Configuration Validation

Configuration validation catches bad environment values before the server starts accepting requests. toni-config integrates with the validator crate via a feature flag.

Setup

[dependencies]
toni-config = { version = "0.1", features = ["validation"] }
validator = { version = "0.20", features = ["derive"] }

Annotating configuration

Combine #[derive(Config)] with #[derive(Validate)] and add #[validate(...)] constraints:

use toni_config::Config;
use validator::Validate;

#[derive(Config, Clone, Validate)]
pub struct AppConfig {
    #[env("PORT", default = "3000")]
    #[validate(range(min = 1024, max = 65535))]
    pub port: u16,

    #[env("DATABASE_URL")]
    #[validate(url)]
    pub database_url: String,

    #[env("JWT_SECRET")]
    #[validate(length(min = 32))]
    pub jwt_secret: String,

    #[env("ALLOWED_ORIGINS")]  // comma-separated list
    #[validate(length(min = 1))]
    pub allowed_origins: String,

    #[env("MAX_UPLOAD_MB", default = "10")]
    #[validate(range(min = 1, max = 100))]
    pub max_upload_mb: u32,
}

Loading with validation

fn main() {
    dotenv::dotenv().ok();

    let config = AppConfig::load_and_validate()
        .expect("Invalid configuration — check environment variables");

    // If we get here, all validations passed
}

Validation errors

When validation fails, the error message lists every constraint violation:

Configuration error:
  port: Value 80 is not in range [1024, 65535]
  jwt_secret: Value must be at least 32 characters long

This makes configuration errors visible immediately at startup, not buried in runtime panics.

Custom validators

Use validator's custom validation support for constraints beyond the built-in ones:

use validator::ValidationError;

fn validate_database_url(url: &str) -> Result<(), ValidationError> {
    if !url.starts_with("postgresql://") && !url.starts_with("postgres://") {
        return Err(ValidationError::new("must be a PostgreSQL URL"));
    }
    Ok(())
}

#[derive(Config, Clone, Validate)]
pub struct AppConfig {
    #[env("DATABASE_URL")]
    #[validate(custom(function = "validate_database_url"))]
    pub database_url: String,
}