[floofy222]

Most programming languages don't have a stable / unstable distinction at all, and unless one is "in the Rust loop", stating something like "_unstable_ Rust can do X" won't probably mean what the reader think it means.

Unstable Rust sounds very dangerous, like something that breaks every day. Definitely more dangerous than stable Rust.

Yet if one is in the Rust loop, one knows that this is often not the case. I've been using some unstable features on nightly, like const fn, specialization, function traits, etc. for years (3 years?), and I've never had a CI build job fail due to a change to the implementation of these features.

Yet some features in stable Rust like Rust2018 uniform_paths or stable SIMD have caused many build job breaks and undefined behavior due to bugs in the compiler over the last months.

So whatever stability means, it does not mean "using this feature won't result in your code not breaking". It also doesn't mean "you have to use a nightly toolchain to use the feature".

An unstable Rust feature is more like a "compiler extension" in C / C++. It is just something that hasn't fully gone through the process of standardization.

I don't think it is a fair characterization that code that uses this extensions is not Rust. Pretty much all C++ code uses compiler extensions, and nobody says that this code is not C++ just because it uses one of them.

Explaining all of this when telling someone "Rust is a technology that allows you to solve problem X nicely" isn't helpful.

Many people vocal about Rust seem to think that Rust is the end in of itself. The goal isn't solving a problem, but using Rust to solve it. I see many of these people argue that unstable Rust isn't Rust, and that people should be using stable Rust etc. For most people, using Rust isn't the goal, solving their problem is. Whether one or many compiler extensions have to be enabled for that is pretty much irrelevant to them. Sure it would be nice if one didn't need to do that, but it isn't a big deal either. The big embedded community is living proof of that. Only a small minority of this community cares about the language enough to participate in its evolution. Most people don't care enough about that, they have more interesting problems to solve.

[steveklabnik]

You happened to pick some features where there hasn’t been much development work, since other things were prioritized. And one feature that’s not mostly compiler internal.

This is not the general case for unstable features. And promoting the use of them too heavily can cause a lot of problems. It undermines trust in the language, especially given rust’s pre-1.0 reputation (which was well deserved at the time.)

Stuff that’s unstable isn’t in Rust; that’s why it can be changed or even wholesale removed at any time. The distinction is very important.

[floofy222]

> Stuff that’s unstable isn’t in Rust; that’s why it can be changed or even wholesale removed at any time. The distinction is very important.

I've seen you talk about "writing an OS kernel in Rust", but never heard you phrase that as "writing an OS in kernel in _unstable_ Rust". I've never seen you stating: "correction: what you are using for embedded development, networking, etc. is not Rust, _but unstable Rust_" on any of the many blog posts, announcements, news, etc. about these topics over the past couple of years. I've seen you reply with that argument every now and then, when someone like me downplays the importance of the distinction, but I've never seen you address the source of that behavior.

If the distinction between Rust, and unstable Rust, is important. Why are the people at the top not making it? If you are working on the compiler, servo, etc. you are actually not programming in Rust, but in _unstable_ Rust all of the time. Are they hypocrites? I don't think so.

If I reflect on why I feel that this distinction is not important, the first thing I realize is that I do think the distinction is important. But this distinction is not binary _to me_, as opposed to how you and burntsushi are putting it.

As you mentioned, some unstable features change more than others. There is a wide range of how much continues breakage does using certain unstable features cause downstream users. Some features break every day, some features haven't broken anything in 3 years.

Are unstable features that haven't broken anything in 3 years stable? No, by definition, they aren't.

Are they practical to use? The answer isn't yes or no, the answer is "depends on how much breakage you are willing to accept". We upgrade C++ compiler ~twice per year, and even though we only write 100% standard compliant code, we have to always fix breakage due to the upgrade. Yet I wouldn't say that standard compliant C++ is an unstable programming language.

So, if consider bi-yearly breakage is stable enough for our professional C++ projects in practice, why would I judge Rust stable / unstable features using a different bar? This does not mean that I believe that using unstable (or only stable) features will never cause breakage, since that is impossible.

I've had stable Rust CI jobs break because the standard library added some new trait method, and that caused an ambiguity that broke in my stable Rust code. The answer was: your code was correct, but we are allowed to break it in this way.

In my opinion, it is not "stable vs unstable", but 99% vs what degree of stability does your project need, where choosing more stability than what it needs puts it at a technical disadvantage. It doesn't matter whether one is talking here about Rust unstable features, or using the super unstable next-gen stable Rust web framework.

The stability line does not lie where I or anybody else decides to arbitrarily put it. It lies exactly on the amount of stability that a particular project can tolerate, and it is up to the judgement of the developers of that particular project to find out where that is.

Telling someone that a particular project is not Rust because the stability line for that project does not fall where your line does feels just wrong. Particularly when those doing it don't make that distinctions about themselves and the projects their work on.