string to cstring conversions produce the empty C string for nil; this will be consistent with alternative string implementations

changelog.md

@@ -11,7 +11,7 @@
   to deal with!
 - Indexing into a ``cstring`` for the JS target is now mapped
   to ``charCodeAt``.
-- Assignments that would "slice" an object into its supertype are not prevented
+- Assignments that would "slice" an object into its supertype are now prevented
   at runtime. Use ``ref object`` with inheritance rather than ``object`` with
   inheritance to prevent this issue.
 
@@ -81,6 +81,9 @@
   Imported exceptions can be raised and caught just like Nim exceptions.
   More details in language manual.
 
+- ``nil`` for strings/seqs is finally gone. Instead the default value for
+  these is ``"" / @[]``.
+
 ### Tool changes
 
 - ``jsondoc2`` has been renamed ``jsondoc``, similar to how ``doc2`` was renamed

compiler/ccgcalls.nim

@@ -125,15 +125,15 @@ proc openArrayLoc(p: BProc, n: PNode): Rope =
     of tyString, tySequence:
       if skipTypes(n.typ, abstractInst).kind == tyVar and
             not compileToCpp(p.module):
-        result = "(*$1)->data, (*$1)->$2" % [a.rdLoc, lenField(p)]
+        result = "(*$1)->data, (*$1 ? (*$1)->$2 : 0)" % [a.rdLoc, lenField(p)]
       else:
-        result = "$1->data, $1->$2" % [a.rdLoc, lenField(p)]
+        result = "$1->data, ($1 ? $1->$2 : 0)" % [a.rdLoc, lenField(p)]
     of tyArray:
       result = "$1, $2" % [rdLoc(a), rope(lengthOrd(a.t))]
     of tyPtr, tyRef:
       case lastSon(a.t).kind
       of tyString, tySequence:
-        result = "(*$1)->data, (*$1)->$2" % [a.rdLoc, lenField(p)]
+        result = "(*$1)->data, (*$1 ? (*$1)->$2 : 0)" % [a.rdLoc, lenField(p)]
       of tyArray:
         result = "$1, $2" % [rdLoc(a), rope(lengthOrd(lastSon(a.t)))]
       else:
@@ -143,7 +143,7 @@ proc openArrayLoc(p: BProc, n: PNode): Rope =
 proc genArgStringToCString(p: BProc, n: PNode): Rope {.inline.} =
   var a: TLoc
   initLocExpr(p, n.sons[0], a)
-  result = "$1->data" % [a.rdLoc]
+  result = "($1 ? $1->data : \"\")" % [a.rdLoc]
 
 proc genArg(p: BProc, n: PNode, param: PSym; call: PNode): Rope =
   var a: TLoc

compiler/ccgexprs.nim

@@ -1740,7 +1740,7 @@ proc genConv(p: BProc, e: PNode, d: var TLoc) =
 proc convStrToCStr(p: BProc, n: PNode, d: var TLoc) =
   var a: TLoc
   initLocExpr(p, n.sons[0], a)
-  putIntoDest(p, d, n, "$1->data" % [rdLoc(a)],
+  putIntoDest(p, d, n, "($1 ? $1->data : \"\")" % [rdLoc(a)],
               a.storage)
 
 proc convCStrToStr(p: BProc, n: PNode, d: var TLoc) =