Rust format! double referencing performance impact

format.rs

// Place this page as /benches/format.rs in a rust project created with `cargo new fmttest --lib`

// Add to Cargo.toml:
//
// [dev-dependencies]
// criterion = { version = "0.4", features = ["html_reports"] }
//
// [[bench]]
// name = "format"
// harness = false

// To benchmark:   cargo bench
// To view html:   open target/criterion/report/index.html

use criterion::{criterion_group, criterion_main, Criterion};
use std::fmt::Write;

const ELEMENTS: usize = 100;

pub fn bench_format(c: &mut Criterion) {
    let mut buffer = String::with_capacity(ELEMENTS * 10);
    let mut g = c.benchmark_group("Summary");
    let i = 100;

    g.bench_function("value", |b| {
        b.iter(|| {
            buffer.clear();
            for i in 0..ELEMENTS {
                _ = write!(buffer, "{}", i);
            }
        })
    });
    g.bench_function("reference", |b| {
        b.iter(|| {
            buffer.clear();
            for i in 0..ELEMENTS {
                _ = write!(buffer, "{}", &i);
            }
        })
    });

    g.finish();
}

criterion_group!(benches, bench_format);
criterion_main!(benches);