Rust: Stabilize `#[bench]` and `Bencher`?

[SimonSapin]

I’ll take the liberty of copying a section of @hsivonen’s Rust 2020 blog post:

Non-Nightly Benchmarking

The library support for the cargo bench feature has been in the state “basically, the design is problematic, but we haven’t had anyone work through those issues yet” since 2015. It’s a useful feature nonetheless. Like I said a year ago and the year before, it’s time to let go of the possibility of tweaking it for elegance and just let users use it on non-nighly Rust.

Indeed the existing benchmarking support has basically not changed in years, and I’m not aware of anyone planning to work on it. To keep reserving the right to make breaking changes is not useful at this point. Custom test frameworks offer another way forward for when someone does want to work on better benchmarking.

So I’d like to propose a plan:

• Move test::Bencher to std::bench::Bencher. I have a PR coming soon that https://github.com/rust-lang/rust/pull/66290 demonstrates that this is possible. This move avoids the need to stabilize the test crate.
• Optionally, make some surface-only API tweaks to Bencher. For example, the public bytes field could become a parameter to some method.
  
  
  
  • Although not strictly necessary for changes to an unstable type, we can keep the existing bytes field and iter method unchanged as unstable + deprecated for a while.
  
  
• Stabilize the #[bench] attribute and just enough of Bencher to make it usable with #[bench]. (For example, no stable constructor.)

@rust-lang/libs, @rust-lang/lang, do you feel this needs an RFC?

[Centril]

do you feel this needs an RFC?

Given that "basically, the design is problematic" I'd say yes, we should have an RFC that justifies and explains the overall design (as if it was an unimplemented feature).

cc @rust-lang/dev-tools

[alexcrichton]

I'd like to paste parts of this comment here:

I'm personally pretty wary and against stabilizing something just because progress isn't happening on it. Stabilization takes real work and has a cost. Simply because an API exists doesn't mean we should stabilize it, even if it's been sitting for months. I think it's pretty bad design to get things into the standard library, then point out it's sat with no feedback for months and propose stabilization.

I would ask again that the justification for stabilization has "it's not been touched for years" only taken as an input of whether to start the stabilization process, not an implicit ratification that the current design is "obviously good enough because no one has changed it".

There have been repeated attempts to stabilize the benchmarking infrastructure in the past and they've all been stymied because the libtest benchmarking support just isn't that great. It's great for quick and dirty things, but like #[test], it does not have extensibility hooks, it's not always the easiest to use, etc. Stabilization often also gets quickly into the weeds of "what to do about std::test?" or "what about custom test frameworks?" and such. I personally think it's very difficult to avoid these questions when proposing stabilization.

Given the history of this feature I would personally expect an RFC to justify why the current design is suitable for stabilization (or the proposal here). Again, though, the RFC will need to justify its design irrespective of the amount of time the design has sat in libtest for now.

[Manishearth]

Note that we already have a landed eRFC for custom test frameworks, and
some implementation work there. It would be nice to focus efforts on
getting that finished.

On Mon, Nov 11, 2019, 7:12 AM Alex Crichton [e-mail]
wrote:

I'd like to paste parts of this comment here
https://github.com/rust-lang/rust/issues/48043#issuecomment-544581895:

I'm personally pretty wary and against stabilizing something just because
progress isn't happening on it. Stabilization takes real work and has a
cost. Simply because an API exists doesn't mean we should stabilize it,
even if it's been sitting for months. I think it's pretty bad design to get
things into the standard library, then point out it's sat with no feedback
for months and propose stabilization.

I would ask again that the justification for stabilization has "it's not
been touched for years" only taken as an input of whether to start the
stabilization process, not an implicit ratification that the current design
is "obviously good enough because no one has changed it".

There have been repeated attempts to stabilize the benchmarking
infrastructure in the past and they've all been stymied because the libtest
benchmarking support just isn't that great. It's great for quick and dirty
things, but like #[test], it does not have extensibility hooks, it's not
always the easiest to use, etc. Stabilization often also gets quickly into
the weeds of "what to do about std::test?" or "what about custom test
frameworks?" and such. I personally think it's very difficult to avoid
these questions when proposing stabilization.

Given the history of this feature I would personally expect an RFC to
justify why the current design is suitable for stabilization (or the
proposal here). Again, though, the RFC will need to justify its design
irrespective of the amount of time the design has sat in libtest for now.

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[SimonSapin]

@Manishearth Yes, this proposal specifically does not involve opening a design discussion beyond small API-surface-only tweaks, so as to not divert efforts and limited team bandwidth from custom test frameworks or other work areas. (And I don’t feel up to personally leading such a discussion at the moment.)

[Manishearth]

Oh, yeah, I agree, I actually think we should find a path for stabilizing bench itself at this point.

One thing I will point out is that a _lot_ of people want some form of bench framework (and are likely willing to put in the work to design and stabilize just that), whereas far fewer people want custom test frameworks, and the work for that is proportionally larger.

I don't want to say that the justification for stabilization is "it hasn't been touched for years", but I do want to say that the fact that benching has been blocked on custom test frameworks has led to this annoying impasse where there just aren't enough interested people to do the huge amount of work to complete custom test frameworks. I'd still _prefer_ if that happened, but that doesn't seem likely. I feel like as a community we have a tendency to fall into this trap, so I do see this issue as a push for a way out.

That said, I already opened an RFC for this and it got closed: https://github.com/rust-lang/rfcs/pull/2287 . The reasoning there hasn't changed much, though I'd argue that the languishing of custom test frameworks is a valid argument to say that we should revisit this.

[SimonSapin]

"It hasn't been touched for years" isn’t an argument for stabilization, it’s a sign that that more time as unstable is unlikely to lead to improvements to this benchmarking framework. Instead, I suspect that improvements will come as something entirely new that can be used through custom frameworks.

I think this leaves two realistic outcomes for #[bench]:

• It is eventually removed, perhaps after alternatives become available through custom frameworks. The pFCP to close the previous RFC planned for this: https://github.com/rust-lang/rfcs/pull/2287#issuecomment-362431922

• It is stabilized with its current design. People who are already happy enough with it can now use it on the Stable channel. It can then slowly fall out of use when something better comes up later. The Rust project is stuck with maintaining it "forever", but #[bench] is very similar to #[test] that we’re already stuck with. I feel that the additional maintenance burden might be light enough that the benefit until an alternative is available may be worth it.

  I understand there’s not a lot of appetite for this and that we generally set the bar high, but perhaps we can live with something known not to be perfect.