Incorrect `lljson.slencode()` behavior around `nil` / `lljson.null`

[HaroldCindy]

In lljson.slencode() we can encode the distinction between nil holes in arrays and literal lljson.null, and we should do so, rather than always turning nil into lljson.null, since that changes semantics.

[WolfGangS]

I would suggest a new prefix for slencode

I can pr this if wanted, or you can pinch the included patch

!0 nil

Same number of characters as null and differentiates it from lljson.null

Example

[1,"!0",2]

becomes

{1,nil,2}

implemented like this

diff --git a/VM/src/cjson/lua_cjson.cpp b/VM/src/cjson/lua_cjson.cpp
index 592c896e..e52cef77 100644
--- a/VM/src/cjson/lua_cjson.cpp
+++ b/VM/src/cjson/lua_cjson.cpp
@@ -1202,7 +1202,10 @@ static int json_append_data(lua_State *l, json_config_t *cfg,
         }
         break;
     case LUA_TNIL:
-        strbuf_append_mem(json, "null", 4);
+        if (cfg->sl_tagged_types)
+            strbuf_append_string(json, "\"!0\"");
+        else
+            strbuf_append_mem(json, "null", 4);
         break;
     case LUA_TLIGHTUSERDATA: {
         if (void* json_internal_val = lua_tolightuserdatatagged(l, -1, LU_TAG_JSON_INTERNAL))
@@ -1604,6 +1607,11 @@ static bool json_parse_tagged_string(lua_State *l, const char *str, size_t len)
         lua_newbuffer(l, 0);
         return true;
     }
+    case '0':
+    {
+        lua_pushnil(l);
+        return true;
+    }
 
     default:
         luaL_error(l, "unknown tag '!%c' in string: %s", tag, str);

Produces the following output