Implements RFCs #209 (#13995)

* add test
* add changelod entry
Co-authored-by: cooldome <ariabushenko@bk.ru>

changelog.md

@@ -12,7 +12,34 @@
   one in an instantiation context while `-d:nimIntHash1` recovers it globally.
 
 ## Language changes
+- In newruntime it is now allowed to assign discriminator field without restrictions as long as case object doesn't have custom destructor. Discriminator value doesn't have to be a constant either. If you have custom destructor for case object and you do want to freely assign discriminator fields, it is recommended to refactor object into 2 objects like this: 
+  ```nim
+  type
+    MyObj = object
+      case kind: bool
+        of true: y: ptr UncheckedArray[float]
+        of false: z: seq[int]
 
+  proc `=destroy`(x: MyObj) =
+    if x.kind and x.y != nil:
+      deallocShared(x.y)
+      x.y = nil
+  ```
+  Refactor into:
+  ```nim
+  type
+    MySubObj = object
+      val: ptr UncheckedArray[float]
+    MyObj = object
+      case kind: bool
+      of true: y: MySubObj
+      of false: z: seq[int]
+
+  proc `=destroy`(x: MySubObj) =
+    if x.val != nil:
+      deallocShared(x.val)
+      x.val = nil
+  ```
 
 ## Compiler changes