Pass a static string to a macro_rule in rust - rust

Question

I wrote the following code :
macro_rules! my_macro{
("A") => {
println!("Macro called !")
}
}
fn main(){
static test: &'static str = "A";
my_macro!(test);
}
but I have the following error :
error: no rules expected the token `test`
--> test.rt:9:19
|
1 | macro_rules! my_macro{
| --------------------- when calling this macro
...
9 | my_macro!(test);
| ^^^^ no rules expected this token in macro call
error: aborting due to previous error
However, it works fine if I directly call my_macro("A"). Is it possible to fix this ?

Answer 1

Is it possible to fix this ?
No. Macros are expanded at compile time before item names are resolved, therefore your macro has no idea what the value of test is (and would have no idea even if it were a const rather than a static).

Answer 2

so the first problem here is that you macro expects a pattern of "A" not a variable that contains "A"
when you create macros you define certain patterns and follow those patterns in your case your macro must always have "A" in it but it is not a string a it is a pattern of double quote followed by capital a followed by another double quote
If you want to pass a value you should use variable syntax and define what it should expect such as ($a:expr)=>{...}
here you can see all magic tokens possible just scroll down a bit on that docs there are a lot of great examples
PS. here is a macro I use for responding from my endpoints
macro_rules! resp {
(ok) => {
|_| actix_web::HttpResponse::Ok().body(r#"{"success":true}"#)
};
(ok,$data:expr) => {
|_| actix_web::HttpResponse::Ok().json(serde_json::json!({"success":true,"data":$data}))
};
(ok,) => {
|d| actix_web::HttpResponse::Ok().json(serde_json::json!({"success":true,"data":d}))
};
}