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Comment by suby | Hacker News Reader

Perhaps worth mentioning is that someone attempted to port this to Rust and got about 60,000 lines of code into it before archiving the project. I feel like comparing these two efforts would be an interesting case study on the impacts / benefits / limitations or difficulties, etc involved in rewriting from C++ to Rust.

https://github.com/sidkshatriya/rd/

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From the perspective of an rr maintainer, Sid's work was good and we were supportive of it. The main issues with migrating to that as the "blessed" version are that 1) rr has accumulated a decade of very hairy fixes for crazy kernel/process behavior that we feared could be lost during a port and 2) there's a closed source project (remix[0]) built on top of rr that would have needed to be ported too.

[0] https://robert.ocallahan.org/2020/12/rr-remix-efficient-repl...

I don't understand the "rewrite X/Y/Z in Rust" trend that has been going for a few years.

I'm not familiar with Rust, but I'm almost sure it has a good C interoperability. If a certain piece of software is working well, what is the benefit of rewriting it in Rust?

>If a certain piece of software is working well, what is the benefit of rewriting it in Rust?

Presumablly the idea here is to support Rust replay debugging, not just rewrite a C/C++ targetting replay debugger in Rust

It works with pretty much any compiled language.

Does it understand the semantics, primitives, and structures of any compiled language?

Or is it more of a lowest common denominator experience, where e.g. all of them are constrained to common semantics of C/C++?

That's up to debuggers to implement. lldb, gdb, and delve all support rr traces.

I'm not so sure of this. Most code in a debugger doesn't have much to do with the source language. Line mapping and figuring out values for variables happens via symbol information, eg. DWARF, and compilers for multiple languages can produce that in the same format.

Does that neutral way of debugging work as well as having first-class explicit support for the language, or it's more of a lowest common denominator (kind of like what e.g. language support over LSP can offer and how conveniently, compared to what a native Lisp or Smalltalk environment's language support can offer)?

One debugger feature I thought of while writing the comment was the ?? command in windbg. That takes an expression and evaluates it. Gdb also does this which I've used with the "print" command to print a C expression, including pointer casts and such. That would obviously require language support.

But then again you don't need to code everything in the same language either. You could write a rust parser in another language. Or a modular interface to dispatch knowledge of a programming language (does Microsoft's "language server" concept work this way?)

The latter.

The neutral way of debugging is really debugging the raw machine code of a process. This requires OS integration for your low level manipulation primitives. To add language support, you then need to figure out how to define the semantics of your manipulations in terms of the low-level primitives.

If you have a rich runtime, you can add language-level debugging facilities that can operate at a higher level. However, this requires you to implement portions of a debugger in your runtime. Now you have to maintain a language, runtime, and debugger. It also means that if new debugger techniques are invented, such as time travel debugging, you do not get them for free since you embedded a debugger of your own design. So, like many similar things, it is a trade-off of specialization versus maintenance. The perennial question of use a library, or do it yourself.

>If a certain piece of software is working well, what is the benefit of rewriting it in Rust?

The "working" C program has a high risk of undiscovered bugs relating to concurrency and memory safety. Rust lets you rule out a large swathe of them by construction. Rust's type system is also far more expressive, which in many cases enables cleaner domain modelling.

You're correct that this is the stated justification most of the time.

Should be nuanced though, because the working C program has a risk, but the the risk is a function of the size of the codebase, its age, and the number of audits it has undergone.

It is definitely easier to write bugs in C due to the additional freedom you have, but it is not necessarily a "high" risk for mature C libraries.

It is definitely not as advisable to just replace all C with Rust, but it is advisable to prefer memory safety in new projects.

Definitely. Once you aren't adding new features, the only thing left to do is fix bugs. New rewrites can make the same old mistakes. But there is the potential to have a lower "bug floor" in a safer language.

Recently the federal government issued a security advisory encouraging all new development to be done in Rust. I'm not sure the extent of which agencies this was meant to cover but it struck me as very unrealistic having just done years of java development for a state agency. https://www.nextgov.com/cybersecurity/2024/02/white-house-ur...

It doesn't say that, no. Even in the title it says "memory-safe languages", which of course includes Java.

Rust is "only" mentioned in the context of a C / C++ replacement, which tends to be a different area.

Yes that is correct, I had read a different article that had a more rust first focus. I just tried to google for a reference to the news release I was referencing, but yea it doesn't really support my comment :(

> the federal government issued a security advisory

Well, I guess that's it...

Maybe rust will take the same path as Ada.

There's a bit of a higher abstraction ceiling in Rust so in theory if you are successful rewriting a thing in Rust then you now have a codebase that's easier to change confidently.

This sort of property is nice to have in huge codebases where you really start losing confidence in shipping changes that don't subtly break things. But of course a huge codebase is hard to rewrite in general...

Sounds like a perfect situation for a strangler pattern? Wrap or transpile the original code into a language with stronger refactoring support and the rest should become incrementally easier.

The advantages of rust only come when you actually use the rust-provides abstraction, especially those around allocation and concurrency. Even if transpiling is possible, the code would still not be structured I the rust way, and you wouldn't have any of the benefits. Same goes for wrapping.

> There's a bit of a higher abstraction ceiling in Rust

Compared to C, yes, but not compared to C++.

This isn't really true. Rust has a much better type system. When writing generic code the impact is enormous.

C++ doesn't have a real Empty Type, and it thinks Units have non-zero size. In practical terms this makes it incredibly wasteful and in terms of a clear abstraction it encourages you to come up with a hack that's unclear but efficient.

…or you can just waste a few bytes? It's not a big deal.

You can, but that makes the type system worse. Also depending on how these few bytes are used, they can add up and drag down performance.

No, my point is that it doesn't. If your zero-sized types are big your type system is not any worse: it's just less efficient.

If you're focused on just the theoretical correctness of the type system, go back to my first critique: C++ does not have Empty Types. So immediately a whole class of problems that are just a type system question in Rust are imponderable, you can't even say what you meant in C++

Copying a bunch of stuff because the borrow checker won't let you share it can drag down performance as well. Yes, I do understand why one might conclude that tradeoff is worth it. But it is a tradeoff.

The original topic was the abstraction ceiling. There are a bunch of abstractions which C++ just can't express.

Funnily enough, because the borrow checker is so strict I feel more confident writing complex borrowing logic that I wouldn't dare attempting in C or C++ because even if I were to get everything right (a big if), there's no assurance that a later refactor wouldn't subtilty break the code. The borrow checker sometimes makes you copy data that you thought you didn't, but more often than not it is enforcing an actual edge case that would have been a bug, had the borrow checker not be present. If the copy is indeed so critical, you can also ease your pain with runtime checks instead using Rc/Arc, but that's another discussion.

Do you have a link to an example where this matters?

C++20 added `[[no_unique_address]]`, which lets a `std::is_empty` field alias another field, so long as there is only 1 field of that `is_empty` type. https://godbolt.org/z/soczz4c76 That is, example 0 shows 8 bytes, for an `int` plus an empty field. Example 1 shows two empty fields with the `int`, but only 4 bytes thanks to `[[no_unique_address]]`. Example 2 unfortunately is back up to 8 bytes because we have two empty fields of the same type...

`[[no_unique_address]]` is far from perfect, and inherited the same limitations that inheriting from an empty base class had (which was the trick you had to use prior to C++20). The "no more than 1 of the same type" limitation actually forced me to keep using CRTP instead of making use of "deducing this" after adopting c++23: a `static_assert` on object size failed, because an object grew larger once an inherited instance, plus an instance inherited by a field, no longer had different template types.

So, I agree that it is annoying and seems totally unnecessary, and has wasted my time; a heavy cost for a "feature" (empty objects having addresses) I have never wanted. But, I still make a lot of use of empty objects in C++ without increasing the size of any of my non-empty objects.

C++20 concepts are nice for writing generic code, but (from what I have seen, not experienced) Rust traits look nice, too.

It's probably mean for me to say "empty type" to C++ people because of course just as std::move doesn't move likewise std::is_empty doesn't detect empty types. It can't because C++ doesn't have any.

You may need to sit down. An empty type has no values. Not one value, like the unit type which C++ makes a poor job of as you explain, but no values. None at all.

Because it has no values we will never be called upon to store one, we can't call functions which take one as a parameter, operations whose result is an empty type must diverge (ie control flow escapes, we never get to use the value because there isn't one). Code paths which are predicated on the value of an empty type are dead and can be pruned. And so on.

Rust uses this all over the place. C++ can't express it.

Help me out here.

What is this empty type for? Could you provide an old man with a nice concrete example of this in action? I've used empty types in C++ to mark the end of recursive templates - which I used implement typelists before variadic templates were available.

But then you mention being unable to call functions which take an empty type as a parameter. At which point I cease to understand the purpose.

I don't know that I'll be able to convince you but I'll give a couple of examples.

What is the type of the expression "return x" ? Rust says that's ! pronounced Never, an empty type. This expression never had a value, control flow diverges.

So this means we can just use simple type arithmetic to decide that a branch which returns contributed nothing to the type of the expression - it has no possible value. This wasn't a special case, it's just type arithmetic.

Ok, lets introduce another. Rust has a suite of conversion traits. From, Into, TryFrom and TryInto. They're chained, so if I implement From<Goose> for Doodad, everybody gets the three other implied conversions. But the Try conversions are potentially fallible, hence the word Try. So they have an error type. Generic Code handling the Error type of potentially failing conversion will thus be written, even if in some cases the conversion undertaken chained back to my From<Goose> code. But wait, that conversation can't fail! Sure enough the chained TryFrom and TryInto produced will have the error type Infallible, which is an Empty Type.

So the compiler can trim all the error handling code, it depends upon this value which we know can't exist, therefore it never executes.

void is an empty type in C++. It's less useful than it could be, but it does exist.

void isn't a type. If you try to use it as a type you'll be told "incomplete type".

People who want void to be a type in C++ (proponents of "regular void") mostly want it to be a unit type. If they're really ambitious they want it to have zero size. Generally a few committee meetings will knock that out of them.

Thanks for the clarification.

Can you instantiate an empty type? If yes, are all instances unique? Years ago, I was surprised to learn how C++ handles the (essentially) empty type (no data): A single byte to differentiate each instance.

that's unit. the empty type is a type you cannot instantiate

> If a certain piece of software is working well, what is the benefit of rewriting it in Rust?

One benefit is it's easier to hide malicious code thanks to Rust's complicated syntax.

It's far easier to hide malicious code in C or C++: just write some subtle undefined behavior that you can write an exploit against. Developers do that all the time even when they're not trying to be malicious. In Rust you'd have to wrap it in "unsafe" which draws attention.

It would be a good but difficult analysis; at a quick check, rr 1.0 took 3 years and signficant contributions from around 3 or 4 people (I saw at least 5 people contributing), and the rr we have today is 10 years further work on that.

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