The Rust JSON package needs the entire JSON text read, then parsed, and only then can you do anything. This uses a lot of memory at peak. Streaming XML parsers are common, but streaming JSON parsers are not. Is there a Rust streaming parser for JSON?
I don't think you can even read "back to back" JSON sequentially where multiple JSON outputs are concatenated.
(I have a program which reads a huge JSON file (tens of gigabytes), and can go over 32GB at peak. Once the data is loaded, it needs only a few hundred MB. It's mostly repeats of a modest size item, so all I really need is some serialization format that's readable sequentially. Do any of the already-implemented Serde formats offer that?)