rust-syntax-iterators-closures

The mechanics of Rust's iterator and closure systems, treated as one topic because closures are predominantly authored as arguments to iterator adapters and they share the same lifetime and trait-bound mental model. Covers the Iterator trait contract, the standard adapter set, lazy evaluation, custom Iterator impls, IntoIterator vs Iterator, the Fn / FnMut / FnOnce trait hierarchy, move closures, edition-2021 disjoint captures, async closures (Rust 1.85), and returning closures.

Cross-references: [[rust-syntax-traits]] (Iterator IS a trait, closure traits are traits) [[rust-syntax-async-await]] (async closures return impl Future) [[rust-syntax-ownership]] (move captures, by-value vs by-reference) [[rust-syntax-generics]] (generic bounds T: Iterator, F: Fn(...)) [[rust-syntax-trait-objects]] (Box<dyn Fn> vs impl Fn).

When to use this skill

User writes any iterator chain: .map(...).filter(...).collect()
User asks "why doesn't my loop / iterator do anything?" (lazy evaluation)
User implements a custom Iterator for their own type
User confuses .iter(), .iter_mut(), and .into_iter()
User picks between Fn, FnMut, FnOnce for a closure-taking API
User hits "closure may outlive borrowed value" and needs move
User writes async || { ... } (Rust 1.85+) and needs the trait story
User wants to return a closure from a function
User asks about collect::<Vec<_>>() turbofish syntax