Kostya's Boring Codec World

One of the Rust language features is explicit object lifetimes that help compiler correctly track memory usage and free objects without using garbage collector. A neat idea but it leads to lifetime specifiers being used everywhere including places where compiler should be smart enough to deal with them without explicit mentions in every place.

Maybe I’m using Rust wrong but in most of the cases I create objects that have no need for lifetime specifier or the objects that have the same lifetime for both its members and itself. Thus I argue that in addition to generic lifetime specifier 'a (or whatever the name you give it) and obviously named 'static there should be 'self that specifies the lifetime to be exactly the same as the object itself.

So, instead of current:

struct Foo<'a> {
  myref: &'a [u8],
  subobj: Bar<'a>,
}

impl<'a> Foo<'a> {
  pub fn new(myref: &'a [u8], subobj: Bar<'a>) -> Self { ... }
}

it should be possible to write:

struct Foo {
  myref: &'self [u8],
  subobj: Bar,
}

impl Foo {
  pub fn new(myref: &'self [u8], subobj: Bar) -> Self { ... }
}

I am not sure whether compiler needs to perform some additional things in such objects compared to objects without no lifetime specifier but it should be easy to assign proper lifetime after parsing the structure definition anyway and I’m pretty sure the compiler does something like this anyway.

And I see only these reasons why this has not been done yet:

• Considerations for compiler simplicity (i.e. parsing process should be kept as simple as possible)—I still think it should be easy for compiler to recognize the lifetime definition by the time structure declaration parsing is over and it’s used externally (i.e. for objects using this one);
• Considerations for language clarity and consistency (i.e. it’s immediately obvious when you look at the object that it deals with lifetimes but not with the proposed change). I’d argue that explicit lifetimes should be kept for complex cases only, when you have to juggle lifetimes from several complex sources, and the objects with references not outliving themselves should be fine;
• Simple oversight (i.e. “we did not think of such simplification”) or developers’ bias (i.e. “we got used to writing lifetime specifiers everywhere that we didn’t think it annoys anybody”). You should be able to guess what I have to say about such argument.

So all in all I’d be happy to either hear why it cannot be done (beside the compatibility with the existing code) or see it implemented. But most likely this will be ignored (and I’m fine with that too).

Today I wanted to talk about two features that are quite important for multimedia decoding but are quite inconvenient in the current state.

First, macros. I know that macros in Rust are both very powerful and quite flexible but they are hard to use for data definition and I ranted about it before. The problem is that quite often you have tables with some internal structure that would benefit from macro substitutions: if you have a codebook constructed from entries following patterns like a, b, -a, -b and a, b, a, -b, -a, b, -a, -b it would be easier and less error-prone to represent them as e.g. FLIP2#(a, b) and FLIP4#(a, b) inside the data definition. The problem is that macro! does not allow you to do that easily since it’s supposed to expand into valid statements (i.e. code or full data definitions). Of course you can work it around by making a set of macros to define the whole array and some bits inside it but that’s what makes it unwieldy. And that’s why I believe there should be another macro substitution mechanism, maybe named macro#, that would work just on data but it’d be much easier to use in that particular case.

The second issue is assembly integration. Despite Rust being fast and such it’s still better to write small critical functions in assembly. And obviously it would be better if Cargo supported including assembler files into crate. You can point out there’s stdsimd for using the power of SIMD without much hassle. I can point out that compiler-generated code is still far from being perfect even with intrinsics and assembly is still better; supporting querying SIMD capabilities via standard package is good though. And you can point out that there’s a special crate for supporting various files with various compilers/assemblers already. I’d say that it’s a bit too generic but at least it can serve as a base for what I need. Again, there’s more or less standard way to deal with assembly files so making a common standard is not hard.

And in the unlikely case somebody reads this and asks why I don’t form an RFC—from what I heard it involves proposing code as well and I don’t want to study the compiler nor waste days compiling it.

Disclaimer: obviously it’s my opinion, feel free to prove me wrong or just ignore.

Now I should qualify for zoidberg (slang name for lowly programmer in Rust who lives somewhere in a dumpster and who is also completely ignored—perfect definition for me) I want to express my thoughts about programming experience with Rust. After all, NihAV was restarted to find out how modern languages fare for my favourite task and there was about one language that was promising enough. So here’s a short rant about the aspects of this programming language that I found good and not so good.

Good things

• Modern language features: standard library containers, generics, units and their visibility etc etc. And at least looks like Rust won’t degrade into metaprogramming language any time soon (that’s left for upcoming Rust+=1 programming language);
• Reasonable encapsulation: I mean both (sub)modules organisation and the fact that functions can be implemented just for some structure;
• Powerful enums that can act both as plain C set of values and also as tagged objects, e.g. the standard Result enum has two values—Ok(result) and Err(error) where both result and error are two different user-defined types, so returned value can contain either while being the same type (Result);
• More helpful error messages (e.g. it tries to suggest a correction for mistyped variable name or explains an error a bit more detailed). Sure, Real Programmers™ don’t need that but it’s still nice;
• No need for dependency resolving: you can have stuff in one module referencing stuff in another module and vice versa at the same time, same for no need
• Traits (standard interfaces for objects) and the fact that operations are implemented as specific traits (i.e. if you need to have a + b with your custom object you can implement std::ops::Add for it and it will work). Also it’s nice to extend functionality of some object by making an implementation for some trait: e.g. my bitstream reader is defined in one place but in another module I made another trait for it for reading codebooks so I can invoke let val = bitread.read_codebook(&cb)?; later.

Unfortunately, it’s not all rosy and peachy, Rust has some things that irritate me. Some of them are following from the advantages (i.e. you pay for many features with compilation time) and other are coming from language design or implementation complexity.

Irritating things that can probably be fixed

• Compilation time is too large IMO. While the similar code in Libav is recompiled in less than a second, NihAV (test configuration) is built in about ten seconds. And any time above five seconds is irritating to wait. I understand why it is so and I hope it will be improved in the future but for now it’s irritating;
• And, on the similar note, benchmarks. While overall built-in testing capabilities in Rust are good (file it under good things too), the fact that benchmarking is available only for limbo nightly Rust is annoying;
• No control over allocation. On one hoof I like that I can not worry about it, on the other hoof I’d like to have an ability to handle it.
• Poor primitive types functionality. If you claim that Rust is systems programming language then you should care more about primitive types than just relying on as keyword. If you care about systems programming and safety you’d have at least one or two functions to convert type into a smaller one (e.g. i16/u16 -> u8) and/or check whether the result fits. That’s one of the main annoyances when writing codecs: you often have to convert result into byte with range clipping;
• Macros system is lacking. It’s great for code but if you want to use macros to have more compact data representation—tough luck. For example, in Indeo3 codebooks have sequences like (a,b), (-a,-b), (b,a), (-b,-a) which would be nice to shorten with a macro. But the best solution I saw in Rust was to declare whole array in a macro using token tree manipulation for proper submacro expansion. And I fear it might be the similar story with implementing motion compensation functions where macros are used generate required functions for specific block sizes and operations (simple put or average). I’ve managed to work it around a bit in one case with lambdas but it might not work so well for more complex motion compensation functions;
• Also the tuple assignments. I’d like to be able to assign multiple variables from a tuple but it’s not possible now. And maybe it would be nice to be able to declare several variables with one let;
• There are many cases where compiler could do the stuff automatically. For example, I can’t take a pointer to const but if I declare another const as a pointer to the first one it works fine. In my opinion compiler should be able to generate an intermediate second constant (if needed) by itself. Same for function calling—why does bitread.seek(bitread.tell() - 42); fail borrow check while let pos = bitread.tell() - 42; bitread.seek(pos); doesn’t?
• Borrow checker and arrays. Oh, borrow checker and arrays.

This is probably the main showstopper for implementing complex video codecs in Rust effectively. Rust is anti-FORTRAN in a sense that FORTRAN was all about arrays and could operate arrays safely while Rust safely prevents you from operating arrays.

Video codecs usually operate on planes and there you’d like to operate with different chunks of the frame buffer (or plane) at the same time. Rust does not allow you to mutably borrow parts of the same array even when it should be completely safe like let mut a = &mut arr[0..pivot]; let mut b = &mut arr[pivot..];. Don’t tell me about ChunksMut, it does not allow you to work with them both simultaneously. And don’t tell me about Bytes crate—it should not be a separate crate, it should be a core language functionality. In result I have to resort to using indices inside frame buffer and Rc<RefCell<...>> for frames themselves. And only dream about being able to invoke mem::swap(&mut arr[idx1], &arr[idx2]);.

Update: so there’s slice::split_at_mut() which does some of the things I want, thanks Tomas for pointing it out.

And it gets even more annoying when I try to initialise an array of codebooks for further user. The codebook structure does not implement Clone because there’s no good reason for it to be cloned or copied around, but when I initialise an array of them I cannot simply declare it and fill the contents in a loop, I have to resort to unsafe { arr = mem::uninitialized(); for i in 0..arr.len() { ptr::write(&arr[i], Codebook::new(...); } }. I know that if there’s an error creating new element compiler won’t be able to ensure that it drops only already initialised elements but it’s still a problem for compiler not being smart enough yet. Certain somebody had an idea of using generator to initialise arrays but I’m not sure even that will be implemented any time soon.

And speaking about cloning, why does compiler refuse to generate Clone trait for a structure that has a pointer to function?

And that’s why C is still the best language for systems programming—it still lets you to do what you mean (the problem is that most programmers don’t really know what they mean) without many magical incantations. Sure, it’s very good to have many common errors eliminated by design but when you can’t do basic things in a simple way then what it is good for?

Annoying things that cannot be fixed

• type keyword. Since it’s a keyword it can’t be used as a variable name and many objects have type, you know. And it’s not always reasonable to give a longer name or rewrite using enum. Similar story with ref but I hardly ever need it for a variable name and ref_<something> works even better. Still, it would be better if language designers picked typedef instead of type;
• Not being able to combine if let with some other condition (those nested conditions tend to accumulate rather fast);
• Sometimes I fear that compilation time belongs to this category too.

Overall, Rust is not that bad and I’ll keep developing NihAV using it but keep in mind it’s still far from being perfect (maybe about as far as C but in a different direction).

P.S. I also find the phrase “rewrite in Rust” quite stupid. Rust seems to be significantly different from other languages, especially C, so while “Real Programmers can write FORTRAN program in any language” it’s better to use new language features to redesign interfaces and make new overall design instead of translating the same mistakes from the old code. That’s why NihAV will lurch where somebody might have stepped before but not necessarily using the existing roads.