How to set a compile time condition in macros

Question

I want to set the compile-time condition in macros when it generates code outside the functions. I need something like that:

macro_rules! cond {
    ( $cond_el:expr ) => {
         #[if $cond_el == "i32"]
             struct A {
                 x: i32
             }
         #[else]
             struct A {
                 x: f64
             }
    }
}

cond!("i32");
cond!("f64");

fn main() {}

Or maybe something like that:

macro_rules! cond {
    ( $cond_el:expr ) => {
         match $cond_el {
            "i32" => {
                struct A {
                   x: i32
                }
            }
            _ => {
                struct A {
                    x: f64
                }
            }
        }
    }
}

cond!("i32");
cond!("f64");

fn main() {}

Is it possible with current Rust macros state?

Answer 1

What you're asking for is impossible. Thankfully, what you're asking for and what your examples suggest you want are two different things:

macro_rules! cond {
    ("i32") => {
        struct A {
            x: i32,
        }
    };
    ($el:expr) => {
        struct B {
            x: f64
        }
    };
}

cond!("i32");
cond!("f64");

fn main() {}

Taking your question literally, however: no, there is no way to do any sort of complex testing of conditions in macros, nor can macros set or test any kind of state outside of what's passed in to them. You can do literal matches of the direct input to the macro, or you can parse some input as one of a limited number of syntax constructs (which you then cannot match against), and that's about it.