Why is an invalid literal not handled by another arm in a macro?

help
1

I want to write a macro to instantiate a big integer in a clean manner, and the first step is to read the integer literal correctly. The first version I have is

macro_rules! bigint {
    ($tt:tt) => {
        println!("caught {}", ::core::stringify!($tt)); // for debug
        // some logic to create a big integer from the string
    };
}

and this macro can nicely handle very long literals like bigint!(100000000000000000000000000000000000000000000000000000000000000000000000).

One drawback of this approach is that it has to invocate the string parser even if the integer is small enough to fit in a native integer type (i.e. u128), so then what I tried next is

macro_rules! bigint {
    ($val:literal) => {
        println!("caught literal {}", $val);
        let val: u128 = $val;
        // some logic to create a big integer from val
    };
    ($tt:tt) => {
        println!("caught {}", ::core::stringify!($tt));
        // some logic to create a big integer from the string
    };
}

but this time bigint!(100000000000000000000000000000000000000000000000000000000000000000000000) will cause an compilation error: error: integer literal is too large. I'm wondering if the first matching arm failed, why wouldn't the second arm be tried?

2

Because the literal did match the first clause. By the time the "integer literal is too big" error is emitted, macro expansion is likely over – that error is not of a syntactic nature, it requires actually evaluating the literal.

3

Yes the first arm did match, but I thought that the compiler will try other arms if compilation error happened in one of the arm..

In this specific case, is there any workaround for this?

4

The other arms are retried if they don't match. This is exactly what I'm talking about – it's not possible to detect a const evaluation error at macro expansion time, because macro expansion is over at that point.

Write a const function instead.

1 Like
5

Thanks for the help, I guess I would follow your suggestion.

6

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