A lot of languages claim to be a C replacement, but Zig is the second language I've seen that seemed like it had a reasonable plan to do so at any appreciable scale. The language makes working with the C ABI pretty easy, but it also has a build system that can seamlessly integrate Zig and C together, as well as having a translate-c that actually works shockingly well in the code I've put through it.
The only thing it didn't do was be 99% compatible with existing C codebases...which was the C++ strategy, the first language I can think of with such a plan. And frankly, I think Zig keeping C's relative simplicity while avoiding some of the pitfalls of the language proper was the better play.
D can compile a project with a C and a D source file with:
dmd foo.d bar.c
./fooI do like D. I've written a game in it and enjoyed it a lot. I would encourage others to check it out.
But it's not a C replacement. BetterC feels like an afterthought. A nice bonus. Not a primary focus. E.g. the language is designed to use exceptions for error handling, so of course there's no feature for BetterC dedicated to error handling.
Being a better C is the one and only focus of Zig. So it has features for doing error handling without exceptions.
D is not going to replace C, perhaps for the same reasons subsets of C++ didn't.
I don't know if Zig and Rust will. But there's a better chance since they actually bring a lot of stuff to the table that arguably make them better at being a C-like language than C. I am really hyped to see how embedded development will be in Zig after the new IO interface lands.