int i = f(3); // evaluate at run time
enum i = f(3); // evaluate at compile time
enum res = toggle_gpio(FRONT_LED_2);
Then, suppose we have all these requirements
- We want a separate compilation model where the code which is calling f(3) at either compile or run time knows nothing about how f is defined, only how it's declared.
- We want not to include the compiled image of f in the target program, if if is only called at compile time, only if at least one call to it somewhere in the program is staged to run-time.
- We want to make sure that if f is called at compile time, nobody can change its definition to accidentally call for run-time semantics like toggle_gpio(FRONT_LED_2).
and that's how we probably arrive at a declarative mechanism like constexpr that goes on the function, rather than trying to orchestrate thing remotely/indirectly by placing a call to the function into a compile-time context.
Not including functions never called for runtime execution in the runtime is a normal compiler optimization. No user input is necessary.
If the function's semantics change so it is no longer runnable at compile time, the compiler will let you know when you try it. If you want to force the detection, just call it at compile time with dummy declaration.
> that's how we probably arrive at a declarative mechanism like constexpr that goes on the function, rather than trying to orchestrate thing remotely/indirectly by placing a call to the function into a compile-time context.
This issue is raised now and then, and in 16 years of CTFE in D it has never ever been reported as causing an actual problem.
Great we have many good languages to choose from