Defaults for list of nested objects about figment HOT 2 CLOSED

The Default trait doesn't integrate with Figment in any particular way. If you're reading the tips for app authors, you'll see that Default is implemented as an avenue for having a structure that can be merged into the figment as a default for config values, and it doesn't do anything beyond that.

Having said that, this line in your code is almost certainly not what you want. It's placing the address and timeout fields in your top-level config.

Back to the problem at hand. If we look at your figment with its defaults:

        let mut config = Figment::new().merge(Serialized::defaults(Config::default()));
        config = config.merge(Serialized::defaults(ListenerConfig::default()));

We'll have a figment with the following data after this runs:

default.listeners = [{address: 127.0.0.1:8080, timeout: 300_000}}
default.storage_path = "storage"
default.address = 127.0.0.1:8080
default.timeout = 300_000

Your TOML file as read contains:

default.listeners = [{address: 127.0.0.1:8088}]

When we merge this into the previous figment, we get:

default.listeners = [{address: 127.0.0.1:8088}]
default.storage_path = "storage"
default.address = 127.0.0.1:8080
default.timeout = 300_000

Since the TOML default.listeners array overrides the previous one. As you can see, the only listener in the array is incomplete, missing the address field.

If you want this to work, you probably want to specify a default via serde:

#[derive(Debug, Deserialize, Serialize)]
pub struct ListenerConfig {
    pub address: String,
    #[serde(default = "ListenerConfig::default_timeout")]
    pub timeout: u32,
}

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Hope the above resolved your question! If not, please feel free to reopen.

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