Validation

facet-validate defines validation attributes for Facet fields: numeric bounds, length checks, string checks, regexes, and custom validators. Reach for it when a value is only valid inside a smaller domain than its Rust type can express on its own.

Install

Add facet and facet-validate to your crate. Format crates and builders that use facet's partial-construction path can then enforce the validators while constructing the value.

Minimal example

use facet::Facet;
use facet_validate as validate;

#[derive(Facet)]
struct Product {
    #[facet(validate::min_length = 1, validate::max_length = 100)]
    title: String,

    #[facet(validate::min = 0)]
    price_cents: i64,

    #[facet(validate::email)]
    contact_email: String,
}

The attributes are stored in the type's facet shape, so validation-aware deserializers can reject bad input at the point where the offending field is read.

Built-in constraints

Custom validators

A custom validator takes a reference to the field value and returns Result<(), String>:

use facet::Facet;
use facet_validate as validate;

fn validate_currency(s: &str) -> Result<(), String> {
    match s {
        "USD" | "EUR" | "GBP" => Ok(()),
        _ => Err(format!("invalid currency code: {s}")),
    }
}

#[derive(Facet)]
struct Price {
    #[facet(validate::custom = validate_currency)]
    currency: String,
}

The validator message becomes the useful part of the construction error, so make it short and specific.

Related

• facet-json — deserialize external data into facet-shaped values
• facet-default — combine defaults with validation constraints
• facet-error — model validation failures as typed errors when needed
• Ecosystem — other facet derive plugins and format crates