String::from_utf16 using a pre-allocated push loop vs. collect()

main.rs

#![feature(test)]
extern crate test;
use test::{Bencher, black_box};

use std::string::FromUtf16Error;
use std::char::decode_utf16;

const S15: &[u16]= &[0xd800, 0xdf45, 0xd800, 0xdf3f, 0xd800, 0xdf3b, 0xd800, 0xdf46, 0xd800,
                     0xdf39, 0xd800, 0xdf3b, 0xd800, 0xdf30, 0x000a];

const S41: &[u16] = &[0xd801, 0xdc8b, 0xd801, 0xdc98, 0xd801, 0xdc88, 0xd801, 0xdc91, 0xd801,
                      0xdc9b, 0xd801, 0xdc92, 0x0020, 0xd801, 0xdc95, 0xd801, 0xdc93, 0x0020,
                      0xd801, 0xdc88, 0xd801, 0xdc9a, 0xd801, 0xdc8d, 0x0020, 0xd801, 0xdc8f,
                      0xd801, 0xdc9c, 0xd801, 0xdc92, 0xd801, 0xdc96, 0xd801, 0xdc86, 0x0020,
                      0xd801, 0xdc95, 0xd801, 0xdc86, 0x000a];

fn old(str_u16: &[u16]) -> Result<String, FromUtf16Error> {
    String::from_utf16(str_u16)
}

fn new(str_u16: &[u16]) -> Result<String, ()> {
    let mut ret = String::with_capacity(str_u16.len());
    for c in decode_utf16(str_u16.iter().cloned()) {
        if let Ok(c) = c {
            ret.push(c);
        } else {
            return Err(());
        }
    }
    Ok(ret)
}

#[bench]
fn bench_old(b: &mut Bencher) {
    b.iter(|| old(black_box(S41)))
}

#[bench]
fn bench_new(b: &mut Bencher) {
    b.iter(|| new(black_box(S41)))
}