• Just a wrapper of Serde (100% compatible),
• implementing serde::{Serialize, Deserialize} for your structs,
• with providing #[serde(validate = "...")] for declarative validation in #[derive(Deserialize)].

#[derive(serde::Deserialize)]
struct Point {
    x: i32,
    y: i32
}

#[derive(serde::Deserialize)]
#[serde(try_from = "Point")]
struct ValidPoint(Point);

impl TryFrom<Point> for ValidPoint {
    // ...
}

struct Point {
    x: i32,
    y: i32
}

impl<'de> serde::Deserialize<'de> for Point {
    // ...
}

[dependencies]
serdev     = { version = "0.3", features = ["derive"] }
serde_json = "1.0"

use serdev::{Serialize, Deserialize};

#[derive(Serialize, Deserialize, Debug)]
#[serde(validate = "Self::validate")]
struct Point {
    x: i32,
    y: i32,
}

impl Point {
    fn validate(&self) -> Result<(), impl std::fmt::Display> {
        if self.x * self.y > 100 {
            return Err("x * y must not exceed 100")
        }
        Ok(())
    }
}

fn main() {
    let point = serde_json::from_str::<Point>(r#"
        { "x" : 1, "y" : 2 }
    "#).unwrap();

    // Prints point = Point { x: 1, y: 2 }
    println!("point = {point:?}");

    let error = serde_json::from_str::<Point>(r#"
        { "x" : 10, "y" : 20 }
    "#).unwrap_err();

    // Prints error = x * y must not exceed 100
    println!("error = {error}");
}

#[serde(validate = "...")] works with:

• other validation tools like validator crate or something similar. (working example: validator.rs)

  use serdev::Deserialize;
  use validator::{Validate, ValidationError};
  
  #[derive(Deserialize, Debug, PartialEq, Validate)]
  #[serde(validate = "Validate::validate")]
  struct SignupData {
      #[validate(email)]
      mail: String,
      #[validate(url)]
      site: String,
      #[validate(length(min = 1), custom(function = "validate_unique_username"))]
      #[serde(rename = "firstName")]
      first_name: String,
      #[validate(range(min = 18, max = 20))]
      age: u32,
      #[validate(range(min = 0.0, max = 100.0))]
      height: f32,
  }
  
  fn validate_unique_username(username: &str) -> Result<(), ValidationError> {
      if username == "xXxShad0wxXx" {
          // the value of the username will automatically be added later
          return Err(ValidationError::new("terrible_username"));
      }
  
      Ok(())
  }

• inlined closure like |p| if p.x * p.y <= 100 {Ok(())} else {Err("...")}, not only a method path. (working example: closure.rs)

  use serdev::{Serialize, Deserialize};
  
  #[derive(Serialize, Deserialize, Debug)]
  #[serde(validate = r#"|p| (p.x * p.y <= 100).then_some(()).ok_or("x * y must not exceed 100")"#)]
  struct Point {
      x: i32,
      y: i32,
  }

• #[serde(validate = "function")]

  Automatically validate the deserialized struct by the function. The function must be an expression that is callable as type fn(&self) -> Result<(), impl Display> (of course the error type must be known at compile time).

  (expression: an inlined closure as above, or name/path to a fn or a method, or even a block expression or function calling or anything that are finally evaluated as fn(&self) -> Result<(), impl Display>)

  Errors are internally converted to a String and passed to serde::de::Error::custom.

• #[serde(validate(by = "function", error = "Type"))]

  Using given Type for the validation error, without conversion. The function signature must be fn(&self) -> Result<(), Type>.

  This will be preferred in no-std use, or, maybe when you need better performance in error cases.

Both "function" and "Type" above accept path e.g. "Self::validate" or "crate::util::validate".

Additionally, #[serdev(crate = "path::to::serdev")] is supported for reexport from another crate.

Licensed under MIT LICENSE ( LICENSE or https://opensource.org/licenses/MIT ).