fix string literal assignment with different lengths on ARC (#24083)

fixes #24080

lib/system/strs_v2.nim

@@ -166,7 +166,7 @@ proc setLengthStrV2(s: var NimStringV2, newLen: int) {.compilerRtl.} =
   s.len = newLen
 
 proc nimAsgnStrV2(a: var NimStringV2, b: NimStringV2) {.compilerRtl.} =
-  if a.p == b.p: return
+  if a.p == b.p and a.len == b.len: return
   if isLiteral(b):
     # we can shallow copy literals:
     frees(a)

tests/arc/tstringliteral.nim

@@ -0,0 +1,17 @@
+discard """
+  matrix: "--mm:arc; --mm:orc"
+"""
+
+block: # issue #24080
+  var a = (s: "a")
+  var b = "a"
+  a.s.setLen 0
+  b = a.s
+  doAssert b == ""
+
+block: # issue #24080, longer string
+  var a = (s: "abc")
+  var b = "abc"
+  a.s.setLen 2
+  b = a.s
+  doAssert b == "ab"