Collections

How Rust's lazy iterator chains work, how iter/iter_mut/into_iter map onto ownership, and what "zero-cost abstraction" actually buys you.

Choosing among `Vec`, `HashMap`, `BTreeMap`, `HashSet`.

Growth, capacity, indexing, and slice operations.

Entries, ordering, and the `entry` API.

Specialized containers and their uses.

`next`, laziness, and building custom iterators.

`map`, `filter`, `fold`, `zip`, `flat_map`, and chaining.

`collect`, `sum`, `count`, and `for` loops.

`iter`, `iter_mut`, and `into_iter`.

Idiomatic, allocation-aware pipelines.