fixes #20422; emit nimPrepareStrMutationV2 for toOpenArray to keep th… (#21459)

fixes #20422; emit nimPrepareStrMutationV2 for toOpenArray to keep the abstraction of mutable strings which have immutable string literals

compiler/ccgcalls.nim

@@ -156,7 +156,7 @@ proc reifiedOpenArray(n: PNode): bool {.inline.} =
   else:
     result = true
 
-proc genOpenArraySlice(p: BProc; q: PNode; formalType, destType: PType): (Rope, Rope) =
+proc genOpenArraySlice(p: BProc; q: PNode; formalType, destType: PType; prepareForMutation = false): (Rope, Rope) =
   var a, b, c: TLoc
   initLocExpr(p, q[1], a)
   initLocExpr(p, q[2], b)
@@ -164,6 +164,8 @@ proc genOpenArraySlice(p: BProc; q: PNode; formalType, destType: PType): (Rope,
   # but first produce the required index checks:
   if optBoundsCheck in p.options:
     genBoundsCheck(p, a, b, c)
+  if prepareForMutation:
+    linefmt(p, cpsStmts, "#nimPrepareStrMutationV2($1);$n", [byRefLoc(p, a)])
   let ty = skipTypes(a.t, abstractVar+{tyPtr})
   let dest = getTypeDesc(p.module, destType)
   let lengthExpr = "($1)-($2)+1" % [rdLoc(c), rdLoc(b)]

compiler/ccgexprs.nim

@@ -2408,7 +2408,10 @@ proc genDispose(p: BProc; n: PNode) =
       lineCg(p, cpsStmts, ["#nimDestroyAndDispose($#)", rdLoc(a)])
 
 proc genSlice(p: BProc; e: PNode; d: var TLoc) =
-  let (x, y) = genOpenArraySlice(p, e, e.typ, e.typ.lastSon)
+  let (x, y) = genOpenArraySlice(p, e, e.typ, e.typ.lastSon,
+    prepareForMutation = e[1].kind == nkHiddenDeref and
+                         e[1].typ.skipTypes(abstractInst).kind == tyString and
+                         p.config.selectedGC in {gcArc, gcOrc})
   if d.k == locNone: getTemp(p, e.typ, d)
   linefmt(p, cpsStmts, "$1.Field0 = $2; $1.Field1 = $3;$n", [rdLoc(d), x, y])
   when false:

tests/arc/topenarray.nim

@@ -6,6 +6,8 @@ Nim
 '''
   matrix: "--gc:arc -d:useMalloc; --gc:arc"
 """
+{.experimental: "views".}
+
 block: # bug 18627
   proc setPosition(params: openArray[string]) =
     for i in params.toOpenArray(0, params.len - 1):
@@ -38,3 +40,13 @@ block: # bug #20954
       var v: seq[int]
 
       echo len(toOpenArray(v, 20, 30))
+
+# bug #20422
+
+proc f(a: var string) =
+  var v = a.toOpenArray(1, 3)
+  v[0] = 'a'
+
+var a = "Hello"
+f(a)
+doAssert a == "Hallo"