From your post, though, it sounds like Bun may have been a pretty direct rewrite, without too many hard choices along the way. Is that fair?
That’s exactly the type of thing that is needed is to optimize projects for modern compatibility, portability and safety when other modernization efforts or forks don’t exist.
That said, I suspect this rewrite went so quickly and so optimally because it had the benefit of (effectively) 100% test coverage already in place in a really well defined system. Most open source project spawn from efforts of a single developer who frequently never waste time writing tests for a little side project. Later as it grows, they rarely stop and go back to implement testing. So if you’re truly working with an old dead project, there is a really good chance there are zero tests to be found. That is far more difficult to reach the same completeness unless the goal is simply to port all of those same problems to a new language and hope type safety fixes them.
(Not specific to ngspice, just mean generally.)
Lifetimes can get pretty hard in very complex code bases... even if other aspects of burrow checking may be more common, this is where I've had and seen the biggest gaps in understanding in practice. That said, you can usually do inefficient things to work around these issues with the opportunity to come back later. Often inefficient Rust with lots of clone operations is still faster, smaller, lighter than the same services in Java or C# as an example.