C++ backend: use .noInit pragma for non-public default constructors; refs #4687

compiler/cgen.nim

@@ -616,6 +616,24 @@ proc closureSetup(p: BProc, prc: PSym) =
   linefmt(p, cpsStmts, "$1 = ($2) ClEnv;$n",
           rdLoc(env.loc), getTypeDesc(p.module, env.typ))
 
+proc easyResultAsgn(n: PNode): PNode =
+  const harmless = {nkConstSection, nkTypeSection, nkEmpty, nkCommentStmt} +
+                    declarativeDefs
+  case n.kind
+  of nkStmtList, nkStmtListExpr:
+    var i = 0
+    while i < n.len and n[i].kind in harmless: inc i
+    if i < n.len: result = easyResultAsgn(n[i])
+  of nkAsgn, nkFastAsgn:
+    if n[0].kind == nkSym and skResult == n[0].sym.kind:
+      incl n.flags, nfPreventCg
+      return n[1]
+  of nkReturnStmt:
+    if n.len > 0:
+      result = easyResultAsgn(n[0])
+      if result != nil: incl n.flags, nfPreventCg
+  else: discard
+
 proc genProcAux(m: BModule, prc: PSym) =
   var p = newProc(prc, m)
   var header = genProcHeader(m, prc)
@@ -627,11 +645,17 @@ proc genProcAux(m: BModule, prc: PSym) =
     var res = prc.ast.sons[resultPos].sym # get result symbol
     if not isInvalidReturnType(prc.typ.sons[0]):
       if sfNoInit in prc.flags: incl(res.flags, sfNoInit)
-      # declare the result symbol:
-      assignLocalVar(p, res)
-      assert(res.loc.r != nil)
+      if sfNoInit in prc.flags and p.module.compileToCpp and (let val = easyResultAsgn(prc.getBody); val != nil):
+        var decl = localVarDecl(p, res)
+        var a: TLoc
+        initLocExprSingleUse(p, val, a)
+        linefmt(p, cpsStmts, "$1 = $2;$n", decl, rdLoc(a))
+      else:
+        # declare the result symbol:
+        assignLocalVar(p, res)
+        assert(res.loc.r != nil)
+        initLocalVar(p, res, immediateAsgn=false)
       returnStmt = rfmt(nil, "\treturn $1;$n", rdLoc(res.loc))
-      initLocalVar(p, res, immediateAsgn=false)
     else:
       fillResult(res)
       assignParam(p, res)
@@ -791,12 +815,12 @@ proc addIntTypes(result: var Rope) {.inline.} =
 proc getCopyright(cfile: string): Rope =
   if optCompileOnly in gGlobalOptions:
     result = ("/* Generated by Nim Compiler v$1 */$N" &
-        "/*   (c) 2015 Andreas Rumpf */$N" &
+        "/*   (c) 2016 Andreas Rumpf */$N" &
         "/* The generated code is subject to the original license. */$N") %
         [rope(VersionAsString)]
   else:
     result = ("/* Generated by Nim Compiler v$1 */$N" &
-        "/*   (c) 2015 Andreas Rumpf */$N" &
+        "/*   (c) 2016 Andreas Rumpf */$N" &
         "/* The generated code is subject to the original license. */$N" &
         "/* Compiled for: $2, $3, $4 */$N" &
         "/* Command for C compiler:$n   $5 */$N") %