Envfig

A flexible and type-safe system for defining, documenting, and validating environment variables.

The EnvVarDef type provides a builder-style interface to define environment variables with optional default values, human-friendly metadata (title, description, example), and custom validation logic.

It supports loading required or optional variables using EnvVarDef::load and EnvVarDef::load_option, and will return rich error types when parsing or validation fails.

Example

use std::{env, str::FromStr};

use envfig::{EnvVarDef, LoadError, validator::Validator};

// A simple validator that ensures the value is greater than zero
struct Positive;

impl Validator<i32> for Positive {
    type Err = String;

    fn validate(
        self,
        val: i32,
    ) -> Result<i32, Self::Err> {
        if val > 0 {
            Ok(val)
        } else {
            Err("Value must be positive".into())
        }
    }
}

// Set an environment variable
unsafe {
    env::set_var("APP_PORT", "8080");
}

let port = EnvVarDef::new("APP_PORT")
    .with_title("Application Port")
    .with_description("The port the application listens on")
    .with_example(8080)
    .with_validator(Positive)
    .load()
    .unwrap();

Features

• Type-safe parsing from environment variables.
• Optional default value fallback.
• Full metadata support (title, description, example).
• Custom validation support via the Validator trait.
• Granular error types via LoadError.

Traits

• Validator<T> is required to perform validation of environment variable values.