Configuration as part of a library

[cprieto]

I will start by saying I really like how simple config-rs is, in theory you don't depend on anything for configuration values so it can be included as part of a library easily, great!
Now, there is one problem I found though, let's say I have a simple configuration value in one of my libraries: MySettings and this is defined as:

pub struct MySettings {
    host: String,  // this prop is required
    port: u16, // this can have a default value, let's say 8000
}

And later in our app just include it as part of our app configuration:

struct MyAppConfig {
    name: String,
    settings: MySettings,
}

And use our sources and all to fill our configuration, but then what happen with those required fields (like host) and those with default values (like port)? In things like Figment the guide for library developers just solves this exposing the configuration to your providers (and depending on Figment in the process, see https://docs.rs/figment/latest/figment/#for-library-authors), but what about here?

How can I ensure that:

• Required fields (like host) fail loudly if not provided by any source
• Optional fields (like port) fall back to library-defined defaults
• This works when MySettings is nested in larger configurations

I am aware I can use ConfigBuilder::set_default but that will happen on the main caller (in this case, the app using MyAppConfig), and I cannot use Default for MySettings because while port has a default value, host doesn't. Do you see my dilemma?

For really big configurations (let's say, very deep hierarchies and a lot of config values with default or required values) this is sort of crucial.

Is there an out-of-the-box way to handle this in config-rs? I have a few ideas for workarounds, but I want to know if there's an idiomatic approach before implementing something custom.