allow addressing elements of openArray[char] in VM (#22045)

allow addressing elements of openArray[char]

(cherry picked from commit a8d0dda833)

compiler/vm.nim

@@ -772,6 +772,8 @@ proc rawExecute(c: PCtx, start: int, tos: PStackFrame): TFullReg =
       else:
         if src.kind notin {nkEmpty..nkTripleStrLit} and idx <% src.len:
           takeAddress regs[ra], src.sons[idx]
+        elif src.kind in nkStrKinds and idx <% src.strVal.len:
+          regs[ra] = takeCharAddress(c, src, idx, pc)
         else:
           stackTrace(c, tos, pc, formatErrorIndexBound(idx, src.safeLen-1))
     of opcLdStrIdx:

tests/vm/tvmmisc.nim

@@ -657,3 +657,17 @@ proc macroGlobal =
 
 static: macroGlobal()
 macroGlobal()
+
+block:
+  proc swap[T](x: var T): T =
+    result = x
+    x = default(T)
+
+  proc merge[T](a, b: var openArray[T]) =
+    a[0] = swap b[0]
+
+  static:
+    var x = "abc"
+    var y = "356"
+    merge(x, y)
+    doAssert x == "3bc"