Auto merge of #9237 - yaymukund:serde-expecting, r=ehuss

Use serde's error message option to avoid implementing `Deserialize`.

This is a cleanup based on https://github.com/serde-rs/serde/issues/1883, which fell out of https://github.com/rust-lang/cargo/pull/8664

It looks like this changes the resulting error messages. I'll leave it up to you to decide if the tradeoff makes sense here. Maybe the correct answer here is to make `serde`'s error messages include more details (e.g. `invalid type: int`).