Unsafe is sometimes useful

Your code can go faster!

rust,editable

# // Copyright 2026 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
fn iter_sum(xs: &[u64]) -> u64 {
    xs.iter().sum()
}

fn fast_sum(xs: &[u64]) -> u64 {
    let mut acc = 0;
    let mut i = 0;
    unsafe {
        while i < xs.len() {
            acc += *xs.get_unchecked(i);
            i += 1;
        }
    }
    acc
}

fn main() {
    let data: Vec<_> = (0..1_000_000).collect();

    let baseline = iter_sum(&data);
    let unchecked = fast_sum(&data);

    assert_eq!(baseline, unchecked);
}

Code using unsafe might be faster.

fast_sum() skips bounds checks. However, benchmarking is necessary to validate performance claims. For cases like this, Rust's iterators can usually elide bounds checks anyway.

Optional: show identical generated assembly for the two functions.

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