Consuming Iterators

Consumption runs the pipeline: for, collect, sum, count, fold, reduce, last, any, all.

Recipe

fn main() {
    let total: i32 = (1..=5).sum();
    let evens: Vec<_> = (0..10).filter(|n| n % 2 == 0).collect();
    println!("{total} {:?}", evens);
}

When to reach for this: Final step after adapters - pick consumer matching output need.

Working Example

fn stats(nums: impl IntoIterator<Item = f64>) -> (f64, f64) {
    let mut count = 0u64;
    let sum: f64 = nums.into_iter().inspect(|_| count += 1).sum();
    (sum, sum / count as f64)
}
 
fn main() {
    println!("{:?}", stats([1.0, 2.0, 3.0]));
}

What this demonstrates:

• sum consumer
• inspect side effect counting
• IntoIterator flexible input

Deep Dive

collect builds many types via FromIterator. fold is general reducer. for desugars to while let Some.

Gotchas

• collect without type hint - Ambiguous. Fix: collect::<Vec<_>>().
• sum on empty - Returns zero for numeric types - know identity.
• reduce on empty - Returns None. Fix: fold with seed.
• for mutates while iterating adapter - Borrow rules on source.
• needless collect - clippy flags extra allocations.

Alternatives

FAQs

Related

• Iterator Adapters
• IntoIterator & Owned vs Borrowed
• Fallible Iterators & collect()
• Control Flow

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+.