RSTR-PERF-002 — Rust `for x in xs.clone()`

Summary

A for loop iterates over xs.clone() (or xs.to_vec(), etc.). The clone walks the entire collection and allocates a new owned copy just for the iteration — wasted memory and wasted CPU. Borrowing &xs does the same job with zero allocations.

The clone is usually copy-paste from "I needed to satisfy the borrow checker once, then never revisited" or "the IDE auto-suggested .clone()."

Severity

Low. Correctness-neutral; the impact is purely overhead.

Languages

Rust.

What rastray flags

#![allow(unused)]
fn main() {
for item in items.clone() {      // ← flagged
    process(item);
}

for row in rows.to_vec() {       // ← flagged
    print(row);
}
}

What rastray deliberately does not flag

for item in &items / for item in items.iter() — borrowing.
for item in items where items: Vec<T> and the loop consumes items (moves) — the original cannot be used after the loop.
Clones followed by mutation: let mut copy = xs.clone(); copy.push(...) — clone is doing real work.

How to fix it

Borrow:

#![allow(unused)]
fn main() {
for item in &items {            // borrows; iteration over &T
    process(item);
}
}

If you need to consume the value but also keep the collection, clone inside the loop body for the specific elements that need it:

#![allow(unused)]
fn main() {
for item in &items {
    consumer.give(item.clone());   // clone only the elements consumer needs to own
}
}

rastray rules