I can work on these as I've had experience with hashmaps in the past, and I just completed a small project where I had to work out some nuances with hashmaps and lifetimes (I was trying to use references [specifically string slices] as keys).

Topics (ref is an arbitrary reference type, struct or enum):

• HashMaps, basics and use cases
• Alternate key types, glossing over primitives and focusing on structs with #[deriving(Eq, Hash)]
• HashSet<ref>
• SmallIntMap<ref>

Not sure whether I should go into the underlying implementation of a common hashmap or just how to use it. Maybe compare HashMap<int, int/ref> to Vec<int/ref> and why one would prefer one over the other, and then introduce SmallIntMap<ref> as a specialized alternative. I could present my use case for Hashmaps, an RPN calculator with user-defined variables, or maybe a basic memoizer app (setting values to keys via CLI and retrieving).