Not saying that we shouldn't change the definition but you could have written this as

For sure, I'm happy with that too if we'd rather not change the generated code.

From the documentation, it's not clear to me which element dedup_by_with_count keeps if multiple elements map to the same key: Is it the first, the last, or any?

We would want to flag all except the first, which I believe the implementation currently isn't handling (it only flags one).

In which case we probably can't use dedup_by_with_count anymore, in which case I would suggest writing this entire check without using itertools (I don't mind it but I also don't love it because it hides allocations)

if type_params.len() > 1 {
	let mut sorted = type_params.clone();
	sorted.sort_by_key(|t| &*t.name());

	let last_name = None;

	for type_param in sorted {
		if last_name.is_some_and(|last| last == type_param.name()) {
			// push diagnostic
		}
		last_name = Some(type_param.name());
	}
}

Ah that's a good point. I assumed it was taking the first, which is why I reversed the list, but that doesn't handle the case with more than two duplicates. I also felt like I was going overboard with itertools here, so your implementation makes sense to me.

I went with an inner linear search instead of sorting at all, but I'm happy to go with your approach if you prefer it.

Let's add a few examples where:

• With more than two duplicate type params
• With non duplicate type params