This (arrays implementing IntoIterator) is probably the improvement with broadest impact of the last 12 months.

It even makes lots of Rust's documentation examples more readable. Previously if there's an iterator involved somewhere, the doc example ends up constructing say vec![1,2,3,4,5] and a reasonable novice might ask why do we use a vector here? Today Rust's documentation just says [1,2,3,4,5] because sure, an array of five elements 1, 2, 3, 4, 5 that makes sense why use any other data structure?