Iterator & Extension Traits

Extension traits add methods to existing types (str, Iterator, third-party types) without wrapping. Iterator adapters compose zero-cost data pipelines.

Recipe

trait IterExt: Iterator {
    fn collect_vec(self) -> Vec<Self::Item> where Self: Sized { self.collect() }
}
impl<I: Iterator> IterExt for I {}

When to reach for this: Ergonomic helpers on types you do not own; reusable iterator utilities.

Working Example

trait StrExt {
    fn trimmed_nonempty(&self) -> Option<&str>;
}
impl StrExt for str {
    fn trimmed_nonempty(&self) -> Option<&str> {
        let t = self.trim();
        if t.is_empty() { None } else { Some(t) }
    }
}
 
let ids: Vec<u64> = records.iter().filter_map(|r| r.id).collect();

What this demonstrates:

• Extension trait + blanket impl pattern
• Iterator chains lazy until collect/sum
• Prefer std adapters (filter_map, try_fold) before custom
• Keep extension traits in prelude module of your crate

Deep Dive

Blanket impls: impl<T: AsRef<str>> StrExt for T {} for wider coverage. For iterators, Iterator::map + collect beats manual loops unless profiled.

Gotchas

• Orphan rule on foreign types - cannot impl foreign trait for foreign type. Fix: extension trait you own.
• Name collisions - methods clash with std future additions. Fix: prefix methods or use modules.
• Over-custom iterators - reinventing filter. Fix: use std adapters.
• Collect without type - inference failures. Fix: collect::<Vec<_>>() or turbofish.
• Iterator of references clone - hidden allocs. Fix: copied() for Copy items.

Alternatives

FAQs

Related

• Zero-Cost Abstractions
• Rust Patterns Basics
• Iterator Closure Patterns
• Common Anti-Patterns

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.