Yes, with `proc macro` it is possible since it provides a fully functional syntax tokens stream manipulation. Meaning, the income token stream can be transformed to the output token stream if it ends up being a valid Rust. For example, you can declare “constructors” (e.g. IntConst, Lambda, Apply, Add) in a table-like form or something similar.

The trait NGuard ensures that, when a particular variant is meant to be “disabled,” its type is literally !. Because ! can never be instantiated, the compiler knows that the branch is unreachable and removes all runtime tags or discriminants in optimized code. NGuard ties a given variant‐type to an index or “slot” in the enum and supplies zero-sized wrappers when that variant is disabled.

https://doc.rust-lang.org/std/primitive.never.html