Rust Design Patterns

Start from the compiler error, API decision, or code under review. Read only the reference sections that match; the catalog is not a checklist to apply everywhere.

Establish applicability

• Check the crate's edition, MSRV, and whether it is a library or binary. Library crates carry compatibility constraints (non_exhaustive, lint policy, public types) that binaries do not.
• Follow the crate's existing conventions for errors, builders, and unsafe boundaries before introducing a new pattern.
• For borrow checker errors, reshape ownership before reaching for clone, Rc, or Arc. Treat a clone as a deliberate cost, not a way to silence the compiler.
• When a lifetime is too short and no borrow reshaping fits, prefer an owned type over widening lifetimes or adding 'static bounds.

Read by problem

Problem or decision	Reference
Move a value out of &mut, or swap enum variants	ownership-patterns.md — mem::take and mem::replace
Borrow two fields of a struct independently	ownership-patterns.md — Struct Decomposition
Choosing shared ownership, or tempted to clone	ownership-patterns.md — Rc/Arc Decision Guide
Guaranteed cleanup (locks, files, transactions)	ownership-patterns.md — RAII Guards, Finalisation in Destructors
Unsafe code needs a safe boundary	ownership-patterns.md — Contain Unsafety in Small Modules
Function parameter types (&str vs &String)	api-design.md — Borrowed Types for Arguments
Many optional constructor parameters	api-design.md — Builder Pattern, Default Trait
Distinct types over the same representation	api-design.md — Newtype Pattern
Compile-time state transitions	api-design.md — Type State Pattern
Public types that must evolve without breaking callers	api-design.md — non_exhaustive, Constructor Conventions
FFI errors, strings, or object lifetimes	api-design.md — FFI Patterns
Dynamic dispatch without boxing	idioms.md — On-Stack Dynamic Dispatch
Closure captures, post-setup immutability, Option in iterator chains	idioms.md
A consuming call fails and the caller needs the value back	idioms.md — Return Consumed Argument on Error
Reviewing existing Rust for common mistakes	common-pitfalls.md — start with Review Checklist

Examples illustrate the shape of a pattern, not code to paste. Verify version-sensitive behavior against the crate's toolchain.

Complete the change

Use the applicable AGENTS.md test mode. cargo clippy flags most of the unnecessary clones and borrowed-type issues these references describe.