When comparing Zig vs D, the Slant community recommends D for most people. In the question“What are the best (productivity-enhancing, well-designed, and concise, rather than just popular or time-tested) programming languages?” D is ranked 12th while Zig is ranked 18th. The most important reason people chose D is:
With few exceptions, D will either compile C code to work exactly as when compiled as C, or it won't compile - it won't quietly change the semantics of C code and give unexpected results. This means that D contains an improved C, as it fails compilation where type safety is missing in C. This allows learning the same machine operations available in C and other low-level languages.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Some safety
It's safer than C, at least.
Pro C Interop
Zig programs can import C libraries and export header files to be used in C programs.
Pro Performance
Zig is pretty fast, in some cases even faster than C.
Pro Control flow is simple and obvious
No operator overloading, property methods, runtime dispatch, macros, etc.
Pro Cross-compilation is easy
The Zig compiler can build artifacts for any Tier 3 Supported platform without additional downloads.
Pro Small binary
Produces binary smaller than C.
Pro Compile-time execution
Zig can execute code at compile-time, allowing for more performant and readable programs.
Pro No Hidden Control flow
Zig will not do anything on your back that might slow down your program & make you loose your control over your program.
Pro Has an improved C subset
With few exceptions, D will either compile C code to work exactly as when compiled as C, or it won't compile - it won't quietly change the semantics of C code and give unexpected results. This means that D contains an improved C, as it fails compilation where type safety is missing in C.
This allows learning the same machine operations available in C and other low-level languages.
Pro Easy to read and understand code

Pro Doesn't force you to deal with memory management
When you're just starting out, dealing with manual memory management and its bugs is a huge pain! D is garbage collected by default, which removes a huge class of things that could go wrong. And later on, if you decide you don't want or need the GC, you can turn it off.
Pro Very fast compilation
D is usually up to 10 times faster than C++. Having a language that compiles this fast means that you are free to write highly optimized code because of the relatively low cost of experimentation.
Pro Unit testing built-in
D provides unittest blocks to insert code that verifies functions preform their expected behavior and document edge cases. Since these are enabled with a compiler switch, there is no need to teach new programmers how to install new libraries and import modules, instead training on test driven design can start from the very first function.
Pro Provides a powerful data structure on top of C's arrays called slices
D provides a structure that builds on C's arrays called slices. A slice is a segment of an array that tracks the pointer and the length of the segment.
Slices are extremely powerful because they combine the protection of knowing the length of the data with the garbage collector that manages the memory backing that data, thus avoiding most memory corruption issues.
Pro It's a state-of-art evolution of C
Pro Static with type inference
For a new user adding types can feel tedious, and takes focus off the meaning of the code, but they are also important for checking logic. D provides static types, and a good system to infer types, so types are checked when calling functions, but do not need to be specified everywhere, making it feel more dynamic.
Pro Provable purity and immutability
The compiler can check that functions don't have side effects, extremely important for functional programming in concurrent scenarios, and can check immutability.
Therefore, the compiler will prove that your programs are functionally pure and respect immutable data, if you want it to.
Pro Compile-time Function Execution
Pro Built-in Unicode support
Pro Industrial quality

Pro Asynchronous I/O that doesn’t get in your way
Because all types can be treated as objects, all files can call functions in the same manner -- even stdin
and stdout
. stdout.writeln();
stdin.readln();
file.writeln();
file.readln();
Pro Easy to integrate with C and C++
D practically has the same memory structure as C and C++; all D does it build a structure around that. The entire C standard library is accessible at no cost (syntactic or speed) and it's being worked on allowing the same for the C++ standard library.

Pro Designed for concurrency and parallelism
Supports first-class functionality for both concurrency and parallelism, offered as part of the standard library.

Pro Supports calling functions from types in an object-oriented manner.
if (exists(file)) {}
may be written as if (file.exists) {}.
writeln(file);
may be written as file.writeln();
isDivisibleBy(10, 2);
may be written as 10.isDivisibleBy(2);
writeln(isEven(add(5, 5)));
may be written as 5.add(5).isEven().writeln();
Cons
Con Convoluted syntax
Claims to be an improvement over C, but in this area, not really.
Con No lambdas
Missing many key and useful features other languages have.
Con Deceptively gives impression it's near being production ready
No plans to hit 1.0 until 2025 (3 years later), according to Zig Roadmap speech. Impression is given that (0.9.1) language was close to ready, when it's not.
Con Creator admits to not knowing what he's doing
Creator admits his shortcomings during Zig Roadmap speech. Very disturbing. Not a language to invest in or take seriously, outside personal experimentation.
Con No closures
Does not have closures.
Con Almost no community
Lacking in libraries and users.
Con No interfaces/traits
Features that are useful are missing, where you can find them in other languages.
Con Fundraising looks suspicious
Why do we keep seeing an overhyped unfinished alpha level language showing up everywhere? That answer looks like a financial incentive to promote the language, the Zig Software Foundation, that is making hundreds of thousands of dollars a year from donations. To aggressively maintain cash flow, it appears any opinions that goes against their narrative is suppressed, attacked, or eliminated. All while the hype machine runs rampant. That isn't a language for the people, that looks to be a cash grab for the few that will result in nothing useful. Save yourself the headache and games, better to just use C or other languages with C-like syntax, can interface with C, and are actually useful. Better to not waste your valuable time on crap like Zig, false media hype, or getting scammed.
”If it sounds too good to be true, it probably is.”
This blog has published 2 excellent articles on how Zig's claims do not live up to reality after more than 6 years of development.
Con Poor compiler errors
Several years into development, language still a mess, and no hope of fixing itself for yet more years later.
Con No standard package manager
Several years into development, and still no standard package manager is ridiculous.
Con Exhibits cult-like behavior and animosity towards other programming languages
Strange culture that bashes other languages and are not open to criticism about the faults of their language. Very close-minded, sometimes scary.

Con Poor adoption even after many years of existence
There's a widely accepted perception of D as a language that has been poorly adopted. Since adoption is driven by perception this becomes a fact. So managers and engineers start becoming nervous in adopting a language that has such a perception among the community and that has been so unsuccessful for so long.
Con Failed at becoming alternative to C or C++
Almost as confused and complicated as C++, but without the popularity and widespread corporate usage. Also failed at becoming a good cross-platform GUI application development language like Object Pascal. Many missed past opportunities, and now newer languages are better alternatives.
Con Lack of vision
D is community-driven and lacks the support of any large corporation. While this increases the amount of talent and engineering abilities of the people working on D, it also brings a severe lack of charisma, leadership and vision.
Con Garbage Collection
Memory is not managed directly.
Con All the downsides of garbage collection without any of its benefits
When D decided to implement garbage collection it instantly alienated a large community of developers (C and C++ programmers). For them, the official policy has been: "Don't want garbage collection? Use D with RAII or manual management style!".
While true, it's also absolutely pointless because there's little to none support for alternate memory management styles in the standard library, which means that a new user will have to start with a language that is stripped down of the core infrastructure.
On the other hand, for those people who want to use garbage collection, the implementation of it is lackluster.
