> For instance, in C, qsort() takes a function pointer for the comparison function, in Rust and C++, the standard library sorting functions are templated on the comparison function.

That's more of a critique of the standard libraries than the languages themselves.

If someone were writing C and cared, they could provide their own implementation of sort such that the callback could be inlined (LLVM can inline indirect calls when all call sites are known), just as it would be with C++'s std::sort.

Further, if the libc allows for LTO (active area of research with llvm-libc), it should be possible to optimize calls to qsort this way.