Rust Error Identity Boundaries

Treat diagnostic text, concrete error identity, and the source chain as separate contracts. An informative message does not prove that callers can still classify the failure.

Preserve the information needed by callers

1. Identify the classification contract before changing an error boundary: a domain variant, a typed operation marker, an underlying source kind, or a combination. Do not branch on formatted messages.
2. Preserve existing errors with ?, anyhow::Error::new, or an explicit domain wrapper.
3. Attach operation information using anyhow context. Typed context can classify the operation while ordinary text explains it. Additional context preserves anyhow's existing downcasts. Context values require Display + Send + Sync + 'static; a marker need not implement std::error::Error. Use with_context for lazily prepared details.
4. Select the traversal the contract requires. Inspect a domain error directly, or use chain() and downcast each cause when classification genuinely depends on a lower-level type. Keep operation context as a separate classification condition when necessary.
5. Use explicit map_err construction when a thiserror wrapper must retain both operation fields and its original source.

Regression evidence

Test direct typed-context downcasts, domain-wrapper traversal, and the broken stringification alternative. Add unrelated and different-kind failures as negative controls. Change human-readable context without changing the classification. Assert retained fields and source kinds independently of display text.

The original fixture uses synthetic errors with no filesystem or network operations. See provenance and validation limits before running it. Its direct dependencies are version-pinned; compilation and runtime are unverified.

Pitfalls

• Anyhow context downcasts are not arbitrary source-chain searches. Introducing a domain wrapper changes the concrete underlying error; direct downcasts to its source can fail.
• Stringification destroys original types and causes. Later context cannot reconstruct that information.
• #[from] implies #[source] but permits only the source field and an optional backtrace, not extra operation fields. #[error(transparent)] forwards Display and source behavior rather than adding a diagnostic layer.