Find some way for serde to fail to deserialize inputs that break invariants

[vks]

Deserializing does not currently check for all invalid inputs. Also see #6 and #7.

[vks]

Here is some official advice for how this could be done:

use serde::{de, Deserialize, Deserializer};

#[derive(Deserialize, Debug)]
#[serde(remote = "Self")]
struct Invariant {
    /// len() >= 3
    s: String,
}

impl<'de> Deserialize<'de> for Invariant {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: Deserializer<'de>,
    {
        let unchecked = Invariant::deserialize(deserializer)?;
        if unchecked.s.len() < 3 {
            return Err(de::Error::custom("length of `s` must be >= 3"));
        }
        Ok(unchecked)
    }
}

fn main() {
    let j = r#" {"s": "abc"} "#;
    println!("{:?}", serde_json::from_str::<Invariant>(j).unwrap());
}