Type-indexed scoped singletons, propagated through an implicit backing context.

Requiring references from the environment

The from_env macro provides an attribute for functions that require access to a singleton in their environment. Here, the contrived function requires a u8 to add one to:

#[illicit::from_env(num: &u8)]
fn env_num_plus_one() -> u8 {
num + 1
}

illicit::child_env!(u8 => 1).enter(|| {
assert_eq!(env_num_plus_one(), 2u8);
});

This provides convenient sugar for values stored in the current Env as an alternative to thread-locals or a manually propagated context object. However this approach incurs a significant cost in that the following code will panic without the right type having been added to the environment:

# #[illicit::from_env(num: &u8)]
# fn env_num_plus_one() -> u8 {
#    num + 1
# }
// thread 'main' panicked at 'expected a value from the environment, found none'
env_num_plus_one();

See the attribute's documentation for more details, and please consider whether this is appropriate for your use case before taking it on as a dependency.