Yeah, having done this, it's extremely verbose if you have all your error handling in there due to the verbosity of dealing with JSON and explicit return values instead of exceptions. By far my greatest annoyance is that []interface{} cannot be directly cast to []concreteType. Having to make a sized slice and type assert each value is annoying. Require validation of the values for even more "if err != nil" fun.

Many Go advocates seem to consider the verbosity a feature, because it's explicit and forbids any clever-but-confusing tricks.

How would you express deserializing these 'polymorphic JSON' objects using serde/Rust?

Using a rust enum I guess.

You're right, this does seem to work [1]. I wasn't aware that serde would attempt to deserialize multiple enum variants until something matches.

[1] - https://serde.rs/enum-representations.html#untagged

Yeah that's a rather common scheme out there so Serde does provide built-in support for this deserialisation. Probably better for deserialisation performances to use properly tagged enums, but if you don't have a choice Serde's got your back.