rust_string_join_bench.rs

#![feature(test)]

extern crate test;
extern crate itertools;

use test::Bencher;
use itertools::Itertools;

fn strings_vec() -> Vec<String> {
    vec![String::from("again"); 512]
}

#[bench]
fn consume_collect_join(b: &mut Bencher) {
    b.iter(|| {
        strings_vec()
            .into_iter()
            .collect::<Vec<String>>()
            .join(" and ")
    });
}

#[bench]
fn consume_itertools_join(b: &mut Bencher) {
    b.iter(|| strings_vec().into_iter().join(" and "));
}

#[bench]
fn borrow_collect_join(b: &mut Bencher) {
    let strings = strings_vec();
    b.iter(|| {
        strings
            .iter()
            .map(|s| &**s)
            .collect::<Vec<&str>>()
            .join(" and ")
    });
}

#[bench]
fn borrow_itertools_join(b: &mut Bencher) {
    let strings = strings_vec();
    b.iter(|| strings.iter().join(" and "));
}

// Results:
// 
// test borrow_collect_join    ... bench:      24,071 ns/iter (+/- 11,873)
// test borrow_itertools_join  ... bench:      41,002 ns/iter (+/- 6,126)
// test consume_collect_join   ... bench:      63,260 ns/iter (+/- 18,295)
// test consume_itertools_join ... bench:      82,640 ns/iter (+/- 30,842)

perf shows the consume benches calls malloc heavily whereas the borrow cases don't. Probable itertools explanation in rust-itertools/itertools#577 . I don't have a reference for the core lib case.