OpenAPI with utoipa + Axum

utoipa provides compile-time OpenAPI documentation for Rust REST APIs. Code-first approach: annotate handlers and types, get OpenAPI 3.1 spec automatically.

Core Concepts

1. ToSchema — Derive for request/response body types
2. #[utoipa::path] — Annotate handlers with path, method, params, responses
3. OpenApi — Combine all paths and schemas into spec
4. Scalar — Serve interactive API documentation

Cargo.toml Setup