Interpolate string in const function

Question

Im trying to make a function similar to this

pub const fn insert(num1:i32, num2:i32) -> &'static str { 
    formatcp!("n1:{}, n2:{}" , num1, num2) 
}

But num1/num2 are not const. I think this would be possible as a macro but im not experienced in macros.

Does this answer your question? [How to format a const string](https://stackoverflow.com/questions/32279858/how-to-format-a-const-string) — Chayim Friedman, May 16 '22 at 06:22
Chayim Friedman Not really, since the answer is from a 2015 where const fn where in nightly and not finished. By now you can create constants as a result of a const fn. My question focuses on string interpolation with arguments in a compile time context. — RedCrafter LP, May 16 '22 at 06:32
The crates it refers to, `const_format`, still works (I don't know about inside `const fn`, but it should work). — Chayim Friedman, May 16 '22 at 06:34
The macro is complaining that the parameters(num1, num2) are not const. I also tried Generic constants () not working. — RedCrafter LP, May 16 '22 at 06:44
Is that _must_ be the signature of the function, or it can be a little different (for example, return an array that will be converted to `&str` at call site)? — Chayim Friedman, May 16 '22 at 06:59
Where will the result get used? If you can adjust the signature, maybe you can just use `const_format`? It is not much different, and uses the same ideas. — Chayim Friedman, May 16 '22 at 07:06
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/244765/discussion-between-redcrafter-lp-and-chayim-friedman). — RedCrafter LP, May 16 '22 at 07:07

score 0 · Accepted Answer · answered May 19 '22 at 07:35

0

Does it help?

macro_rules! insert {
    ($n1:expr, $n2:expr) => {
        concat!("n1: ", $n1, " , n2: ", $n2)
    };
}

const TEST_1: &str = insert!(1, 2);
const TEST_2: &str = insert!(2, 3);

fn main() {
    println!("{}", TEST_1);
    println!("{}", TEST_2);
}

answered May 19 '22 at 07:35

Konstantin Grech

403
4
8

That's a great solution. Didn't know that you could use concat! in a const context. The only thing missing is, that the macro parameters are not i32. But that's easily fixable by adding 2 constants of i32 in the macro and assign the parameters to it. – RedCrafter LP May 21 '22 at 09:28

Interpolate string in const function

1 Answers1