nil strings are movable

compiler/ccgexprs.nim

@@ -151,7 +151,7 @@ proc getStorageLoc(n: PNode): TStorageLoc =
     result = getStorageLoc(n.sons[0])
   else: result = OnUnknown
 
-proc canMove(n: PNode): bool =
+proc canMove(p: BProc, n: PNode): bool =
   # for now we're conservative here:
   if n.kind == nkBracket:
     # This needs to be kept consistent with 'const' seq code
@@ -159,6 +159,10 @@ proc canMove(n: PNode): bool =
     if not isDeepConstExpr(n) or n.len == 0:
       if skipTypes(n.typ, abstractVarRange).kind == tySequence:
         return true
+  elif optNilSeqs notin p.options and
+    n.kind in nkStrKinds and n.strVal.len == 0:
+    # Empty strings are codegen'd as NIM_NIL so it's just a pointer copy
+    return true
   result = n.kind in nkCallKinds
   #if result:
   #  echo n.info, " optimized ", n
@@ -286,7 +290,7 @@ proc genAssignment(p: BProc, dest, src: TLoc, flags: TAssignmentFlags) =
   of tySequence:
     if p.config.selectedGC == gcDestructors:
       genGenericAsgn(p, dest, src, flags)
-    elif (needToCopy notin flags and src.storage != OnStatic) or canMove(src.lode):
+    elif (needToCopy notin flags and src.storage != OnStatic) or canMove(p, src.lode):
       genRefAssign(p, dest, src, flags)
     else:
       linefmt(p, cpsStmts, "#genericSeqAssign($1, $2, $3);$n",
@@ -295,7 +299,7 @@ proc genAssignment(p: BProc, dest, src: TLoc, flags: TAssignmentFlags) =
   of tyString:
     if p.config.selectedGC == gcDestructors:
       genGenericAsgn(p, dest, src, flags)
-    elif (needToCopy notin flags and src.storage != OnStatic) or canMove(src.lode):
+    elif (needToCopy notin flags and src.storage != OnStatic) or canMove(p, src.lode):
       genRefAssign(p, dest, src, flags)
     else:
       if dest.storage == OnStack or not usesWriteBarrier(p.config):

tests/ccgbugs/tassign_nil_strings.nim

@@ -0,0 +1,13 @@
+discard """
+  cmd: "nim $target --nilseqs:off $options $file"
+  output: "Hello"
+  ccodecheck: "\\i@'a = ((NimStringDesc*) NIM_NIL)'"
+"""
+
+proc main() =
+  var a = "Hello"
+  echo a
+  a = ""
+  doAssert a.len == 0
+
+main()