> One (maybe the) distinguishing feature between comptime in Zig and Rust macros seems to me to be access to type information. In Zig you have it[1] in Rust you don't and that makes a big difference.
There are other differences. First, comptime functions aren't syntactic macros. This makes them much easier to reason about and debug. You could think about them as if they were regular functions running at runtime in a partially-typed language with powerful reflection (their simplicity also means they're weaker than macros, but the point is that you can get very far with that, without taking on the difficulties associated with macros). Second, I think that comptime's uniqueness comes not from what it does in isolation, but that it makes other language features redundant, keeping the entire language simple. This means that with one simple yet just-powerful-enough feature you can do away with several other features.
The end result is that Zig is a very simple language with the expressivity of far more complicated languages. That on its own is not super unusual; in a way, JavaScript is like that, too. But Zig does it in a low-level language, and that's revolutionary. It is because of its simplicity that people compare Zig to C, but it's as expressive as C++ while also being safer than C++, let alone C.
Adding comptime to an already-complex language misses out on its greatest benefit.
There are other differences. First, comptime functions aren't syntactic macros. This makes them much easier to reason about and debug. You could think about them as if they were regular functions running at runtime in a partially-typed language with powerful reflection (their simplicity also means they're weaker than macros, but the point is that you can get very far with that, without taking on the difficulties associated with macros). Second, I think that comptime's uniqueness comes not from what it does in isolation, but that it makes other language features redundant, keeping the entire language simple. This means that with one simple yet just-powerful-enough feature you can do away with several other features.
The end result is that Zig is a very simple language with the expressivity of far more complicated languages. That on its own is not super unusual; in a way, JavaScript is like that, too. But Zig does it in a low-level language, and that's revolutionary. It is because of its simplicity that people compare Zig to C, but it's as expressive as C++ while also being safer than C++, let alone C.
Adding comptime to an already-complex language misses out on its greatest benefit.