Rust Iterator Collections

Use this skill to turn collection code into clear, idiomatic Rust. Prefer iterator and collection APIs that express the operation directly while keeping ownership, allocation, and error behavior visible.

Core Workflow

1. Identify the input ownership mode: borrowed iteration, mutable iteration, or consuming iteration.
2. Pick the simplest iterator shape: iter, iter_mut, into_iter, ranges, drain, or a collection-specific method.
3. Replace manual loops with adapters only when the resulting code is clearer. Keep explicit loops for complex branching, early mutation, or debugging.
4. Use fallible iterator consumers such as collect::<Result<Vec<_>, _>>(), try_fold, and try_for_each when errors should short-circuit.
5. Use HashMap::entry or BTreeMap::entry for insert-or-update logic.
6. Avoid unnecessary intermediate Vecs. Chain iterators or extend an existing collection when possible.
7. Test empty input, single item input, duplicate keys, ordering expectations, and error short-circuiting.

Iterator Rules

• Accept impl IntoIterator<Item = T> when the function only needs iteration.
• Use &[T] when slice semantics are enough; avoid &Vec<T>.
• Use filter_map for map-then-discard-None; use flat_map when each item expands to zero or more items.
• Use map_while or scan for stateful transformations when they make stopping behavior explicit.
• Add type annotations at collect boundaries, not throughout the pipeline.
• Prefer cloned or copied over map(|x| x.clone()) when cloning iterator items is intentional.

Collection Rules

Read references/iterator-collection-patterns.md when refactoring a loop or reviewing collection mutation.

• Use Vec for contiguous ordered data, VecDeque for queue-like push/pop at both ends, BinaryHeap for priority queues, HashMap for unordered lookup, and BTreeMap for sorted lookup or range queries.
• Use retain, drain, splice, and split_off instead of mutating a collection while separately iterating over borrowed elements.
• Use Entry::or_insert_with when default construction is expensive.
• Preserve ordering deliberately. Do not swap BTreeMap for HashMap when iteration order is part of behavior.
• Reserve capacity only when size is known or profiling shows reallocation matters.

Review Checklist

• The pipeline communicates ownership and error behavior clearly.
• No contains_key followed by insert where entry would be simpler.
• No avoidable collect::<Vec<_>>() just to iterate again.
• Indexing is used only when indices are the domain concept.
• Tests cover duplicates, empty input, and deterministic ordering where needed.