Bencher: Add method to run once?

[joliss]

I hope it's OK to post a feature request here—feel free to just close if you want to declutter the issue tracker.

I've found myself wishing I had a function to benchmark something just once (or a fixed number of times), e.g. b.once, in addition to the b.iter method:

#[bench]
fn foo_bench(b: &mut Bencher) {
    b.once(|| {
        // Do something slow and expensive
    });
}

Alternatively, perhaps libtest could allow for printing timings for the entire function. It could do that either by detecting that we're not calling b.iter, or by inspecting the function signature to check if there is no Bencher parameter.

#[bench]
fn foo_bench(_b: &mut Bencher) {
    // Do something slow and expensive
}

Right now if we do this, it just prints bench: 0 ns/iter (+/- 0), even if the function is actually quite slow.

[Mark-Simulacrum]

I'm actually going to close this in favor of #11010 since I think that's the underlying problem here. Could you add a comment there with your suggestion? It sounds interesting, though I think possibly not quite ideal -- I'd worry that one run may not give us enough information to say how long it takes per iteration....

[joliss]

Sounds good. I've added my use case there and provided some motivation.