validation.rs

pub enum FormError {
    UniqueViolation,
    LengthRange(u32, u32),
}

// impl MyError for FormError { ... }

#[derive(Deserialize)]
pub struct SignupData {
    username: String,
    password: String,
}

impl SignupData {
    // In actual implementation, map keys could be interned strings or enum variants.
    pub fn validate(&self, conn: &GenericConnection) -> MyResult<BTreeMap<String, FormError>> {
        let mut map = BTreeMap::new();

        let n = try!(conn.execute("SELECT 1 FROM users WHERE username = $1;", &[&self.username]));
        if n != 0 {
            map.insert("username", FormError::UniqueViolation);
        }

        let n = self.password.len();
        if n < 8 || n > 32 {
            map.insert("password", FormError::LengthRange(8, 32));
        }

        map
    }
}