fixes #23759; rework move for refc (#23764)

fixes #23759

compiler/ccgexprs.nim

@@ -343,7 +343,7 @@ proc genAssignment(p: BProc, dest, src: TLoc, flags: TAssignmentFlags) =
   of tyString:
     if optSeqDestructors in p.config.globalOptions:
       genGenericAsgn(p, dest, src, flags)
-    elif (needToCopy notin flags and src.storage != OnStatic) or canMove(p, src.lode, dest):
+    elif ({needToCopy, needToCopySinkParam} * flags == {} and src.storage != OnStatic) or canMove(p, src.lode, dest):
       genRefAssign(p, dest, src)
     else:
       if (dest.storage == OnStack and p.config.selectedGC != gcGo) or not usesWriteBarrier(p.config):
@@ -2345,8 +2345,13 @@ proc genMove(p: BProc; n: PNode; d: var TLoc) =
           else:
             linefmt(p, cpsStmts, "$1($2);$n", [rdLoc(b), byRefLoc(p, a)])
     else:
-      let flags = if not canMove(p, n[1], d): {needToCopy} else: {}
-      genAssignment(p, d, a, flags)
+      if n[1].kind == nkSym and isSinkParam(n[1].sym):
+        var tmp = getTemp(p, n[1].typ.skipTypes({tySink}))
+        genAssignment(p, tmp, a, {needToCopySinkParam})
+        genAssignment(p, d, tmp, {})
+        resetLoc(p, tmp)
+      else:
+        genAssignment(p, d, a, {})
       resetLoc(p, a)
 
 proc genDestroy(p: BProc; n: PNode) =

compiler/cgen.nim

@@ -412,6 +412,7 @@ proc rdCharLoc(a: TLoc): Rope =
 type
   TAssignmentFlag = enum
     needToCopy
+    needToCopySinkParam
     needTempForOpenArray
   TAssignmentFlags = set[TAssignmentFlag]
 

tests/refc/tsinkbug.nim

@@ -15,3 +15,12 @@ var obj = AnObject(value: 42)
 echo "Value is: ", obj.value
 mutate(obj)
 echo "Value is: ", obj.value
+
+proc p(x: sink string) = 
+  var y = move(x)
+  doAssert x.len == 0
+  doAssert y.len == 4
+
+p("1234")
+var s = "oooo"
+p(s)