fixes #22597; avoid side effects for call returning openArray types (#23257)

fixes #22597

```nim
proc autoToOpenArray*[T](s: Slice[T]): openArray[T] =
  echo "here twice"
  result = toOpenArray(s.p, s.first, s.last)
```
For functions returning openarray types, `fixupCall` creates a temporary
variable to store the return value: `let tmp = autoToOpenArray()`. But
`genOpenArrayConv` cannot handle openarray assignements with side
effects. It should have stored the right part of the assignment first
instead of calling the right part twice.

(cherry picked from commit d44b0b1869)

compiler/ccgcalls.nim

@@ -83,6 +83,10 @@ proc fixupCall(p: BProc, le, ri: PNode, d: var TLoc,
   # getUniqueType() is too expensive here:
   var typ = skipTypes(ri[0].typ, abstractInst)
   if typ[0] != nil:
+    var flags: TAssignmentFlags = {}
+    if typ[0].kind in {tyOpenArray, tyVarargs}:
+      # perhaps generate no temp if the call doesn't have side effects
+      flags.incl needTempForOpenArray
     if isInvalidReturnType(p.config, typ):
       if params.len != 0: pl.add(", ")
       # beware of 'result = p(result)'. We may need to allocate a temporary:
@@ -130,7 +134,7 @@ proc fixupCall(p: BProc, le, ri: PNode, d: var TLoc,
         var list: TLoc
         initLoc(list, locCall, d.lode, OnUnknown)
         list.r = pl
-        genAssignment(p, d, list, {}) # no need for deep copying
+        genAssignment(p, d, list, flags) # no need for deep copying
         if canRaise: raiseExit(p)
       else:
         var tmp: TLoc
@@ -138,7 +142,7 @@ proc fixupCall(p: BProc, le, ri: PNode, d: var TLoc,
         var list: TLoc
         initLoc(list, locCall, d.lode, OnUnknown)
         list.r = pl
-        genAssignment(p, tmp, list, {}) # no need for deep copying
+        genAssignment(p, tmp, list, flags) # no need for deep copying
         if canRaise: raiseExit(p)
         genAssignment(p, d, tmp, {})
   else:

compiler/ccgexprs.nim

@@ -285,15 +285,23 @@ proc genGenericAsgn(p: BProc, dest, src: TLoc, flags: TAssignmentFlags) =
     linefmt(p, cpsStmts, "#genericAssign((void*)$1, (void*)$2, $3);$n",
             [addrLoc(p.config, dest), addrLoc(p.config, src), genTypeInfoV1(p.module, dest.t, dest.lode.info)])
 
-proc genOpenArrayConv(p: BProc; d: TLoc; a: TLoc) =
+proc genOpenArrayConv(p: BProc; d: TLoc; a: TLoc; flags: TAssignmentFlags) =
   assert d.k != locNone
   #  getTemp(p, d.t, d)
 
   case a.t.skipTypes(abstractVar).kind
   of tyOpenArray, tyVarargs:
     if reifiedOpenArray(a.lode):
-      linefmt(p, cpsStmts, "$1.Field0 = $2.Field0; $1.Field1 = $2.Field1;$n",
-        [rdLoc(d), a.rdLoc])
+      if needTempForOpenArray in flags:
+        var tmp: TLoc
+        getTemp(p, a.t, tmp)
+        linefmt(p, cpsStmts, "$2 = $1; $n",
+                [a.rdLoc, tmp.rdLoc])
+        linefmt(p, cpsStmts, "$1.Field0 = $2.Field0; $1.Field1 = $2.Field1;$n",
+          [rdLoc(d), tmp.rdLoc])
+      else:
+        linefmt(p, cpsStmts, "$1.Field0 = $2.Field0; $1.Field1 = $2.Field1;$n",
+          [rdLoc(d), a.rdLoc])
     else:
       linefmt(p, cpsStmts, "$1.Field0 = $2; $1.Field1 = $2Len_0;$n",
         [rdLoc(d), a.rdLoc])
@@ -392,7 +400,7 @@ proc genAssignment(p: BProc, dest, src: TLoc, flags: TAssignmentFlags) =
     # open arrays are always on the stack - really? What if a sequence is
     # passed to an open array?
     if reifiedOpenArray(dest.lode):
-      genOpenArrayConv(p, dest, src)
+      genOpenArrayConv(p, dest, src, flags)
     elif containsGarbageCollectedRef(dest.t):
       linefmt(p, cpsStmts,     # XXX: is this correct for arrays?
            "#genericAssignOpenArray((void*)$1, (void*)$2, $1Len_0, $3);$n",

compiler/cgen.nim

@@ -401,6 +401,7 @@ proc rdCharLoc(a: TLoc): Rope =
 type
   TAssignmentFlag = enum
     needToCopy
+    needTempForOpenArray
   TAssignmentFlags = set[TAssignmentFlag]
 
 proc genObjConstr(p: BProc, e: PNode, d: var TLoc)

tests/views/tviews1.nim

@@ -105,3 +105,22 @@ block: # bug #22117
     result = aref
 
   doAssert main() == 10
+
+type
+  Slice*[T] = object
+    first, last: int
+    p: ptr UncheckedArray[T]
+
+var i = 0
+
+converter autoToOpenArray*[T](s: Slice[T]): openArray[T] =
+  inc i
+  result = toOpenArray(s.p, s.first, s.last)
+
+proc acceptOpenArray(s: openArray[byte]) = discard
+
+proc bug22597 = # bug #22597
+  acceptOpenArray(Slice[byte]())
+  doAssert i == 1
+
+bug22597()