Allow freezing or locking of configuration from future alteration

[mehcode]

A Config instance should allow locking to prevent all future mutation. Sources would need to support some kind of roll-up.

config::freeze();
config::set("key", "value"); // does nothing and returns an error

[nugend]

Is it possible to make a compile time option to panic or return a Result?

[tmccombs]

What if a Config instance had a method like fn freeze(self) -> ConfigView that returned a ConfigView that only had read methods on it. Since it takes ownership of the config, the type and borrow checkers can enforce that the config is actually frozen.

That wouldn't work for the global config though. But maybe the global config should be removed (as mentioned in #17).

[mehcode]

Another thought to consider is how necessary this is.

#[derive(Deserialize)]
struct AppConfig { ... }

fn main() {
  // Setup config via config::merge [...]
  let c: AppConfig = config::get("$");
}

If something similar to the above is supported, perhaps we don't need a way to freeze the underlying config store.

@SergioBenitez I added this issue because you expressed a strong desire to have a frozen configuration. What do you think?

[mehcode]

Closing this because deserialization is essentially freeze