6 releases

0.2.3 Jul 2, 2022
0.2.2 May 25, 2022
0.1.2 May 4, 2022

#9 in #rule

34 downloads per month
Used in type-rules


192 lines


A tool to easily constrain a struct and recover errors.

Table of Contents

  1. Install
  2. Basic checking
  3. Advanced checking
  4. Make your own rule
  5. Rules list


# Cargo.toml
type-rules = { version = "0.2.3", features = ["derive", "regex", "serde"] }

Basic checking

You can declare a struct and impose some constraints on each field and check the validity like this:

use chrono::prelude::*;
use type_rules::prelude::*;

struct NewUser {
    #[rule(MaxLength(100), RegEx(r"^\S+@\S+\.\S+"))]
    email: String,
    #[rule(MinMaxLength(8, 50))]
    password: String,
    birth_date: Option<DateTime<Utc>>

let new_user = NewUser {
    email: "examples@examples.com".to_string(),
    password: "OPw$5%hJ".to_string(),
    birth_date: None,
let new_user = NewUser {
    email: "examples@examples.com".to_string(),
    password: "O".to_string(),
    birth_date: None,
assert!(new_user.check_validity().is_err()); //Value is too short

Also works with enums :

use type_rules::prelude::*;

enum MyEnum {
    Option1(#[rule(MaxLength(200))] String),
    Option2 {
        #[rule(MinMaxRange(1, 10))]
        integer: u32

Advanced checking

To check recursively, you can use the Validate rule

use type_rules::prelude::*;

struct EmailWrapper(#[rule(MaxLength(100), RegEx(r"^\S+@\S+\.\S+"))] String);

struct User {
    email: EmailWrapper,
    #[rule(MinMaxLength(8, 50))]
    password: String,

You can use expressions directly in rule derive attribute.

For example, you can use const or function directly in the rule parameters:

use type_rules::prelude::*;
use chrono::prelude::*;

struct BirthDate(#[rule(MaxRange(Utc::now()))] DateTime<Utc>);
use type_rules::prelude::*;

struct Range {
    min: u32,
    max: u32,

Or use expressions to express a rule directly. Here is an example of using a rule with more complex values:

use std::env;
use type_rules::prelude::*;

fn generate_max_payload_rule() -> MaxLength {
    MaxLength(match env::var("MAX_PAYLOAD") {
        Ok(val) => val.parse().unwrap_or_else(|_| 10000),
        Err(_) => 10000,

struct Payload(#[rule(generate_max_payload_rule())] String);

In this case the generate_max_payload_rule function is executed at each check

Make your own rule

If you need a specific rule, just make a tuple struct (or struct if you make the declaration outside the struct definition) that implements the Rule feature :

use type_rules::prelude::*;

struct IsEven();

impl Rule<i32> for IsEven {
    fn check(&self, value: &i32) -> Result<(), String> {
        if value % 2 == 0 {
        } else {
            Err("Value is not even".into())

struct MyInteger(#[rule(IsEven())] i32);

Valid wrapper

Valid is a wrapper for any type that implements Validator it permit to ensure at compile time that the inner type as been verified.

With the serde feature, Valid can be serialized and deserialized with validity check.

use type_rules::prelude::*;

struct NewUser {
   #[rule(MinMaxLength(3, 50))]
   username: String,
   #[rule(MinMaxLength(8, 100))]
   password: String,

fn do_something(user: Valid<NewUser>) {
   // No need to check if user is valid

let new_user = NewUser {
   username: "example".to_string(),
   password: "OPw$5%hJJ".to_string(),

Rules list

Here a list of the rules you can find in this crate.

Each rule has its own documentation with examples.

Check the length of any type that implements AsRef<str> such as String or &str:

  • MinLength: Minimum length ex: MinLength(5)
  • MaxLength: Maximum length ex: MaxLength(20)
  • MinMaxLength: Minimum and maximum length ex: MinMaxLength(5, 20)

Check the range for anything that implements PartialOrd<Self> like all numeric/floating types or dates with chrono:

  • MinRange: Minimum range ex: MinRange(5)
  • MaxRange: Maximum range ex: MaxRange(20)
  • MinMaxRange: Minimum and maximum range ex: MinMaxRange(5, 20)

Check the size of a Vec<T> :

  • MinSize: Minimum size ex: MinSize(5)
  • MaxSize: Maximum size ex: MaxSize(20)
  • MinMaxSize: Minimum and maximum size ex: MinMaxSize(5, 20)

others :

  • Opt: Apply another rule to inner value of an Option ex: Opt(MinMaxRange(1, 4))
  • And: Rule to ensure that 2 other rules are Ok ex: And(MaxLength(1000), RegEx(r"^\S+@\S+\.\S+"))
  • Or: Rule to apply an Or condition on two other rules. ex: Or(MaxRange(-1), MinRange(1))
  • Eval: Rule to constrain any type to a predicate ex: Eval(predicate, "Error message")
  • Validate: Recursive checking ex: Validate()
  • In: Rule to constrain a type to be in a collection ex: In(["apple", "banana", "orange", "pear"], "Value need to be a fruit")
  • All: Rule to constrain a collection to valid the specified rule ex: All(MinLength(1), "You can't use empty string")
  • RegEx: check if a type that implement AsRef<str> (String, &str, ...) matches the regex. You need the regex feature to use it. ex: RegEx(r"^\S+@\S+\.\S+")


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