Macro to collect metadata

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1

This is a "pie in the sky" question. I don't think it's possible, and I don't even really know how to ask the question. But I'm hoping that someone may have an answer, or an alternative way of doing the same thing.

I have a number of structs that I wanted to define decoratively, and they each have a lot of boiler plate. So, I have a combination of a derive macro and a couple attribute macros that do the work for me.

In the end, something like this:

#[derive(Instruction)]
#[opcode = 30]
#[signature = "LOAD $D #2"]
pub struct Load {
    operand_values: OperandValues,
}

is expanded out to a struct that implements a couple custom traits and fleshes out a few custom methods. All that works well.

What I'd like to be able to do is -- at build or compile time -- collect information about all of these instructions and create some rust structure somewhere else after all have been expanded.

So, I would end up with (for instance) an enum in a different module with a variant for each instruction.

I have (sorta) gotten this to work with a crappy python script, but I was hoping there is a way to leverage macros. Or build scripts.

Any thoughts would be greatly appreciated. And, I can share more code if that's helpful. I just didn't want to overwhelm the topic with code that is currently working.

1 Like
2

Looks like there's a link-time solution, like Linkme — Rust build util // Lib.rs. Will be interesting to hear if there's anything compile-time, though!

3

Sounds like crate local state.

4

The inventory crate could probably be used for that. It is used in the typetag crate, which could serve as inspiration.

5

Thank you all for the crates to look at. I'm going through them now. I'll post back what I end up doing :slight_smile:

6

Note that both crates are archived. I believe due to an unfixable bug.

7

And this one is also archived

8

As far as I know, there is no good solution for this problem. This is on my wishlist for rust 2027.

9

I really appreciate the conversation. It seems like there is no production-safe way of doing what I'm wanting, but I was able to get the GitHub - dtolnay/linkme: Safe cross-platform linker shenanigans crate working in such a way as to do what I wanted. Since the project I'm working on is really an experiment, I'm comfortable using it for now.

@MichaelV I couldn't see any documentation about why they were archived. What is the bug?

10

I also do not know, but expect it to be related too
https://github.com/dtolnay/linkme/issues/31

I know that the crate works for my toy example, but I fear that it will explode. So I guess I should blog about my personal solution to this problem space
If I would have time ...

11

The linkme solution actually works really well (so far). I was even able to add the distributed_slice into the existing procedural macro. I haven't hit the reported issue, but needless to say I don't think I would want to use this for anything other than this research experiment.

For reference, this is what I'm working on: Add the most basic Assembly Language by electricjones · Pull Request #8 · electricjones/kaylee · GitHub

12

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