rust

Borrow checker errors map to specific fixes: E0499 (two mutable borrows) is fixed by shortening one borrow's scope or splitting into disjoint fields, while E0502 (shared and mutable borrow at once) is fixed by finishing reads before mutating. The rust-ownership sub-skill contains a full error-to-fix table.

Use thiserror with a typed error enum when writing a library, since callers need to match on failure modes. Use anyhow with one opaque error type for applications where you only report and exit. Mixing both is normal: libraries return typed errors and the app collects them into anyhow.

Use OS threads or rayon for CPU-bound work like compute and parsing, since async provides no benefit there. Use tokio async for I/O-bound work with many concurrent connections. For mixed workloads, run async with spawn_blocking for the blocking or CPU-heavy parts.

The quality gate requires cargo fmt --check, cargo clippy --all-targets with -D warnings, and cargo test to be green, plus cargo audit and cargo deny check if installed. If CI already ran a matching check on the PR and it passed, that result can be consumed instead of re-running locally.

This error occurs when a non-Send value like Rc or a std MutexGuard is held across an .await point. Fix it by using Send types like Arc, dropping the value before the await, or never holding a std lock across an await.

The testing sub-skill recommends rstest for table-driven cases and fixtures, proptest for property-based testing, mockall for mocking trait dependencies, insta for snapshots, cargo-fuzz for adversarial input, and cargo nextest with cargo llvm-cov for running and coverage.