How to merge sources with existing default values?

[cbeuw]

Suppose I have this config struct

#[derive(Debug, Deserialize)]
struct MyConfig {
    // ...
}

impl Default for MyConfig {
    fn default() -> MyConfig {
        // some complex logic
    }
}

I want to produce a MyConfig, where values explicitly set in config.toml are used, otherwise, the default implementation is used.

Something like

Config::builder().
    .add_source(File::with_name("config.toml"))
    .set_default(???, MyConfig::default())
    .build()?;

But I couldn't figure out a way to do this. This seems like a very common use case, so I feel like I'm missing something.