Rust config

Rust Config

Rule

Configuration must be explicit, typed, validated, and testable. Keep loading separate from business logic and pass typed config into services.

Hard Stops

Ask before:

• Changing precedence, variable names, defaults, file locations, or compatibility contracts.
• Reading global machine files, user home config, real secrets, or production env by default.
• Adding config libraries for simple env/CLI needs.
• Logging config that may contain secrets.

Defaults

• Prefer Clap/env/std parsing for simple apps.
• Use typed structs with serde when files are needed.
• Use figment or config only for layered config from files/env/CLI and only when approved or already present.
• Define precedence explicitly, commonly: defaults < config file < environment < CLI.
• Validate once at startup and fail fast with actionable errors.
• Use secrecy/redaction wrappers for secret values when they may be logged or debugged.

Workflow

1. Define settings, defaults, required values, validation, and precedence.
2. Implement loaders that accept explicit sources for tests.
3. Avoid reading env/files inside domain logic.
4. Add tests for defaults, overrides, precedence, validation errors, and redaction.
5. Run tests and just check.

Antipatterns

• Global mutable config.
• Hidden environment reads deep in libraries.
• .env loading in production paths without policy.
• Printing full config structs with secrets.

Completion

Report config contract, precedence, secret handling, dependencies, tests, and validation results.