Add "separator" to parse nested structs

[dbofmmbt]

💡 Feature description

In config-rs, there's an option to define a separator which allow users to structure their env vars. I think an example will illustrate better:

#[derive(Deserialize)]
struct Config {
    database: Database
}

#[derive(Deserialize)]
struct Database {
    name: String,
}

If we define a separator, let's say "__", we could create this Config by setting DATABASE__NAME=foo.

Would it be possible and desirable for envy to include it?

Alternatives

It is possible to get the same behavior using the flatten attribute from serde (as noticed in #15). The drawback is that it demands a good amount of boilerplate because we would have to serde's rename every field in the nested struct if we wanted to use this separator idea.

[jonasbb]

I want to mention one alternative using serde_with::with_prefix. It's a bit less boilerplate than applying rename on every field.
It is not a replacement for having it integrated directly into envy though.

serde_with::with_prefix solution

serde_with::with_prefix!(pdatabase "database__");
#[derive(Debug, serde::Deserialize)]
struct Config {
    #[serde(flatten, with = "pdatabase")]
    database: Database,
}
#[derive(Debug, serde::Deserialize)]
struct Database {
    name: String,
}

fn main() {
    std::env::set_var("DATABASE__NAME", "foobar");
    let config: Config = envy::from_env().unwrap();
    dbg!(config);
}

[jayvdb]

Worth noting that flatten doesnt work with non-String datatypes. c.f. #26