TL;DR: Vast culture clash that rust guys didn't perceive and C guys hated and false assertion that "you don't need to learn rust" based on inexplicably naive lack of understanding that maintenance might be necessary.

If someone builds a rust api on top of your C code inside your project, you have exactly five choices: (1) preserve the assumptions the rust code is making (2) only change your code if you have a rust expert to collaborate with handy (3) edit the rust code yourself (4) break the rust assumptions leading to hard to find bugs (5) break the build. The C guys hated all five of those options, and the rust guys told them they didn't need to worry their pretty little heads about it. ON, they weren't as dismissive as that, but they either didn't understand those as issues or didn't care about them or dismissed them.

The rust guys were asking the C guys to tell them the semantics so that they could fix the type signatures for their rust functions and the C guys were reluctant to do that because they wanted to be able to change the semantics of that turned out to be useful to them. They didn't want to commit so something that was documented in a way they weren't familiar with because they felt that even if they wanted to, they couldn't ensure their code was compliant with this specification going forward because they didn't understand the rust type signature fully. (They got hung up on the self argument and launched a rant against OOP.)

The rust guys knew instinctively that the Result return type meant that the operation could fail and could tell from the two arguments to that both in what ways it could fail and every kind of answer it could produce if it succeeded, but the C guys found almost none of that obvious. This was for just one function in the rust API, but it also radically changed the way of doing it. This one rust call replaced the whole algorithms of ask, check answer, if none, check this and that, otherwise do this blah blah blah. The C guys are used to keeping everything lean and simple with a single purpose and were being asked to think of a while collection of procedural knowledge and edge cases with a handle everything monolith. But they were audibly reluctant to commit to that being all the edge cases because they don't think of all of those tests as one thing and instinctively wouldn't write something that checks for all of the edge cases because (a) in a lot of circumstances the code they're writing only needs to know that there was a problem and will give up quickly and move on and (b) they want to be able to freely choose to add other edge cases in the future like they normally do without having to worry about the rust code breaking.

They weren't complaining that they were being asked to write rust, they were complaining that they didn't want to learn rust, and they were complaining this because they could see that to preserve all the rust API type signatures they would have to understand them, the expectations around them and memory safety principles, so that a rust programmer in the future wouldn't have to change the rust type signature.

The rust guys would have gained a lot more traction by just asking the C guys to keep a bunch of comments up to date detailing the semantics and error checking procedures, and promising to edit their rust API if the C code changes, but I suspect they didn't ask for that because they know that no guarantees come from a comment and they want to be sure that the rust code works across all the possible scenarios and in rust culture, that is always documented in the type system where it can be enforced.

The rust guys spoke like it was self evident that having a monolithic API with a bunch of stuff guaranteed by the rust compiler was best, but seem not to have realised that this is a massive culture clash because the C guys come from a culture of rejecting the idea of compiler guarantees anyway (because they have long had confidence in their ability to hand optimize their code to be faster than some prescriptive compiler's output and look down on people who choose to have the guardrails up).

They felt like they were being asked to help write an interface definition in a monolithic style that they have always rejected, to achieve goals that they have long resisted, in a language that they find alien, with no guarantees for them that the rust guys were going to stick around to agree and implement the rust changes necessary if they changed the C code, and with no confidence that they understood what would count as a breaking change at the rust level.

This perceived straightjacket made them particularly cross. They complained about the inability to change their C code and its semantics and the need to learn enough rust to understand quickly what not to change, but they didn't want to not change things and would need to edit the rust API at the same time as editing the C code if they didn't want the rust build to break, and then there would be even more downstream changes from that, so realistically they would need not only to be able to understand the rust type signatures, they would need to be able to edit both the type signatures and the functions themselves, and basically maintain all the downstream rust, and they would want to be sure they were writing efficient rust, well aware that it took them decades to get to the level of extreme efficiency they write in pure C, a much simpler language.

The rust guys said "Just tell us what your code means so we can write our type signatures", but the C guys didn't want to help create for themselves a prison whose walls were of a strange and intricate design they found hard to perceive, made out of materials they didn't have experience working with. They felt like the first guys were asking them how all the doors, windows, chimneys, air vents etc of the house that they built by hand would ever be used, so they could encase it in a stainless steel shell and make it part of a giant steel city. The C guys said "but I might want to build an extension or a wider garage!" They claimed that the C guys didn't have to learn how to weld or manufacture steel sheets, and that their house would be much safer, but for some reason this didn't win the C guys round to the plan, and there's a bunch of people online calling the C guys tech luddites for not liking the whole thing and saying that they were incorrect that they needed to learn rust just because the rust guys made that claim, but that claim is actually completely incorrect unless you think that it's OK to stop the project compiling with your pull request or you think that changes to the C code should be banned wherever a rust API is built on top of it.