Garde  

A Rust validation library

• Basic usage example
• Validation rules
• Inner type validation
• Handling Option
• Custom validation
• Implementing rules
• Implementing Validate
• Integration with web frameworks
• Feature flags
• Why garde?

Basic usage example

To get started, use the Validate derive macro and add some validation rules to your type. This generates an implementation of the Validate trait for you. To use it, call the validate method on an instance of the type.

Here's what that looks like in full:

use garde::{Validate, Valid};

#[derive(Validate)]
struct User<'a> {
    #[garde(ascii, length(min=3, max=25))]
    username: &'a str,
    #[garde(length(min=15))]
    password: &'a str,
}

let user = User {
    username: "test",
    password: "not_a_very_good_password",
};

if let Err(e) = user.validate(&()) {
    println!("invalid user: {e}");
}

Garde can also validate enums:

use garde::{Validate, Valid};

#[derive(Validate)]
enum Data {
    Struct {
        #[garde(range(min=-10, max=10))]
        field: i32,
    },
    Tuple(
        #[garde(ascii)]
        String
    ),
}

let data = Data::Struct { field: 100 };
if let Err(e) = data.validate(&()) {
    println!("invalid data: {e}");
}

Available validation rules

Additional notes:

• required is only available for Option fields.
• For length and range, either min or max may be omitted, but not both.
• length and range use an inclusive upper bound (min..=max).
• length uses .chars().count() for UTF-8 strings instead of .len().
• For contains, prefix, and suffix, the pattern must be a string literal, because the Pattern API is currently unstable.
• Garde does not enable the default features of the regex crate - if you need extra regex features (e.g. Unicode) or better performance, add a dependency on regex = "1" to your Cargo.toml.

If most of the fields on your struct are annotated with #[garde(skip)], you may use #[garde(allow_unvalidated)] instead:

#[derive(garde::Validate)]
struct Foo<'a> {
    #[garde(length(min = 1))]
    a: &'a str,

    #[garde(skip)]
    b: &'a str, // this field will not be validated
}

#[derive(garde::Validate)]
#[garde(allow_unvalidated)]
struct Bar<'a> {
    #[garde(length(min = 1))]
    a: &'a str,

    b: &'a str, // this field will not be validated
                // note the lack of `#[garde(skip)]`
}

Inner type validation

If you need to validate the "inner" type of a container, such as the String in Vec<String>, then use the inner modifier:

#[derive(garde::Validate)]
struct Test {
    #[garde(
        length(min = 1),
        inner(ascii, length(min = 1)), // wrap the rule in `inner`
    )]
    items: Vec<String>,
}

The above type would fail validation if:

• the Vec is empty
• any of the inner String elements is empty
• any of the inner String elements contains non-ASCII characters

Handling Option

Every rule works on Option<T> fields. The field will only be validated if it is Some. If you additionally want to validate that the Option<T> field is Some, use the required rule:

#[derive(garde::Validate)]
struct Test {
    #[garde(required, ascii, length(min = 1))]
    value: Option<String>,
}

The above type would fail validation if:

• value is None
• the inner value is empty
• the inner value contains non-ASCII characters

Custom validation

Validation may be customized via the custom rule, and the context attribute.

The context may be any type without generic parameters. By default, the context is ().

#[derive(garde::Validate)]
#[garde(context(PasswordContext))]
struct User {
    #[garde(custom(is_strong_password))]
    password: String,
}

struct PasswordContext {
    min_entropy: f32,
    entropy: cracken::password_entropy::EntropyEstimator,
}

fn is_strong_password(value: &str, context: &PasswordContext) -> garde::Result {
    let bits = context.entropy.estimate_password_entropy(value.as_bytes())
        .map(|e| e.mask_entropy)
        .unwrap_or(0.0);
    if bits < context.min_entropy {
        return Err(garde::Error::new("password is not strong enough"));
    }
    Ok(())
}

let ctx = PasswordContext { /* ... */ };
let user = User { /* ... */ };
user.validate(&ctx)?;

The validator function may accept the value as a reference to any type which it derefs to. In the above example, it is possible to use &str, because password is a String, and String derefs to &str.

Implementing rules

Say you want to implement length checking for a custom string-like type. To do this, you would implement the garde::rules::length::HasLength trait for it.

#[repr(transparent)]
pub struct MyString(pub String);

impl garde::rules::length::HasLength for MyString {
    fn length(&self) -> usize {
        self.0.chars().count()
    }
}
#[derive(garde::Validate)]
struct Foo {
    // Now the `length` check may be used with `MyString`
    #[garde(length(min = 1, max = 1000))]
    field: MyString,
}

Each rule comes with its own trait that may be implemented by custom types in your code. They are all available under garde::rules.

Implementing Validate

In case you have a container type for which you'd like to support nested validation (using the #[garde(dive)] rule), you may implement Validate for it:

#[repr(transparent)]
struct MyVec<T>(Vec<T>);

impl<T: garde::Validate> garde::Validate for MyVec<T> {
    type Context = T::Context;

    fn validate(&self, ctx: &Self::Context) -> Result<(), garde::Errors> {
        garde::Errors::list(|errors| {
            for item in self.0.iter() {
                errors.push(item.validate(ctx));
            }
        })
        .finish()
    }
}

#[derive(garde::Validate)]
struct Foo {
  #[garde(dive)]
  field: MyVec<Bar>,
}

#[derive(garde::Validate)]
struct Bar {
  #[garde(range(min = 1, max = 10))]
  value: u32,
}

To make implementing the trait easier, the Errors type supports a nesting builders.

• For list-like or tuple-like data structures, use Errors::list, and its .push method to attach nested Errors.
• For map-like data structures, use Errors::fields, and its .insert method to attach nested Errors.
• For a "flat" error list, use Errors::simple, and its .push method to attach individual errors.

The ListErrorBuilder::push and ListErrorBuilder::insert methods will ignore any errors which are empty (via Errors::is_empty).

Integration with web frameworks

• axum: https://crates.io/crates/axum_garde

Feature flags

Why garde?

Garde means guard in French. I am not French, nor do I speak the language, but guard was taken, and this is close enough :).

Development

Contributing to garde only requires a somewhat recent version of Rust.

This repository also makes use of the following tools, but they are optional:

• insta for snapshot testing (tests/rules).
• just for running recipes defined in the justfile. Run just -l to see what recipes are available.

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

Acknowledgements

This crate is heavily inspired by the validator crate. It is essentially a full rewrite of validator. The creation of this crate was prompted by this comment and a few others talking about a potential rewrite.

[^1]: HTML5 forms - valid email address