It also covers the async-fn-in-traits Send problem — why async fn in traits does not give Send futures by default and how to work around it.

Out of scope: HTTP service structure (routing, extractors, middleware) is owned by rust-web-backend; sync-only ownership and borrowing questions belong to rust-core-language.

Rust Async & Concurrency

Agent Workflow (MANDATORY)

Before writing async code, spawn in parallel: