Rust Serde

A #[derive(Deserialize)] is a claim that anything it accepts is already valid for the domain — the parse is the only place that claim is cheap to enforce.

Deserialization is a parse

The type you deserialize into is the type the rest of the program trusts, so it is the last place validation is cheap. A Config that deserializes with a String port and checks it in run() has moved the failure past every layer that could have reported it usefully. What the validated type should be is /type-driven-design; this skill owns the boundary that type crosses.

#[serde(try_from = "...")] is the mechanism

Deserialize into a raw shape, convert with TryFrom into the validated type, and the conversion failure becomes a deserialization error — the read fails, not the first use of the value. The error must implement std::error::Error.