[simonask]

LTO cannot change the layout of structs. For something like a hash map implementation, it matters whether inner nodes store a pointer to the key and value, or whether it stores a pointer to each. To achieve this in C, you have no other options than emulating templates using macros.

[flohofwoe]

The question is whether a hash-map implementation that works on a general `[key, index]` item and where index references at separate array of values isn't actually better for some access patterns ;)

And of course the other alternative to macros is code-generation (but macros are actually often fine).

But this also only matters for actually reusable generic code. If I'd implement a super-hot-path hashmap in C, I would stamp out a specialized version by hand instead of relying on a generic implementation. But for 90% of cases, a solution like in stb_ds.h is probably good enough.

[simonask]

Sure, but now you're actually moving the goal posts. We're talking about the practicalities - you can always achieve the same by doing more work, but it makes a difference that Rust gives you `HashMap` in the standard library that you can just use and get best-in-class performance, every time, with zero work, zero maintenance. The only choice you have to make is which hash function you want to plug into it, and since it is generic, that gets optimized and inlined as well (even with LTO disabled).