Basically, the other system is a dynamic library which sends and receives C structures from my application. Those structures are then mapped into a buffer that is supposed to have the same size and there are pointers with metadata pointing into the buffer that are supposed to be exactly like the struct elements. Those structures can have arbitrary complexity, and are passed around through type erasure (essentially char*).
I wrote a "reflection" code for the other team, which runs when they register the struct instance to be sent, checks if there's a matching PDB [0] around, reads it, and outputs a json including the metadata needed, which can then be used to define the structures' metadata on our side correctly.
This is all in C/C++ since in some contexts we have soft real-time requirements, else I would have used any of the many RPC frameworks available.
This has been working for several years now.
This is not a generic solution but it's good enough for in-house communication between 2 systems that are maintained by different parts of the organization, where the API between them, that like I said is based on passing around char* buffers, has been more or less set in stone a long time ago. Conway's law [1] and all that. Sigh.
[0] We are a Windows shop although the same thing should work with DWARF info, same as the OP library works. In fact he says "It may never work on Windows, which does not use DWARF to encode debug info" but I can say that the same approach does work on Windows, for C++ at least. The PDB format might be a tad undocumented, but its documentation has been improved in the last decade or so since I started working on my library. Writing some small test programs is enough to understand how to access it, if all you need is meta info on C-style structures. Other stuff is more... challenging. But it wasn't necessary for my use-case.
Yeah, I know, it's our problem, not theirs. It's something I cannot fix on my own without a huge effort. I've tried pushing for it for more than a decade, and at some point my wish was sort of abducted by my boss' boss as an excuse to create a DSL [0]. This did solve some huge problems but also created many others. It didn't solve that char* / h file problem since it doesn't really have an FFI.
[0] Domain specific language, custom-made for our own internal users. I've come to hate DSLs since I have to support that one, which never wanted.
Regardless, quite an ingenious use of standard ELF features, I didn't think this would be possible in Rust without adding some kind of VM around reflection code.
* invokes local_type_id to get the memory address of your value’s static implementation of local_type_id
* maps that memory address to an offset in your application’s binary
* searches your application’s debug info for the entry describing the function at that offset
* parses that debugging information entry (DIE) to determine the type of local_type_id’s &self parameter.
This is a rather strange thing to bolt onto a language. I could see this as an external tool. The use case seems to be programs which used "async" so much they can't figure out the resulting state machine. External debug tools to view and examine the async state machine might be helpful.
My experience with Rust has been that debugging of safe code is just not a problem. Print statements and logging are enough.
It is an external tool. This is a crate, not a part of the compiler.
This seems like a valuable library. It’s impressive that it can be so powerful in a compiled language. C and C++ are much older but don’t have anything quite like this.
It can just be an extremely fun and cute demo, without practical application.
I don't have the context to judge the probability of that in this specific case (lots of technical nitty-gritty comes in to this sort of thing), but I've certainly seen similar things happen in other communities.
Very impressive work!
Reflection seems more helpful when the programming language is little unsounded.
My crate is suitable for cases where you cannot know (or control) the set of types you might need to reflect on in advance. It's primary use-cases are related to debugging.
Compile time reflection AFAIK is available in D and Zig, and is planned for C++.
Ruts does not support inheritance either. But I have never missed either feature in a Rust program.
Thx OP for providing an example.
I don't think you understand how people in dynamic languages use any types at all.
Even if not, this is still very useful for debugging
[0]: https://doc.rust-lang.org/cargo/reference/profiles.html#rele...
