Module setting 

Closed-set settings — Setting and setting_enum!.

A driver name is not a string. It is one of a handful of values the code actually knows how to build, and every other string is a mistake. Spelling it as a String moves that mistake from the compiler to production:

CACHE_DRIVER=redys        # boots fine, caches in-process, forever

setting_enum! declares the closed set once and derives everything that follows from it — the wire spelling, Display, FromStr, serde, and a parse error that lists what was expected:

use rainier_support::{setting_enum, Setting};

setting_enum! {
    /// Where cached values live.
    pub enum CacheDriver: "cache driver" {
        /// This process only.
        #[default]
        Memory = "memory",
        /// One Redis server.
        Redis = "redis",
        /// A sharded Redis Cluster.
        RedisCluster = "redis-cluster",
    }
}

assert_eq!(CacheDriver::parse("redis-cluster").unwrap(), CacheDriver::RedisCluster);
assert_eq!(CacheDriver::default(), CacheDriver::Memory);

let err = CacheDriver::parse("redys").unwrap_err();
assert_eq!(
    err.message(),
    "`redys` is not a valid cache driver; expected one of `memory`, `redis`, `redis-cluster`",
);

Why parsing fails rather than falling back

A default is for a value nobody set. It is not for a value somebody set wrong — silently substituting one there means the deployment that typed redys runs on an in-process cache, and the first symptom is a rate limiter that lets through N × its limit across N instances.

Failing at boot puts the error where the mistake was made, with the list of valid values in the message.

Every setting has a default

setting_enum! always derives Default, so exactly one variant must carry #[default] and the macro will not compile otherwise. That is deliberate: a setting with no default is a setting that has to be named in every environment, and if that is genuinely what you want, it belongs in Env::require rather than here.

Traits

Setting

A setting whose values are a closed set of wire spellings.