df12 Productions — Additional notes

Vector merging – For Vec<T> fields the default merge strategy is append, meaning that values from the configuration file appear first, then environment variables and finally CLI arguments. Use merge_strategy = "append" explicitly for clarity. When overrides should discard earlier layers entirely (for example, to replace a default list with a CLI-provided value) apply merge_strategy = "replace" instead.
Map merging – Map fields (such as BTreeMap<String, _>) default to keyed merges, where later layers update only the entries they define. Apply merge_strategy = "replace" when later layers must replace the entire map. The hello_world example exposes a greeting_templates map that uses this strategy, so declarative configuration files can swap the full template set at once.
Option<T> fields – Fields of type Option<T> are not treated as required. They default to None and can be set via any source. Required CLI arguments can be represented as Option<T> to allow configuration defaults while still requiring the CLI to provide a value when defaults are absent; see the vk example above.
Changing naming conventions – Currently, only the default snake/hyphenated (underscores → hyphens)/upper snake mappings are supported. Future versions may introduce attributes such as file_key or env to customize names further.
Testing – Because the CLI and environment variables are merged at runtime, integration tests should set environment variables and construct CLI argument vectors to exercise the merge logic. The figment crate makes it easy to inject additional providers when writing unit tests.
Sanitized providers – The sanitized_provider helper returns a Figment provider with None fields removed. It aids manual layering when bypassing the derive macro. For example:

  use figment::{Figment, providers::Serialized};
  use ortho_config::sanitized_provider;

  let fig = Figment::from(Serialized::defaults(&Defaults::default()))
      .merge(sanitized_provider(&cli)?);
  let cfg: Defaults = fig.extract()?;