zig comptime is not a macro system and you can't really generate code in a way that makes hygeine a thing to worry about (there is no ast manipulation, you can't "create variables"). the only sort of codegen you can do is via explicit conditionals (switch, if) or loops conditioned on compile time accessible values.
thats still powerful, you could probably build a compile time ABNF parser, for example.
Zig is also not the only language that has interesting compile-time features. V (Vlang)[1] and D (Dlang) (among others), has them too. It's kind of weird that this is promoted so much with Zig, as if other languages don't have a lot of this.
Surely there's a way to generate code by manipulating an AST structure? Is there some reason this can't be done in Zig or is it just that no one has bothered?
Doing it this way is more verbose but sidesteps all hygiene issues.
Zig lets you inspect type info (including, eg, function signatures), but it doesn't give you raw access to the AST. There's no way to access the ast of the body of the function. As highlighted by view 0 in my article, I consider this a good thing. Zig code can be read without consideration for which pieces are comptime or not, something that heavy AST manipulation loses.
Though, if you really wanted to do stupid things, you could use @embedFile to load a Zig source file, then use the Zig compiler's tokenizer/ast parser (which are in the standard library) to parse that file into an AST. Don't do that, but you could.
thats still powerful, you could probably build a compile time ABNF parser, for example.