But that's not what people usually mean with zero-cost, right? They mean that assuming you do use the same algorithm, then the abstracted version is just as fast and memory efficient as the specialized version.

And that is a bar that C++ at least clearly meets, and, by the looks of it, rust is in practice much closer than C. Other languages have various features that address this need too; it's not that rare. C really is particularly poor here, but even C has macros, and although it's horrible in terms of usability, people nevertheless manage to do some pretty nifty stuff with macros.

Still; it's fair to say C is pretty bad at zero-cost abstractions, and that widely used alternatives support that a lot better (even C++ alone is a solid chunk of the market, more than C - https://redmonk.com/sogrady/2020/02/28/language-rankings-1-2...) Implementing a custom quicksort in C is a pain, enough pain that I expect most hand-tuned special case quicksorts are likely slower than general versions available for C++ simply because people aren't going to bother doing a good job; it's too much work. And that's just quicksort; the same goes for lots and lots and lots of other things too; reimplementing everything from scratch is just too much pain - in practice people are likely to not bother, and just pay the costs.

So while "zero-cost" abstractions are a slightly nebulous goal, clearly C in particular is worse at providing those than all of its low level competitors and wannabe competitors (C++, Rust, D, Nim, etc).

Lots of other languages make it way easier than C to write "fused quicksort". Although it doesn't pay much regard to programming language theory, Jai is another interesting language in this area, made by a guy who's mostly concerned with being able to express the fully specialized/fused version of everything without creating extra work for the programmer.