +33,8 @@ proc getPackageName*(conf: ConfigRef; path: string): string =
         result = file.splitFile.name
         break packageSearch
   # we also store if we didn't find anything:
-  if result.isNil: result = ""
+  when not defined(nimNoNilSeqs):
+    if result.isNil: result = ""
   for d in myParentDirs(path):
     #echo "set cache ", d, " |", result, "|", parents
     conf.packageCache[d] = result
diff --git a/compiler/renderer.nim b/compiler/renderer.nim
index c3e151f5a8..8a25d76e8c 100644
--- a/compiler/renderer.nim
+++ b/compiler/renderer.nim
@@ -175,7 +175,7 @@ proc put(g: var TSrcGen, kind: TTokType, s: string) =
     g.pendingWhitespace = s.len
 
 proc putComment(g: var TSrcGen, s: string) =
-  if s.isNil: return
+  if s.len == 0: return
   var i = 0
   let hi = len(s) - 1
   var isCode = (len(s) >= 2) and (s[1] != ' ')
@@ -216,7 +216,7 @@ proc putComment(g: var TSrcGen, s: string) =
   optNL(g)
 
 proc maxLineLength(s: string): int =
-  if s.isNil: return 0
+  if s.len == 0: return 0
   var i = 0
   let hi = len(s) - 1
   var lineLen = 0
diff --git a/compiler/sempass2.nim b/compiler/sempass2.nim
index bdea07ea8a..0a9de674b5 100644
--- a/compiler/sempass2.nim
+++ b/compiler/sempass2.nim
@@ -323,13 +323,13 @@ proc catches(tracked: PEffects, e: PType) =
       dec L
     else:
       inc i
-  if not isNil(tracked.exc.sons):
+  if tracked.exc.len > 0:
     setLen(tracked.exc.sons, L)
   else:
     assert L == 0
 
 proc catchesAll(tracked: PEffects) =
-  if not isNil(tracked.exc.sons):
+  if tracked.exc.len > 0:
     setLen(tracked.exc.sons, tracked.bottom)
 
 proc track(tracked: PEffects, n: PNode)
diff --git a/compiler/semstmts.nim b/compiler/semstmts.nim
index 3a1278137b..6cf2e2d969 100644
--- a/compiler/semstmts.nim
+++ b/compiler/semstmts.nim
@@ -1609,7 +1609,7 @@ proc semProcAux(c: PContext, n: PNode, kind: TSymKind,
     n.sons[pragmasPos] = proto.ast.sons[pragmasPos]
     if n.sons[namePos].kind != nkSym: internalError(c.config, n.info, "semProcAux")
     n.sons[namePos].sym = proto
-    if importantComments(c.config) and not isNil(proto.ast.comment):
+    if importantComments(c.config) and proto.ast.comment.len > 0:
       n.comment = proto.ast.comment
     proto.ast = n             # needed for code generation
     popOwner(c)
diff --git a/compiler/suggest.nim b/compiler/suggest.nim
index a21d643389..3d3cd2484a 100644
--- a/compiler/suggest.nim
+++ b/compiler/suggest.nim
@@ -47,7 +47,7 @@ template origModuleName(m: PSym): string = m.name.s
 
 proc findDocComment(n: PNode): PNode =
   if n == nil: return nil
-  if not isNil(n.comment): return n
+  if n.comment.len > 0: return n
   if n.kind in {nkStmtList, nkStmtListExpr, nkObjectTy, nkRecList} and n.len > 0:
     result = findDocComment(n.sons[0])
     if result != nil: return
diff --git a/compiler/treetab.nim b/compiler/treetab.nim
index e6eb8c666a..f15974f612 100644
--- a/compiler/treetab.nim
+++ b/compiler/treetab.nim
@@ -29,8 +29,7 @@ proc hashTree(n: PNode): Hash =
     if (n.floatVal >= - 1000000.0) and (n.floatVal <= 1000000.0):
       result = result !& toInt(n.floatVal)
   of nkStrLit..nkTripleStrLit:
-    if not n.strVal.isNil:
-      result = result !& hash(n.strVal)
+    result = result !& hash(n.strVal)
   else:
     for i in countup(0, sonsLen(n) - 1):
       result = result !& hashTree(n.sons[i])
diff --git a/compiler/types.nim b/compiler/types.nim
index 674819bc59..4a16e72075 100644
--- a/compiler/types.nim
+++ b/compiler/types.nim
@@ -759,9 +759,10 @@ proc initSameTypeClosure: TSameTypeClosure =
   discard
 
 proc containsOrIncl(c: var TSameTypeClosure, a, b: PType): bool =
-  result = not isNil(c.s) and c.s.contains((a.id, b.id))
+  result = c.s.len > 0 and c.s.contains((a.id, b.id))
   if not result:
-    if isNil(c.s): c.s = @[]
+    when not defined(nimNoNilSeqs):
+      if isNil(c.s): c.s = @[]
     c.s.add((a.id, b.id))
 
 proc sameTypeAux(x, y: PType, c: var TSameTypeClosure): bool
diff --git a/compiler/typesrenderer.nim b/compiler/typesrenderer.nim
index 4d75d5d058..0c4fe01e1f 100644
--- a/compiler/typesrenderer.nim
+++ b/compiler/typesrenderer.nim
@@ -29,7 +29,6 @@ proc renderPlainSymbolName*(n: PNode): string =
   else:
     result = ""
     #internalError(n.info, "renderPlainSymbolName() with " & $n.kind)
-  assert(not result.isNil)
 
 proc renderType(n: PNode): string =
   ## Returns a string with the node type or the empty string.
@@ -80,7 +79,6 @@ proc renderType(n: PNode): string =
     for i in 1 ..< len(n): result.add(renderType(n[i]) & ',')
     result[len(result)-1] = ']'
   else: result = ""
-  assert(not result.isNil)
 
 
 proc renderParamTypes(found: var seq[string], n: PNode) =
diff --git a/compiler/vm.nim b/compiler/vm.nim
index c49b66b82b..8f74761dc9 100644
--- a/compiler/vm.nim
+++ b/compiler/vm.nim
@@ -242,7 +242,8 @@ template getstr(a: untyped): untyped =
   (if a.kind == rkNode: a.node.strVal else: $chr(int(a.intVal)))
 
 proc pushSafePoint(f: PStackFrame; pc: int) =
-  if f.safePoints.isNil: f.safePoints = @[]
+  when not defined(nimNoNilSeqs):
+    if f.safePoints.isNil: f.safePoints = @[]
   f.safePoints.add(pc)
 
 proc popSafePoint(f: PStackFrame) =
@@ -255,7 +256,7 @@ proc cleanUpOnException(c: PCtx; tos: PStackFrame):
   let raisedType = c.currentExceptionA.typ.skipTypes(abstractPtrs)
   var f = tos
   while true:
-    while f.safePoints.isNil or f.safePoints.len == 0:
+    while f.safePoints.len == 0:
       f = f.next
       if f.isNil: return (-1, nil)
     var pc2 = f.safePoints[f.safePoints.high]
@@ -297,7 +298,6 @@ proc cleanUpOnException(c: PCtx; tos: PStackFrame):
     discard f.safePoints.pop
 
 proc cleanUpOnReturn(c: PCtx; f: PStackFrame): int =
-  if f.safePoints.isNil: return -1
   for s in f.safePoints:
     var pc = s
     while c.code[pc].opcode == opcExcept:
@@ -531,9 +531,7 @@ proc rawExecute(c: PCtx, start: int, tos: PStackFrame): TFullReg =
       decodeBC(rkInt)
       let idx = regs[rc].intVal.int
       let s = regs[rb].node.strVal
-      if s.isNil:
-        stackTrace(c, tos, pc, errNilAccess)
-      elif idx <% s.len:
+      if idx <% s.len:
         regs[ra].intVal = s[idx].ord
       elif idx == s.len and optLaxStrings in c.config.options:
         regs[ra].intVal = 0
@@ -1220,7 +1218,6 @@ proc rawExecute(c: PCtx, start: int, tos: PStackFrame): TFullReg =
         # Note that `nfIsRef` + `nkNilLit` represents an allocated
         # reference with the value `nil`, so `isNil` should be false!
         (node.kind == nkNilLit and nfIsRef notin node.flags) or
-        (node.kind in {nkStrLit..nkTripleStrLit} and node.strVal.isNil) or
         (not node.typ.isNil and node.typ.kind == tyProc and
           node.typ.callConv == ccClosure and node.sons[0].kind == nkNilLit and
           node.sons[1].kind == nkNilLit))
@@ -1396,9 +1393,9 @@ proc rawExecute(c: PCtx, start: int, tos: PStackFrame): TFullReg =
       let ast = parseString(regs[rb].node.strVal, c.cache, c.config,
                             toFullPath(c.config, c.debug[pc]), c.debug[pc].line.int,
                             proc (conf: ConfigRef; info: TLineInfo; msg: TMsgKind; arg: string) =
-                              if error.isNil and msg <= errMax:
+                              if error.len == 0 and msg <= errMax:
                                 error = formatMsg(conf, info, msg, arg))
-      if not error.isNil:
+      if error.len > 0:
         c.errorFlag = error
       elif sonsLen(ast) != 1:
         c.errorFlag = formatMsg(c.config, c.debug[pc], errGenerated,
@@ -1411,9 +1408,9 @@ proc rawExecute(c: PCtx, start: int, tos: PStackFrame): TFullReg =
       let ast = parseString(regs[rb].node.strVal, c.cache, c.config,
                             toFullPath(c.config, c.debug[pc]), c.debug[pc].line.int,
                             proc (conf: ConfigRef; info: TLineInfo; msg: TMsgKind; arg: string) =
-                              if error.isNil and msg <= errMax:
+                              if error.len == 0 and msg <= errMax:
                                 error = formatMsg(conf, info, msg, arg))
-      if not error.isNil:
+      if error.len > 0:
         c.errorFlag = error
       else:
         regs[ra].node = ast
@@ -1726,7 +1723,8 @@ proc rawExecute(c: PCtx, start: int, tos: PStackFrame): TFullReg =
       inc pc
       let typ = c.types[c.code[pc].regBx - wordExcess]
       createStrKeepNode(regs[ra])
-      if regs[ra].node.strVal.isNil: regs[ra].node.strVal = newStringOfCap(1000)
+      when not defined(nimNoNilSeqs):
+        if regs[ra].node.strVal.isNil: regs[ra].node.strVal = newStringOfCap(1000)
       storeAny(regs[ra].node.strVal, typ, regs[rb].regToNode, c.config)
     of opcToNarrowInt:
       decodeBC(rkInt)
diff --git a/compiler/vmmarshal.nim b/compiler/vmmarshal.nim
index eb01b35140..149d2e08f9 100644
--- a/compiler/vmmarshal.nim
+++ b/compiler/vmmarshal.nim
@@ -127,7 +127,7 @@ proc storeAny(s: var string; t: PType; a: PNode; stored: var IntSet;
       storeAny(s, t.lastSon, a, stored, conf)
       s.add("]")
   of tyString, tyCString:
-    if a.kind == nkNilLit or a.strVal.isNil: s.add("null")
+    if a.kind == nkNilLit: s.add("null")
     else: s.add(escapeJson(a.strVal))
   of tyInt..tyInt64, tyUInt..tyUInt64: s.add($a.intVal)
   of tyFloat..tyFloat128: s.add($a.floatVal)
diff --git a/lib/packages/docutils/rst.nim b/lib/packages/docutils/rst.nim
index 7ee071c79f..d35f109e78 100644
--- a/lib/packages/docutils/rst.nim
+++ b/lib/packages/docutils/rst.nim
@@ -1387,7 +1387,7 @@ proc parseSectionWrapper(p: var RstParser): PRstNode =
     result = result.sons[0]
 
 proc `$`(t: Token): string =
-  result = $t.kind & ' ' & (if isNil(t.symbol): "NIL" else: t.symbol)
+  result = $t.kind & ' ' & t.symbol
 
 proc parseDoc(p: var RstParser): PRstNode =
   result = parseSectionWrapper(p)
diff --git a/lib/packages/docutils/rstgen.nim b/lib/packages/docutils/rstgen.nim
index 5b0b6c6eeb..a68ae928c0 100644
--- a/lib/packages/docutils/rstgen.nim
+++ b/lib/packages/docutils/rstgen.nim
@@ -243,7 +243,7 @@ proc dispA(target: OutputTarget, dest: var string,
   else: addf(dest, tex, args)
 
 proc `or`(x, y: string): string {.inline.} =
-  result = if x.isNil: y else: x
+  result = if x.len == 0: y else: x
 
 proc renderRstToOut*(d: var RstGenerator, n: PRstNode, result: var string)
   ## Writes into ``result`` the rst ast ``n`` using the ``d`` configuration.
@@ -387,20 +387,16 @@ proc hash(x: IndexEntry): Hash =
   ## Returns the hash for the combined fields of the type.
   ##
   ## The hash is computed as the chained hash of the individual string hashes.
-  assert(not x.keyword.isNil)
-  assert(not x.link.isNil)
   result = x.keyword.hash !& x.link.hash
-  result = result !& (x.linkTitle or "").hash
-  result = result !& (x.linkDesc or "").hash
+  result = result !& x.linkTitle.hash
+  result = result !& x.linkDesc.hash
   result = !$result
 
 proc `<-`(a: var IndexEntry, b: IndexEntry) =
   shallowCopy a.keyword, b.keyword
   shallowCopy a.link, b.link
-  if b.linkTitle.isNil: a.linkTitle = ""
-  else: shallowCopy a.linkTitle, b.linkTitle
-  if b.linkDesc.isNil: a.linkDesc = ""
-  else: shallowCopy a.linkDesc, b.linkDesc
+  shallowCopy a.linkTitle, b.linkTitle
+  shallowCopy a.linkDesc, b.linkDesc
 
 proc sortIndex(a: var openArray[IndexEntry]) =
   # we use shellsort here; fast and simple
@@ -444,8 +440,8 @@ proc generateSymbolIndex(symbols: seq[IndexEntry]): string =
     while j < symbols.len and keyword == symbols[j].keyword:
       let
         url = symbols[j].link.escapeLink
-        text = if not symbols[j].linkTitle.isNil: symbols[j].linkTitle else: url
-        desc = if not symbols[j].linkDesc.isNil: symbols[j].linkDesc else: ""
+        text = if symbols[j].linkTitle.len > 0: symbols[j].linkTitle else: url
+        desc = if symbols[j].linkDesc.len > 0: symbols[j].linkDesc else: ""
       if desc.len > 0:
         result.addf("""<li><a class="reference external"
           title="$3" data-doc-search-tag="$2" href="$1">$2</a></li>
@@ -528,8 +524,6 @@ proc generateDocumentationTOC(entries: seq[IndexEntry]): string =
         """, [titleTag & " : " & levels[L].text, link, levels[L].text])
     inc L
   result.add(level.indentToLevel(1) & "</ul>\n")
-  assert(not titleRef.isNil,
-    "Can't use this proc on an API index, docs always have a title entry")
 
 proc generateDocumentationIndex(docs: IndexedDocs): string =
   ## Returns all the documentation TOCs in an HTML hierarchical list.
@@ -596,7 +590,7 @@ proc readIndexDir(dir: string):
         fileEntries[F].keyword = line.substr(0, s-1)
         fileEntries[F].link = line.substr(s+1)
         # See if we detect a title, a link without a `#foobar` trailing part.
-        if title.keyword.isNil and fileEntries[F].link.isDocumentationTitle:
+        if title.keyword.len == 0 and fileEntries[F].link.isDocumentationTitle:
           title.keyword = fileEntries[F].keyword
           title.link = fileEntries[F].link
 
@@ -611,11 +605,11 @@ proc readIndexDir(dir: string):
           fileEntries[F].linkDesc = ""
         inc F
       # Depending on type add this to the list of symbols or table of APIs.
-      if title.keyword.isNil:
+      if title.keyword.len == 0:
         for i in 0 .. <F:
           # Don't add to symbols TOC entries (they start with a whitespace).
           let toc = fileEntries[i].linkTitle
-          if not toc.isNil and toc.len > 0 and toc[0] == ' ':
+          if toc.len > 0 and toc[0] == ' ':
             continue
           # Ok, non TOC entry, add it.
           setLen(result.symbols, L + 1)
diff --git a/lib/pure/cgi.nim b/lib/pure/cgi.nim
index e0cdc2ec0a..101146aceb 100644
--- a/lib/pure/cgi.nim
+++ b/lib/pure/cgi.nim
@@ -135,10 +135,9 @@ iterator decodeData*(allowedMethods: set[RequestMethod] =
   ## Reads and decodes CGI data and yields the (name, value) pairs the
   ## data consists of. If the client does not use a method listed in the
   ## `allowedMethods` set, an `ECgi` exception is raised.
-  var data = getEncodedData(allowedMethods)
-  if not isNil(data):
-    for key, value in decodeData(data):
-      yield (key, value)
+  let data = getEncodedData(allowedMethods)
+  for key, value in decodeData(data):
+    yield (key, value)
 
 proc readData*(allowedMethods: set[RequestMethod] =
                {methodNone, methodPost, methodGet}): StringTableRef =
diff --git a/lib/pure/collections/sets.nim b/lib/pure/collections/sets.nim
index fdc3b4b03a..0ef91c3cc3 100644
--- a/lib/pure/collections/sets.nim
+++ b/lib/pure/collections/sets.nim
@@ -74,7 +74,7 @@ proc isValid*[A](s: HashSet[A]): bool =
   ##   proc savePreferences(options: HashSet[string]) =
   ##     assert options.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: HashSet[A]): int =
   ## Returns the number of keys in `s`.
diff --git a/lib/pure/httpclient.nim b/lib/pure/httpclient.nim
index 0192e71e78..139d4bb501 100644
--- a/lib/pure/httpclient.nim
+++ b/lib/pure/httpclient.nim
@@ -185,7 +185,7 @@ proc body*(response: Response): string =
   ## Retrieves the specified response's body.
   ##
   ## The response's body stream is read synchronously.
-  if response.body.isNil():
+  if response.body.len == 0:
     response.body = response.bodyStream.readAll()
   return response.body
 
@@ -198,7 +198,7 @@ proc `body=`*(response: Response, value: string) {.deprecated.} =
 proc body*(response: AsyncResponse): Future[string] {.async.} =
   ## Reads the response's body and caches it. The read is performed only
   ## once.
-  if response.body.isNil:
+  if response.body.len == 0:
     response.body = await readAll(response.bodyStream)
   return response.body
 
diff --git a/lib/pure/json.nim b/lib/pure/json.nim
index 69ffb1796f..3599667a86 100644
--- a/lib/pure/json.nim
+++ b/lib/pure/json.nim
@@ -256,7 +256,6 @@ proc add*(obj: JsonNode, key: string, val: JsonNode) =
 proc `%`*(s: string): JsonNode =
   ## Generic constructor for JSON data. Creates a new `JString JsonNode`.
   new(result)
-  if s.isNil: return
   result.kind = JString
   result.str = s
 
diff --git a/lib/pure/logging.nim b/lib/pure/logging.nim
index cdff1f5484..f2f5cac9ec 100644
--- a/lib/pure/logging.nim
+++ b/lib/pure/logging.nim
@@ -103,14 +103,9 @@ var
 proc substituteLog*(frmt: string, level: Level, args: varargs[string, `$`]): string =
   ## Format a log message using the ``frmt`` format string, ``level`` and varargs.
   ## See the module documentation for the format string syntax.
-  const nilString = "nil"
-
   var msgLen = 0
   for arg in args:
-    if arg.isNil:
-      msgLen += nilString.len
-    else:
-      msgLen += arg.len
+    msgLen += arg.len
   result = newStringOfCap(frmt.len + msgLen + 20)
   var i = 0
   while i < frmt.len:
@@ -137,10 +132,7 @@ proc substituteLog*(frmt: string, level: Level, args: varargs[string, `$`]): str
       of "levelname": result.add(LevelNames[level])
       else: discard
   for arg in args:
-    if arg.isNil:
-      result.add(nilString)
-    else:
-      result.add(arg)
+    result.add(arg)
 
 method log*(logger: Logger, level: Level, args: varargs[string, `$`]) {.
             raises: [Exception], gcsafe,
@@ -338,7 +330,6 @@ template fatal*(args: varargs[string, `$`]) =
 
 proc addHandler*(handler: Logger) =
   ## Adds ``handler`` to the list of handlers.
-  if handlers.isNil: handlers = @[]
   handlers.add(handler)
 
 proc getHandlers*(): seq[Logger] =
diff --git a/lib/pure/marshal.nim b/lib/pure/marshal.nim
index b90d2899c1..9804397597 100644
--- a/lib/pure/marshal.nim
+++ b/lib/pure/marshal.nim
@@ -98,8 +98,7 @@ proc storeAny(s: Stream, a: Any, stored: var IntSet) =
   of akProc, akPointer, akCString: s.write($a.getPointer.ptrToInt)
   of akString:
     var x = getString(a)
-    if isNil(x): s.write("null")
-    elif x.validateUtf8() == -1: s.write(escapeJson(x))
+    if x.validateUtf8() == -1: s.write(escapeJson(x))
     else:
       s.write("[")
       var i = 0
diff --git a/lib/pure/os.nim b/lib/pure/os.nim
index 8fbc20bb59..2b3cf51429 100644
--- a/lib/pure/os.nim
+++ b/lib/pure/os.nim
@@ -233,7 +233,7 @@ proc fileNewer*(a, b: string): bool {.rtl, extern: "nos$1".} =
   ## modification time is later than `b`'s.
   when defined(posix):
     # If we don't have access to nanosecond resolution, use '>='
-    when not StatHasNanoseconds:  
+    when not StatHasNanoseconds:
       result = getLastModificationTime(a) >= getLastModificationTime(b)
     else:
       result = getLastModificationTime(a) > getLastModificationTime(b)
@@ -1343,16 +1343,21 @@ elif defined(windows):
   # is always the same -- independent of the used C compiler.
   var
     ownArgv {.threadvar.}: seq[string]
+    ownParsedArgv {.threadvar.}: bool
 
   proc paramCount*(): int {.rtl, extern: "nos$1", tags: [ReadIOEffect].} =
     # Docstring in nimdoc block.
-    if isNil(ownArgv): ownArgv = parseCmdLine($getCommandLine())
+    if not ownParsedArgv:
+      ownArgv = parseCmdLine($getCommandLine())
+      ownParsedArgv = true
     result = ownArgv.len-1
 
   proc paramStr*(i: int): TaintedString {.rtl, extern: "nos$1",
     tags: [ReadIOEffect].} =
     # Docstring in nimdoc block.
-    if isNil(ownArgv): ownArgv = parseCmdLine($getCommandLine())
+    if not ownParsedArgv:
+      ownArgv = parseCmdLine($getCommandLine())
+      ownParsedArgv = true
     if i < ownArgv.len and i >= 0: return TaintedString(ownArgv[i])
     raise newException(IndexError, "invalid index")
 
diff --git a/lib/pure/parsesql.nim b/lib/pure/parsesql.nim
index e3bab9a8d2..9aef43c1bf 100644
--- a/lib/pure/parsesql.nim
+++ b/lib/pure/parsesql.nim
@@ -593,7 +593,6 @@ proc len*(n: SqlNode): int =
 proc `[]`*(n: SqlNode; i: int): SqlNode = n.sons[i]
 
 proc add*(father, n: SqlNode) =
-  if isNil(father.sons): father.sons = @[]
   add(father.sons, n)
 
 proc getTok(p: var SqlParser) =
diff --git a/lib/pure/unidecode/unidecode.nim b/lib/pure/unidecode/unidecode.nim
index 9d8843f064..63ebd8248f 100644
--- a/lib/pure/unidecode/unidecode.nim
+++ b/lib/pure/unidecode/unidecode.nim
@@ -61,7 +61,6 @@ proc unidecode*(s: string): string =
   ##
   ## Results in: "Bei Jing"
   ##
-  assert(not isNil(translationTable))
   result = ""
   for r in runes(s):
     var c = int(r)
diff --git a/lib/pure/xmldom.nim b/lib/pure/xmldom.nim
index 1a9e4ae26c..82f88a9963 100644
--- a/lib/pure/xmldom.nim
+++ b/lib/pure/xmldom.nim
@@ -172,34 +172,30 @@ proc documentElement*(doc: PDocument): PElement =
 proc findNodes(nl: PNode, name: string): seq[PNode] =
   # Made for getElementsByTagName
   var r: seq[PNode] = @[]
-  if isNil(nl.childNodes): return @[]
-  if nl.childNodes.len() == 0: return @[]
+  if nl.childNodes.len == 0: return @[]
 
   for i in items(nl.childNodes):
     if i.fNodeType == ElementNode:
       if i.fNodeName == name or name == "*":
         r.add(i)
 
-      if not isNil(i.childNodes):
-        if i.childNodes.len() != 0:
-          r.add(findNodes(i, name))
+      if i.childNodes.len() != 0:
+        r.add(findNodes(i, name))
 
   return r
 
 proc findNodesNS(nl: PNode, namespaceURI: string, localName: string): seq[PNode] =
   # Made for getElementsByTagNameNS
   var r: seq[PNode] = @[]
-  if isNil(nl.childNodes): return @[]
-  if nl.childNodes.len() == 0: return @[]
+  if nl.childNodes.len == 0: return @[]
 
   for i in items(nl.childNodes):
     if i.fNodeType == ElementNode:
       if (i.fNamespaceURI == namespaceURI or namespaceURI == "*") and (i.fLocalName == localName or localName == "*"):
         r.add(i)
 
-      if not isNil(i.childNodes):
-        if i.childNodes.len() != 0:
-          r.add(findNodesNS(i, namespaceURI, localName))
+      if i.childNodes.len != 0:
+        r.add(findNodesNS(i, namespaceURI, localName))
 
   return r
 
@@ -233,8 +229,8 @@ proc createAttributeNS*(doc: PDocument, namespaceURI: string, qualifiedName: str
   # Exceptions
   if qualifiedName.contains(':'):
     let qfnamespaces = qualifiedName.toLowerAscii().split(':')
-    if isNil(namespaceURI):
-      raise newException(ENamespaceErr, "When qualifiedName contains a prefix namespaceURI cannot be nil")
+    if namespaceURI.len == 0:
+      raise newException(ENamespaceErr, "When qualifiedName contains a prefix namespaceURI cannot be empty")
     elif qfnamespaces[0] == "xml" and
         namespaceURI != "http://www.w3.org/XML/1998/namespace" and
         qfnamespaces[1] notin stdattrnames:
@@ -312,8 +308,8 @@ proc createElementNS*(doc: PDocument, namespaceURI: string, qualifiedName: strin
   ## Creates an element of the given qualified name and namespace URI.
   if qualifiedName.contains(':'):
     let qfnamespaces = qualifiedName.toLowerAscii().split(':')
-    if isNil(namespaceURI):
-      raise newException(ENamespaceErr, "When qualifiedName contains a prefix namespaceURI cannot be nil")
+    if namespaceURI.len == 0:
+      raise newException(ENamespaceErr, "When qualifiedName contains a prefix namespaceURI cannot be empty")
     elif qfnamespaces[0] == "xml" and
         namespaceURI != "http://www.w3.org/XML/1998/namespace" and
         qfnamespaces[1] notin stdattrnames:
@@ -453,7 +449,7 @@ proc importNode*(doc: PDocument, importedNode: PNode, deep: bool): PNode =
 proc firstChild*(n: PNode): PNode =
   ## Returns this node's first child
 
-  if not isNil(n.childNodes) and n.childNodes.len() > 0:
+  if n.childNodes.len > 0:
     return n.childNodes[0]
   else:
     return nil
@@ -461,8 +457,8 @@ proc firstChild*(n: PNode): PNode =
 proc lastChild*(n: PNode): PNode =
   ## Returns this node's last child
 
-  if not isNil(n.childNodes) and n.childNodes.len() > 0:
-    return n.childNodes[n.childNodes.len() - 1]
+  if n.childNodes.len > 0:
+    return n.childNodes[n.childNodes.len - 1]
   else:
     return nil
 
@@ -482,7 +478,7 @@ proc `namespaceURI=`*(n: PNode, value: string) =
 proc nextSibling*(n: PNode): PNode =
   ## Returns the next sibling of this node
 
-  if isNil(n.fParentNode) or isNil(n.fParentNode.childNodes):
+  if isNil(n.fParentNode):
     return nil
   var nLow: int = low(n.fParentNode.childNodes)
   var nHigh: int = high(n.fParentNode.childNodes)
@@ -514,7 +510,7 @@ proc parentNode*(n: PNode): PNode =
 proc previousSibling*(n: PNode): PNode =
   ## Returns the previous sibling of this node
 
-  if isNil(n.fParentNode) or isNil(n.fParentNode.childNodes):
+  if isNil(n.fParentNode):
     return nil
   var nLow: int = low(n.fParentNode.childNodes)
   var nHigh: int = high(n.fParentNode.childNodes)
@@ -531,8 +527,8 @@ proc `prefix=`*(n: PNode, value: string) =
   if illegalChars in value:
     raise newException(EInvalidCharacterErr, "Invalid character")
 
-  if isNil(n.fNamespaceURI):
-    raise newException(ENamespaceErr, "namespaceURI cannot be nil")
+  if n.fNamespaceURI.len == 0:
+    raise newException(ENamespaceErr, "namespaceURI cannot be empty")
   elif value.toLowerAscii() == "xml" and n.fNamespaceURI != "http://www.w3.org/XML/1998/namespace":
     raise newException(ENamespaceErr,
       "When the namespace prefix is \"xml\" namespaceURI has to be \"http://www.w3.org/XML/1998/namespace\"")
@@ -557,10 +553,9 @@ proc appendChild*(n: PNode, newChild: PNode) =
   ## If the newChild is already in the tree, it is first removed.
 
   # Check if n contains newChild
-  if not isNil(n.childNodes):
-    for i in low(n.childNodes)..high(n.childNodes):
-      if n.childNodes[i] == newChild:
-        raise newException(EHierarchyRequestErr, "The node to append is already in this nodes children.")
+  for i in low(n.childNodes)..high(n.childNodes):
+    if n.childNodes[i] == newChild:
+      raise newException(EHierarchyRequestErr, "The node to append is already in this nodes children.")
 
   # Check if newChild is from this nodes document
   if n.fOwnerDocument != newChild.fOwnerDocument:
@@ -572,7 +567,8 @@ proc appendChild*(n: PNode, newChild: PNode) =
   if n.nodeType in childlessObjects:
     raise newException(ENoModificationAllowedErr, "Cannot append children to a childless node")
 
-  if isNil(n.childNodes): n.childNodes = @[]
+  when not defined(nimNoNilSeqs):
+    if isNil(n.childNodes): n.childNodes = @[]
 
   newChild.fParentNode = n
   for i in low(n.childNodes)..high(n.childNodes):
@@ -597,10 +593,10 @@ proc cloneNode*(n: PNode, deep: bool): PNode =
     newNode = PElement(n)
     # Import the childNodes
     var tmp: seq[PNode] = n.childNodes
-    n.childNodes = @[]
-    if deep and not isNil(tmp):
-      for i in low(tmp.len())..high(tmp.len()):
-        n.childNodes.add(cloneNode(tmp[i], deep))
+    newNode.childNodes = @[]
+    if deep:
+      for i in low(n.childNodes)..high(n.childNodes):
+        newNode.childNodes.add(cloneNode(n.childNodes[i], deep))
     return newNode
   else:
     var newNode: PNode
@@ -610,11 +606,11 @@ proc cloneNode*(n: PNode, deep: bool): PNode =
 
 proc hasAttributes*(n: PNode): bool =
   ## Returns whether this node (if it is an element) has any attributes.
-  return not isNil(n.attributes) and n.attributes.len() > 0
+  return n.attributes.len > 0
 
 proc hasChildNodes*(n: PNode): bool =
   ## Returns whether this node has any children.
-  return not isNil(n.childNodes) and n.childNodes.len() > 0
+  return n.childNodes.len > 0
 
 proc insertBefore*(n: PNode, newChild: PNode, refChild: PNode): PNode =
   ## Inserts the node ``newChild`` before the existing child node ``refChild``.
@@ -624,9 +620,6 @@ proc insertBefore*(n: PNode, newChild: PNode, refChild: PNode): PNode =
   if n.fOwnerDocument != newChild.fOwnerDocument:
     raise newException(EWrongDocumentErr, "This node belongs to a different document, use importNode.")
 
-  if isNil(n.childNodes):
-    n.childNodes = @[]
-
   for i in low(n.childNodes)..high(n.childNodes):
     if n.childNodes[i] == refChild:
       n.childNodes.insert(newChild, i - 1)
@@ -641,7 +634,7 @@ proc isSupported*(n: PNode, feature: string, version: string): bool =
 
 proc isEmpty(s: string): bool =
 
-  if isNil(s) or s == "":
+  if s == "":
     return true
   for i in items(s):
     if i != ' ':
@@ -655,7 +648,7 @@ proc normalize*(n: PNode) =
 
   var newChildNodes: seq[PNode] = @[]
   while true:
-    if isNil(n.childNodes) or i >= n.childNodes.len:
+    if i >= n.childNodes.len:
       break
     if n.childNodes[i].nodeType == TextNode:
 
@@ -679,12 +672,11 @@ proc normalize*(n: PNode) =
 
 proc removeChild*(n: PNode, oldChild: PNode): PNode =
   ## Removes the child node indicated by ``oldChild`` from the list of children, and returns it.
-  if not isNil(n.childNodes):
-    for i in low(n.childNodes)..high(n.childNodes):
-      if n.childNodes[i] == oldChild:
-        result = n.childNodes[i]
-        n.childNodes.delete(i)
-        return
+  for i in low(n.childNodes)..high(n.childNodes):
+    if n.childNodes[i] == oldChild:
+      result = n.childNodes[i]
+      n.childNodes.delete(i)
+      return
 
   raise newException(ENotFoundErr, "Node not found")
 
@@ -695,12 +687,11 @@ proc replaceChild*(n: PNode, newChild: PNode, oldChild: PNode): PNode =
   if n.fOwnerDocument != newChild.fOwnerDocument:
     raise newException(EWrongDocumentErr, "This node belongs to a different document, use importNode.")
 
-  if not isNil(n.childNodes):
-    for i in low(n.childNodes)..high(n.childNodes):
-      if n.childNodes[i] == oldChild:
-        result = n.childNodes[i]
-        n.childNodes[i] = newChild
-        return
+  for i in low(n.childNodes)..high(n.childNodes):
+    if n.childNodes[i] == oldChild:
+      result = n.childNodes[i]
+      n.childNodes[i] = newChild
+      return
 
   raise newException(ENotFoundErr, "Node not found")
 
@@ -764,11 +755,10 @@ proc removeNamedItemNS*(nList: var seq[PNode], namespaceURI: string, localName:
 proc setNamedItem*(nList: var seq[PNode], arg: PNode): PNode =
   ## Adds ``arg`` as a ``Node`` to the ``NList``
   ## If a node with the same name is already present in this map, it is replaced by the new one.
-  if not isNil(nList):
-    if nList.len() > 0:
-      #Check if newChild is from this nodes document
-      if nList[0].fOwnerDocument != arg.fOwnerDocument:
-        raise newException(EWrongDocumentErr, "This node belongs to a different document, use importNode.")
+  if nList.len > 0:
+    #Check if newChild is from this nodes document
+    if nList[0].fOwnerDocument != arg.fOwnerDocument:
+      raise newException(EWrongDocumentErr, "This node belongs to a different document, use importNode.")
   #Exceptions End
 
   var item: PNode = nList.getNamedItem(arg.nodeName())
@@ -788,11 +778,10 @@ proc setNamedItem*(nList: var seq[PNode], arg: PNode): PNode =
 proc setNamedItem*(nList: var seq[PAttr], arg: PAttr): PAttr =
   ## Adds ``arg`` as a ``Node`` to the ``NList``
   ## If a node with the same name is already present in this map, it is replaced by the new one.
-  if not isNil(nList):
-    if nList.len() > 0:
-      # Check if newChild is from this nodes document
-      if nList[0].fOwnerDocument != arg.fOwnerDocument:
-        raise newException(EWrongDocumentErr, "This node belongs to a different document, use importNode.")
+  if nList.len > 0:
+    # Check if newChild is from this nodes document
+    if nList[0].fOwnerDocument != arg.fOwnerDocument:
+      raise newException(EWrongDocumentErr, "This node belongs to a different document, use importNode.")
 
   if not isNil(arg.fOwnerElement):
     raise newException(EInuseAttributeErr, "This attribute is in use by another element, use cloneNode")
@@ -814,11 +803,10 @@ proc setNamedItem*(nList: var seq[PAttr], arg: PAttr): PAttr =
 
 proc setNamedItemNS*(nList: var seq[PNode], arg: PNode): PNode =
   ## Adds a node using its ``namespaceURI`` and ``localName``
-  if not isNil(nList):
-    if nList.len() > 0:
-      # Check if newChild is from this nodes document
-      if nList[0].fOwnerDocument != arg.fOwnerDocument:
-        raise newException(EWrongDocumentErr, "This node belongs to a different document, use importNode.")
+  if nList.len > 0:
+    # Check if newChild is from this nodes document
+    if nList[0].fOwnerDocument != arg.fOwnerDocument:
+      raise newException(EWrongDocumentErr, "This node belongs to a different document, use importNode.")
   #Exceptions end
 
   var item: PNode = nList.getNamedItemNS(arg.namespaceURI(), arg.localName())
@@ -837,11 +825,10 @@ proc setNamedItemNS*(nList: var seq[PNode], arg: PNode): PNode =
 
 proc setNamedItemNS*(nList: var seq[PAttr], arg: PAttr): PAttr =
   ## Adds a node using its ``namespaceURI`` and ``localName``
-  if not isNil(nList):
-    if nList.len() > 0:
-      # Check if newChild is from this nodes document
-      if nList[0].fOwnerDocument != arg.fOwnerDocument:
-        raise newException(EWrongDocumentErr, "This node belongs to a different document, use importNode.")
+  if nList.len > 0:
+    # Check if newChild is from this nodes document
+    if nList[0].fOwnerDocument != arg.fOwnerDocument:
+      raise newException(EWrongDocumentErr, "This node belongs to a different document, use importNode.")
 
   if not isNil(arg.fOwnerElement):
     raise newException(EInuseAttributeErr, "This attribute is in use by another element, use cloneNode")
@@ -868,17 +855,14 @@ proc setNamedItemNS*(nList: var seq[PAttr], arg: PAttr): PAttr =
 # Attributes
 proc name*(a: PAttr): string =
   ## Returns the name of the Attribute
-
   return a.fName
 
 proc specified*(a: PAttr): bool =
   ## Specifies whether this attribute was specified in the original document
-
   return a.fSpecified
 
 proc ownerElement*(a: PAttr): PElement =
   ## Returns this Attributes owner element
-
   return a.fOwnerElement
 
 # Element
@@ -886,14 +870,11 @@ proc ownerElement*(a: PAttr): PElement =
 
 proc tagName*(el: PElement): string =
   ## Returns the Element Tag Name
-
   return el.fTagName
 
 # Procedures
 proc getAttribute*(el: PNode, name: string): string =
   ## Retrieves an attribute value by ``name``
-  if isNil(el.attributes):
-    return ""
   var attribute = el.attributes.getNamedItem(name)
   if not isNil(attribute):
     return attribute.value
@@ -902,8 +883,6 @@ proc getAttribute*(el: PNode, name: string): string =
 
 proc getAttributeNS*(el: PNode, namespaceURI: string, localName: string): string =
   ## Retrieves an attribute value by ``localName`` and ``namespaceURI``
-  if isNil(el.attributes):
-    return ""
   var attribute = el.attributes.getNamedItemNS(namespaceURI, localName)
   if not isNil(attribute):
     return attribute.value
@@ -913,14 +892,10 @@ proc getAttributeNS*(el: PNode, namespaceURI: string, localName: string): string
 proc getAttributeNode*(el: PElement, name: string): PAttr =
   ## Retrieves an attribute node by ``name``
   ## To retrieve an attribute node by qualified name and namespace URI, use the `getAttributeNodeNS` method
-  if isNil(el.attributes):
-    return nil
   return el.attributes.getNamedItem(name)
 
 proc getAttributeNodeNS*(el: PElement, namespaceURI: string, localName: string): PAttr =
   ## Retrieves an `Attr` node by ``localName`` and ``namespaceURI``
-  if isNil(el.attributes):
-    return nil
   return el.attributes.getNamedItemNS(namespaceURI, localName)
 
 proc getElementsByTagName*(el: PElement, name: string): seq[PNode] =
@@ -938,41 +913,34 @@ proc getElementsByTagNameNS*(el: PElement, namespaceURI: string, localName: stri
 proc hasAttribute*(el: PElement, name: string): bool =
   ## Returns ``true`` when an attribute with a given ``name`` is specified
   ## on this element , ``false`` otherwise.
-  if isNil(el.attributes):
-    return false
   return not isNil(el.attributes.getNamedItem(name))
 
 proc hasAttributeNS*(el: PElement, namespaceURI: string, localName: string): bool =
   ## Returns ``true`` when an attribute with a given ``localName`` and
   ## ``namespaceURI`` is specified on this element , ``false`` otherwise
-  if isNil(el.attributes):
-    return false
   return not isNil(el.attributes.getNamedItemNS(namespaceURI, localName))
 
 proc removeAttribute*(el: PElement, name: string) =
   ## Removes an attribute by ``name``
-  if not isNil(el.attributes):
-    for i in low(el.attributes)..high(el.attributes):
-      if el.attributes[i].fName == name:
-        el.attributes.delete(i)
+  for i in low(el.attributes)..high(el.attributes):
+    if el.attributes[i].fName == name:
+      el.attributes.delete(i)
 
 proc removeAttributeNS*(el: PElement, namespaceURI: string, localName: string) =
   ## Removes an attribute by ``localName`` and ``namespaceURI``
-  if not isNil(el.attributes):
-    for i in low(el.attributes)..high(el.attributes):
-      if el.attributes[i].fNamespaceURI == namespaceURI and
-          el.attributes[i].fLocalName == localName:
-        el.attributes.delete(i)
+  for i in low(el.attributes)..high(el.attributes):
+    if el.attributes[i].fNamespaceURI == namespaceURI and
+        el.attributes[i].fLocalName == localName:
+      el.attributes.delete(i)
 
 proc removeAttributeNode*(el: PElement, oldAttr: PAttr): PAttr =
   ## Removes the specified attribute node
   ## If the attribute node cannot be found raises ``ENotFoundErr``
-  if not isNil(el.attributes):
-    for i in low(el.attributes)..high(el.attributes):
-      if el.attributes[i] == oldAttr:
-        result = el.attributes[i]
-        el.attributes.delete(i)
-        return
+  for i in low(el.attributes)..high(el.attributes):
+    if el.attributes[i] == oldAttr:
+      result = el.attributes[i]
+      el.attributes.delete(i)
+      return
 
   raise newException(ENotFoundErr, "oldAttr is not a member of el's Attributes")
 
@@ -991,7 +959,6 @@ proc setAttributeNode*(el: PElement, newAttr: PAttr): PAttr =
       "This attribute is in use by another element, use cloneNode")
   # Exceptions end
 
-  if isNil(el.attributes): el.attributes = @[]
   return el.attributes.setNamedItem(newAttr)
 
 proc setAttributeNodeNS*(el: PElement, newAttr: PAttr): PAttr =
@@ -1009,7 +976,6 @@ proc setAttributeNodeNS*(el: PElement, newAttr: PAttr): PAttr =
       "This attribute is in use by another element, use cloneNode")
   # Exceptions end
 
-  if isNil(el.attributes): el.attributes = @[]
   return el.attributes.setNamedItemNS(newAttr)
 
 proc setAttribute*(el: PElement, name: string, value: string) =
@@ -1057,9 +1023,9 @@ proc splitData*(textNode: PText, offset: int): PText =
 
   var left: string = textNode.data.substr(0, offset)
   textNode.data = left
-  var right: string = textNode.data.substr(offset, textNode.data.len())
+  var right: string = textNode.data.substr(offset, textNode.data.len)
 
-  if not isNil(textNode.fParentNode) and not isNil(textNode.fParentNode.childNodes):
+  if not isNil(textNode.fParentNode) and textNode.fParentNode.childNodes.len > 0:
     for i in low(textNode.fParentNode.childNodes)..high(textNode.fParentNode.childNodes):
       if textNode.fParentNode.childNodes[i] == textNode:
         var newNode: PText = textNode.fOwnerDocument.createTextNode(right)
@@ -1098,11 +1064,10 @@ proc escapeXml*(s: string): string =
 
 proc nodeToXml(n: PNode, indent: int = 0): string =
   result = spaces(indent) & "<" & n.nodeName
-  if not isNil(n.attributes):
-    for i in items(n.attributes):
-      result.add(" " & i.name & "=\"" & escapeXml(i.value) & "\"")
+  for i in items(n.attributes):
+    result.add(" " & i.name & "=\"" & escapeXml(i.value) & "\"")
 
-  if isNil(n.childNodes) or n.childNodes.len() == 0:
+  if n.childNodes.len == 0:
     result.add("/>") # No idea why this doesn't need a \n :O
   else:
     # End the beginning of this tag
@@ -1134,3 +1099,4 @@ proc `$`*(doc: PDocument): string =
   ## Converts a PDocument object into a string representation of it's XML
   result = "<?xml version=\"1.0\" encoding=\"UTF-8\" ?>\n"
   result.add(nodeToXml(doc.documentElement))
+  
\ No newline at end of file
diff --git a/lib/system.nim b/lib/system.nim
index 832a989768..29b472bdc3 100644
--- a/lib/system.nim
+++ b/lib/system.nim
@@ -2292,9 +2292,18 @@ iterator mpairs*(a: var cstring): tuple[key: int, val: var char] {.inline.} =
     inc(i)
 
 
-proc isNil*[T](x: seq[T]): bool {.noSideEffect, magic: "IsNil", deprecated.}
+when defined(nimNoNilSeqs2):
+  when not compileOption("nilseqs"):
+    {.pragma: nilError, error.}
+  else:
+    {.pragma: nilError.}
+else:
+  {.pragma: nilError.}
+
+proc isNil*[T](x: seq[T]): bool {.noSideEffect, magic: "IsNil", nilError.}
 proc isNil*[T](x: ref T): bool {.noSideEffect, magic: "IsNil".}
-proc isNil*(x: string): bool {.noSideEffect, magic: "IsNil", deprecated.}
+proc isNil*(x: string): bool {.noSideEffect, magic: "IsNil", nilError.}
+
 proc isNil*[T](x: ptr T): bool {.noSideEffect, magic: "IsNil".}
 proc isNil*(x: pointer): bool {.noSideEffect, magic: "IsNil".}
 proc isNil*(x: cstring): bool {.noSideEffect, magic: "IsNil".}
diff --git a/lib/system/excpt.nim b/lib/system/excpt.nim
index 7b4979cfa0..d9c7b4c25d 100644
--- a/lib/system/excpt.nim
+++ b/lib/system/excpt.nim
@@ -213,7 +213,7 @@ proc auxWriteStackTrace(f: PFrame; s: var seq[StackTraceEntry]) =
     inc(i)
     it = it.prev
   var last = i-1
-  if s.isNil:
+  if s.len == 0:
     s = newSeq[StackTraceEntry](i)
   else:
     last = s.len + i - 1
@@ -361,10 +361,10 @@ proc raiseExceptionAux(e: ref Exception) =
     else:
       when hasSomeStackTrace:
         var buf = newStringOfCap(2000)
-        if isNil(e.trace): rawWriteStackTrace(buf)
+        if e.trace.len == 0: rawWriteStackTrace(buf)
         else: add(buf, $e.trace)
         add(buf, "Error: unhandled exception: ")
-        if not isNil(e.msg): add(buf, e.msg)
+        add(buf, e.msg)
         add(buf, " [")
         add(buf, $e.name)
         add(buf, "]\n")
@@ -382,7 +382,7 @@ proc raiseExceptionAux(e: ref Exception) =
         var buf: array[0..2000, char]
         var L = 0
         add(buf, "Error: unhandled exception: ")
-        if not isNil(e.msg): add(buf, e.msg)
+        add(buf, e.msg)
         add(buf, " [")
         xadd(buf, e.name, e.name.len)
         add(buf, "]\n")
@@ -397,7 +397,7 @@ proc raiseExceptionAux(e: ref Exception) =
 proc raiseException(e: ref Exception, ename: cstring) {.compilerRtl.} =
   if e.name.isNil: e.name = ename
   when hasSomeStackTrace:
-    if e.trace.isNil:
+    if e.trace.len == 0:
       rawWriteStackTrace(e.trace)
     elif framePtr != nil:
       e.trace.add reraisedFrom(reraisedFromBegin)
@@ -427,7 +427,7 @@ proc getStackTrace(): string =
     result = "No stack traceback available\n"
 
 proc getStackTrace(e: ref Exception): string =
-  if not isNil(e) and not isNil(e.trace):
+  if not isNil(e):
     result = $e.trace
   else:
     result = ""
diff --git a/lib/system/repr.nim b/lib/system/repr.nim
index 45acae7f1b..3dc220b87b 100644
--- a/lib/system/repr.nim
+++ b/lib/system/repr.nim
@@ -266,7 +266,7 @@ when not defined(useNimRtl):
     of tyChar: add result, reprChar(cast[ptr char](p)[])
     of tyString:
       let sp = cast[ptr string](p)
-      reprStrAux(result, if sp[].isNil: nil else: sp[].cstring, sp[].len)
+      reprStrAux(result, sp[].cstring, sp[].len)
     of tyCString:
       let cs = cast[ptr cstring](p)[]
       if cs.isNil: add result, "nil"
diff --git a/lib/system/reprjs.nim b/lib/system/reprjs.nim
index d04d6e12b1..7cb25a252b 100644
--- a/lib/system/reprjs.nim
+++ b/lib/system/reprjs.nim
@@ -232,10 +232,7 @@ proc reprAux(result: var string, p: pointer, typ: PNimType,
   of tyString:
     var fp: int
     {. emit: "`fp` = `p`;\n" .}
-    if cast[string](fp).isNil:
-      add(result, "nil")
-    else:
-      add( result, reprStr(cast[string](p)) )
+    add( result, reprStr(cast[string](p)) )
   of tyCString:
     var fp: cstring
     {. emit: "`fp` = `p`;\n" .}
diff --git a/lib/system/sysio.nim b/lib/system/sysio.nim
index 3168e4cb28..75e26a6c9b 100644
--- a/lib/system/sysio.nim
+++ b/lib/system/sysio.nim
@@ -144,13 +144,7 @@ proc getFileHandle*(f: File): FileHandle = c_fileno(f)
 proc readLine(f: File, line: var TaintedString): bool =
   var pos = 0
   var sp: cint = 80
-  # Use the currently reserved space for a first try
-  if line.string.isNil:
-    line = TaintedString(newStringOfCap(80))
-  else:
-    when not defined(nimscript) and not defined(gcDestructors):
-      sp = cint(cast[PGenericSeq](line.string).space)
-    line.string.setLen(sp)
+  line.string.setLen(sp)
   while true:
     # memset to \L so that we can tell how far fgets wrote, even on EOF, where
     # fgets doesn't append an \L
diff --git a/lib/system/threads.nim b/lib/system/threads.nim
index 2434ea21e3..aaf0164fdd 100644
--- a/lib/system/threads.nim
+++ b/lib/system/threads.nim
@@ -393,8 +393,9 @@ proc onThreadDestruction*(handler: proc () {.closure, gcsafe.}) =
   ## A thread is destructed when the ``.thread`` proc returns
   ## normally or when it raises an exception. Note that unhandled exceptions
   ## in a thread nevertheless cause the whole process to die.
-  if threadDestructionHandlers.isNil:
-    threadDestructionHandlers = @[]
+  when not defined(nimNoNilSeqs):
+    if threadDestructionHandlers.isNil:
+      threadDestructionHandlers = @[]
   threadDestructionHandlers.add handler
 
 template afterThreadRuns() =
diff --git a/tests/manyloc/argument_parser/argument_parser.nim b/tests/manyloc/argument_parser/argument_parser.nim
index 985139f0a6..d42842f932 100644
--- a/tests/manyloc/argument_parser/argument_parser.nim
+++ b/tests/manyloc/argument_parser/argument_parser.nim
@@ -232,7 +232,7 @@ template run_custom_proc(parsed_parameter: Tparsed_parameter,
   if not custom_validator.isNil:
     try:
       let message = custom_validator(parameter, parsed_parameter)
-      if not message.isNil and message.len > 0:
+      if message.len > 0:
         raise_or_quit(ValueError, ("Failed to validate value for " &
           "parameter $1:\n$2" % [escape(parameter), message]))
     except:
diff --git a/tests/manyloc/keineschweine/keineschweine.nim.cfg b/tests/manyloc/keineschweine/keineschweine.nim.cfg
index ca6c75f6ed..f335a0e0e3 100644
--- a/tests/manyloc/keineschweine/keineschweine.nim.cfg
+++ b/tests/manyloc/keineschweine/keineschweine.nim.cfg
@@ -7,3 +7,4 @@ path = "dependencies/genpacket"
 path = "enet_server"
 debugger = off
 warning[SmallLshouldNotBeUsed] = off
+nilseqs = on