Can macros match against constant arguments instead of literals?

Question

Given the macro matching example, this shows how macros can match an argument.

I\'ve made very minor changes here to use numbers:

macro_rules! foo {
         


        

Answer 1

No.

Macros operate on the Abstract Syntax Tree, so they reason at the syntactic level: they reason about tokens and their spelling.

For example:

fn main() {
    let v = 3;
}

In this case, the AST will look something like:

fn main
    \_ let-binding v
        \_ literal 3

If you ask a macro whether v is 3, it will look at you funny, and wonder why you would try comparing a variable name and a literal.

Answer 2

I'm fairly sure the answer is "no"; at macro expansion time all you have are token trees - expansion happens before evaluation, or even type inference/checking.