Rust being slower when optimised?

output

$ rustc slower.rs --test -C no-stack-check -O
$ ./slower --bench

running 1 test
test bench_stdrng_u64 ... bench:       187 ns/iter (+/- 14) = 42 MB/s

test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured

$ rustc slower.rs --test -C no-stack-check
$ ./slower --bench

running 1 test
test bench_stdrng_u64 ... bench:       127 ns/iter (+/- 6) = 62 MB/s

test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured

slower.rs

extern crate test;

use test::Bencher;
use std::rand::Rng;
use std::rand::StdRng;

// Note: StdRng passed by value
fn next_u64(mut gen : StdRng) -> u64 {
    gen.next_u64()
}

// Runs considerably slower with optimisations on…?
#[bench]
fn bench_stdrng_u64(b : &mut Bencher) {
    if let Ok(mut gen) = StdRng::new() {
        b.bytes = 8;
        b.iter(|| {
            next_u64(gen)
        });
    } else {
        panic!("couldn’t create StdRng");
    }
}