See https://medium.com/@ly.lee/safer-simpler-embedded-rust-with-apache-mynewt-on-stm32-blue-pill-d8fcb41969ac?source=friends_link&sk=16a27a4b9910aadcfcb9881c02d84adc

parse-json.rs

macro_rules! parse {
  ...  
  (@$enc:ident @object $obj:ident [$($key:tt)+] ($value:expr) , $($rest:tt)*) => {
    d!(adding key: $($key)+ value: $value to object: $obj);

    //  Append the key and value to the "values" array e.g.
    //    {"key":"device", "value":"010203"},
    $crate::coap_item_str!(@$enc $obj, $($key)+, $value);

    //  Continue parsing the rest of the JSON.
    $crate::parse!(@$enc @object $obj () ($($rest)*) ($($rest)*));
  };
  ...