writing to a location counts as "side effect"; implements https://github.com/nim-lang/RFCs/issues/234 (#15030)

tests/effects/tfuncs_cannot_mutate.nim

@@ -0,0 +1,31 @@
+discard """
+  errormsg: "'mutate' can have side effects"
+  line: 25
+"""
+
+{.experimental: "strictFuncs".}
+
+type
+  Node = ref object
+    le, ri: Node
+    data: string
+
+func len(n: Node): int =
+  var it = n
+  while it != nil:
+    inc result
+    it = it.ri
+
+func doNotDistract(n: Node) =
+  var m = Node()
+  m.data = "abc"
+
+func select(a, b: Node): Node = b
+
+func mutate(n: Node) =
+  var it = n
+  let x = it
+  let y = x
+  let z = y
+
+  select(x, z).data = "tricky"