Rust Error Observability

Use this skill to make Rust service failures understandable to operators while keeping user-facing responses safe. Treat error handling and telemetry as one design surface.

Core Workflow

1. Inventory error flows: handler return types, service/repository errors, worker errors, middleware, logs, and tracing setup.
2. Classify each error by purpose:
   • Domain or validation outcome.
   • Recoverable control flow.
   • Operator diagnostic.
   • User-facing HTTP response.
3. Keep domain errors typed. Use thiserror for expected branches that callers should match on.
4. Add context at infrastructure boundaries. Use anyhow or opaque application errors when callers should not match every dependency failure.
5. Map errors to HTTP responses in one place per framework: ResponseError, Axum IntoResponse, or a small adapter function.
6. Add structured spans before more log lines. Include stable diagnostic fields, not raw payloads or secrets.
7. Ensure errors are logged once. Prefer logging at the outer boundary where request context is available.
8. Test both the public response and the diagnostic path when behavior changed.

Error Boundary Rules

• Domain modules return domain errors, not HTTP status codes.
• Repository modules attach query or operation context, but do not log every error.
• Handlers convert domain outcomes into response types and let unexpected failures become a consistent 500.
• Background workers log failed job IDs, attempt counts, and next action.
• Avoid unwrap, expect, or stringly map_err in service paths unless the invariant is local and the panic message is useful.

Read references/error-boundaries.md when choosing between thiserror, anyhow, opaque errors, and framework response adapters.

Tracing Rules

• Instrument request handlers, service methods, outbound clients, database operations, and worker jobs at boundaries.
• Use #[tracing::instrument(skip(...))] for request bodies, pools, clients, passwords, tokens, and large values.
• Attach fields like request ID, user ID, tenant ID, job ID, upstream name, and idempotency key when safe.
• Do not log secrets, cookies, password hashes, bearer tokens, or full PII.

Read references/tracing.md for spans, subscriber setup, and test logging. Read references/secrets-and-pii.md before touching secret-bearing values.

HTTP Response Pattern

Keep response errors stable and intentional:

pub enum SubscribeError {
    Validation(SubscribeValidationError),
    Unexpected(anyhow::Error),
}

impl actix_web::ResponseError for SubscribeError {
    fn status_code(&self) -> actix_web::http::StatusCode {
        match self {
            Self::Validation(_) => actix_web::http::StatusCode::BAD_REQUEST,
            Self::Unexpected(_) => actix_web::http::StatusCode::INTERNAL_SERVER_ERROR,
        }
    }
}

The response body should be safe for users. The trace event should carry the diagnostic context operators need.

Reference Files

• references/error-boundaries.md: choosing error types and response adapters.
• references/tracing.md: spans, fields, subscriber setup, and test output.
• references/secrets-and-pii.md: redaction rules for secrets and sensitive user data.