Derive Macro ConfigStruct

#[derive(ConfigStruct)]
{
    // Attributes available to this derive:
    #[default]
    #[enumerated]
    #[var_name]
}

This proc macro derives the ConfigStruct trait for a struct, enabling easy extraction of fields from environment variables and parsing them to the correct type.

The macro supports the following attributes for struct fields:

• default: Sets a default value for the field. If not provided, the macro will panic if the environment variable is not set.
• env_var: Sets the name of the environment variable to use for the field. If not provided, the macro will use the name of the field in uppercase as the environment variable name.
• enumerated: Identifies an enum that implements the EnvVar trait. The macro will parse the environment variable to the enum type.

Example usage:

use env_extract::ConfigStruct;
use env_extract::EnvVar;
#[derive(Debug, EnvVar)]
#[var_name = "DATABASE_TYPE"]
#[panic_on_invalid]
#[case(convert = "lowercase")]
enum DatabaseType {
    Postgres,
    Mysql,
    Sqlite,
}

#[derive(ConfigStruct, Debug)]
struct Config {
    db_host: String,
    db_port: u16,
    use_tls: bool,

    #[enumerated]
    db_type: DatabaseType,
}

fn main() {
    std::env::set_var("DB_HOST", "localhost");
    std::env::set_var("DB_PORT", "5432");
    std::env::set_var("USE_TLS", "true");
    std::env::set_var("DATABASE_TYPE", "postgres");

    let config = Config::get();

    assert_eq!(config.db_host, "localhost");
    assert_eq!(config.db_port, 5432);
    assert_eq!(config.use_tls, true);
    assert!(matches!(config.db_type, DatabaseType::Postgres));
}

In the example above, the ConfigStruct trait is derived for the Config struct, allowing easy extraction of fields from environment variables. The db_host, db_port, and use_tls fields are extracted as String, u16, and bool types, respectively. The db_type field is extracted as an enum type DatabaseType, which is parsed from the environment variable named DATABASE_TYPE and converted to lowercase.