Comment by jibal

The compilers for Zig, D, Nim, and perhaps other languages contain an interpreter for the whole language that can run at compile time. It's not circular--compiling code and executing code are two quite different things (the interpreter does not contain yet another parser--it works on the AST). You simply can do calculations the results of which can be used in compile-time only contexts such as initializers. The trick in Zig is that the compile-time interpreter supports types as first-class objects, so you can compute types and implement generics this way. In Nim, the interpreter supports computations over the AST. In D, types can be passed as generic arguments, and computed strings can be "mixed in" to the source code and compiled at that point (on-the-fly code generation).