breaking change: arrays of char do not convert to cstring; ptr to array of char does

compiler/condsyms.nim

@@ -108,3 +108,4 @@ proc initDefines*() =
   defineSymbol("nimHasCppDefine")
   defineSymbol("nimGenericInOutFlags")
   when false: defineSymbol("nimHasOpt")
+  defineSymbol("nimNoArrayToCstringConversion")

compiler/rodutils.nim

@@ -21,9 +21,14 @@ proc toStrMaxPrecision*(f: BiggestFloat, literalPostfix = ""): string =
     if f > 0.0: result = "INF"
     else: result = "-INF"
   else:
-    var buf: array[0..80, char]
-    discard c_snprintf(buf.cstring, buf.len.uint, "%#.16e%s", f, literalPostfix.cstring)
-    result = $buf.cstring
+    when defined(nimNoArrayToCstringConversion):
+      result = newString(81)
+      let n = c_snprintf(result.cstring, result.len.uint, "%#.16e%s", f, literalPostfix.cstring)
+      setLen(result, n)
+    else:
+      var buf: array[0..80, char]
+      discard c_snprintf(buf.cstring, buf.len.uint, "%#.16e%s", f, literalPostfix.cstring)
+      result = $buf.cstring
 
 proc encodeStr*(s: string, result: var string) =
   for i in countup(0, len(s) - 1):

compiler/sigmatch.nim

@@ -1288,12 +1288,13 @@ proc typeRelImpl(c: var TCandidate, f, aOrig: PType,
     of tyString: result = isConvertible
     of tyPtr:
       # ptr[Tag, char] is not convertible to 'cstring' for now:
-      if a.len == 1 and a.sons[0].kind == tyChar: result = isConvertible
-    of tyArray:
-      if (firstOrd(a.sons[0]) == 0) and
-          (skipTypes(a.sons[0], {tyRange}).kind in {tyInt..tyInt64}) and
-          (a.sons[1].kind == tyChar):
-        result = isConvertible
+      if a.len == 1:
+        let pointsTo = a.sons[0].skipTypes(abstractInst)
+        if pointsTo.kind == tyChar: result = isConvertible
+        elif pointsTo.kind == tyArray and firstOrd(pointsTo.sons[0]) == 0 and
+            skipTypes(pointsTo.sons[0], {tyRange}).kind in {tyInt..tyInt64} and
+            pointsTo.sons[1].kind == tyChar:
+          result = isConvertible
     else: discard
 
   of tyEmpty, tyVoid:

compiler/types.nim

@@ -667,14 +667,6 @@ proc lengthOrd*(t: PType): BiggestInt =
     else:
       result = lastOrd(t) - firstOrd(t) + 1
 
-proc isCompatibleToCString*(a: PType): bool =
-  if a.kind == tyArray:
-    if (firstOrd(a.sons[0]) == 0) and
-        (skipTypes(a.sons[0], {tyRange, tyGenericInst, tyAlias}).kind in
-            {tyInt..tyInt64, tyUInt..tyUInt64}) and
-        (a.sons[1].kind == tyChar):
-      result = true
-
 # -------------- type equality -----------------------------------------------
 
 type