Rust design guidelines

Apply these rules while writing Rust, not only in review.

• The card below, plus the on-demand card for the task, is the complete rule set. Read the card before writing Rust. It ends with on-demand cards for project setup, FFI and performance work: open the one whose trigger applies before starting that work. No other file is needed to follow the rules.
• Open a detail file only when you need it. rules/<id>.md holds a rule's full upstream text, examples and exceptions. patterns/<id>.md holds a design pattern; open one before choosing between the designs it covers.
• Scope tags. [lib] rules are binding in crates other crates depend on and a strong recommendation in application-only code. [app] rules apply to application crates. Untagged rules apply everywhere.
• Precedence. The repository's own configuration (rustfmt.toml, clippy.toml, [lints]) and the user's or project's agent instructions override this card. Where the upstream guides disagree, the card already states the resolved rule.
• Gaps. Where the card is silent, follow the Rust Style Guide.
• Exceptions. If you must break a rule, name it and give the reason in one line.

Rule card

Errors and panics