Vec::drain benchmark

bench.rs

#![feature(test)]
extern crate test;

use test::{Bencher, black_box};

type Item = u8;
const TOTAL_SIZE: usize = 10000;
const COPY_SIZE: usize = 1000;

#[bench]
fn vec_drain(b: &mut Bencher) {
    let mut v = black_box(vec![42; TOTAL_SIZE]);

    b.iter(|| {
        unsafe {
            v.set_len(TOTAL_SIZE);
            v.drain(..TOTAL_SIZE - COPY_SIZE);
        };
    });
}

#[bench]
fn ptr_copy(b: &mut Bencher) {
    let mut v = black_box(vec![42; TOTAL_SIZE]);

    b.iter(|| {
        unsafe {
            v.set_len(TOTAL_SIZE);

            std::ptr::drop_in_place(std::ptr::slice_from_raw_parts_mut(v.as_mut_ptr(), TOTAL_SIZE - COPY_SIZE));
            let src = v.as_ptr().add(TOTAL_SIZE - COPY_SIZE);
            let dst = v.as_mut_ptr();
            std::ptr::copy(src, dst, COPY_SIZE);
            v.set_len(COPY_SIZE);
        };
    });
}

result.log

$ cargo +nightly bench
   Compiling vec-drain-issue v0.1.0 (/home/kogia-sima/PG/Rust/vec-drain-issue)
    Finished bench [optimized] target(s) in 1.60s
     Running target/release/deps/vec_drain_issue-e603aeb59eacfab3

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

     Running target/release/deps/bench-66a9fcfcff8f19bc

running 3 tests
test ptr_copy     ... bench:          31 ns/iter (+/- 0)
test vec_drain    ... bench:        5710 ns/iter (+/- 0)

test result: ok. 0 passed; 0 failed; 0 ignored; 3 measured; 0 filtered out