gistfile1.rs

#[derive(Deserialize)]
pub struct SignupData {
   #[validate(email)]
    email: String,
    // this would first run min_length and then call validate_unique_username
    #[validate(min_length=2, custom=validate_unique_username)]
    username: String,
    #[validate(min_length=8, max_length=255)]
    password: String,
}

pub fn validate_unique_username(username String) -> Option<Something> {
  ...
}

//  And to use
// first basic serialization errors
let data: SignupData = serde_json::from_str(&serialized);

// then a magic method added () -> Option<ValidationError> with ValidationError being a hashmap
let errors = data.validate();