When you use serde_json::from_str() it'll try to infer the type to parse your string into. You forgot a semicolon at the end of the line and because main doesn't return anything (i.e. it returns ()) we try to deserialize into the unit value. I'm not exactly sure what this is in JSON.
If you don't actually care what type it deserializes as, just that you can deserializes it, look at the serde_json::Value type. This is essentially a union of all the possible things JSON can represent.