Start with the idea
For a Vec<T>, iter normally yields &T, iter_mut yields &mut T and into_iter on an owned vector yields T. The chosen receiver matters: into_iter on a reference can still borrow. Read the actual item type before deciding whether an operation moves, copies or clones values.
Most iterator adapters are lazy. map describes a transformation, while a consumer such as collect or sum drives it. Returning an iterator can avoid collecting an intermediate vector, but keeps any captured input borrows alive for the returned iterator’s lifetime.
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