Function get_subtable 

pub fn get_subtable(
    state: &mut LuaState,
    idx: i32,
    fname: &[u8],
) -> Result<bool, LuaError>

Ensure state[idx][fname] is a table; push it. Returns true if the table already existed, false if newly created.

C: LUALIB_API int luaL_getsubtable(lua_State *L, int idx, const char *fname)