If you mean in a comparison against AST interpretation, then I'd say that Rust tagged enums (which C doesn't have) are already essentially a bytecode, except properly aligned, so it makes sense that they have same or slightly better performance than you would get with manual bytecode implementation.

I am not sure if I understand this argument. The C "tagged union pattern" is very similar memory-wise to Rust tagged enums, it just is less pleasant to use due to no pattern matching and so on.

I would expect that the advantage of the closure version of your AST interpreter is coming from somewhere else (at the end of the day it is still an AST interpreter). One possibility is that you are passing around the operands to your filters as Rust lexically-closed values instead of using a custom stack for your interpreter, which makes things a bit more "statically typed".

> at the end of the day it is still an AST interpreter

In a pretty remote sense, I guess.

> One possibility is that you are passing around the operands to your filters as Rust lexically-closed values instead of using a custom stack for your interpreter, which makes things a bit more "statically typed".

Yes, that and using native dynamic dispatch instead of walking a tree structure with branching are making this technique much closer to template JITs than AST interpretation, with corresponding performance wins.

The reason I think it is fair to call it an AST interpreter is that there is roughly one function call for each AST node. This isn't necessarily a bad thing though -- AST interpreters are super simple and maintainable (as you have demonstrated) so if you got good performance out of one then it is great!

C certainly has this via unions. It's more manual but provides more control, including choice of alignment.

It's CPU dependent but in practice the cache benefits of packed instructions will probably dominate any misalignment penalty. It's not like x86 instructions are aligned after all.

What's the best way to implement a variable-width bytecode interpreter in Rust?

More control is, as usual, less ability for automatic optimisations. For example, Rust uses knowledge of tag + data combinations for niche-filling enums where possible (and more similar optimisations to come), while LLVM can use this information to more easily deduce which branches are unreachable.

As for alignment - you can control it in Rust enums too, if you wish, by putting custom aligned structures as variant payloads.

In general, I'm sure it would be possible to achieve same (or, who knows, maybe even slightly better) performance than AST interpretation with manual tweaking, but the closure-based approach proved to give significant wins out of the box, while being more elegant to maintain and extend over time.

In a bytecode interpreter's loop, you want your language to be little more than a fancy assembler. Exotic techniques become viable and even critical for performance: computed goto, misaligned reads, pointer arithmetic, unchecked type conversions, etc. This is one place where you want to tightly control your machine code.

Oh, sure, if you're willing to go to unsafe techniques, you can go straight to a template JIT as well. But, for us, the balance is important, and safety is probably even more important than further wins in performance (which we can still easily get with algorithm-level optimisations before doing anything unsafe).

Rust has C unions now, but without safety. I would probably write manual parsing code via the byteorder crate.

This is a bit off-topic, but it has been on my mind for a while.

Has the Rust team considered that choosing cute names like "crate" might turn away some of the target audience?

The same applies to other ecosystems like homebrew, but in the case of Rust some of the target audience certainly includes C/C++ programmers.

C++ is itself a “cute” name. It seems to be doing just fine.

Beyond that, “crate” specifically has utility. The closest existing name is “compilation unit,” but with things like incremental compilation, that’s a bit of a misnomer. We need some kind of name.

C too (from B, which was a stripped down BCPL).