CODEWITHSUNDEEP
rustmacros
Ramesh Kithsiri HettiArachchi
Ramesh Kithsiri HettiArachchi

Reputation: 1540

Macro expansion ignores token `let` and any following

I want to define a variable with a macro ident and pass it to a derive macro, but I get an error:

Error

error: macro expansion ignores token `let` and any following
  --> database/src/models.rs:15:9
   |
15 |           let struct_name = stringify!($name);
   |           ^^^
...
50 | / model!(Person {
51 | |     name: String
52 | | });
   | |___- caused by the macro expansion here
   |
   = note: the usage of `model!` is likely invalid in item context

Code

#[macro_export]
macro_rules! model {
    ( 
        $(#[$met: meta])*
        $name: ident { $($attr: ident : $typ: ty),* } 
    ) => {

        let struct_name = stringify!($name);
        //let table_name = struct_name.to_camel_case();
        
        dbg!("{}", struct_name);
        
        #[derive(Debug)]
        struct $name {
            name: String
        };
    };
}

model!(Person {
        name: String
});

Playground

Upvotes: 7

Views: 6459

Answers (1)

madlaina
madlaina

Reputation: 373

You use the macro model! where the compiler expects an item (function/struct/trait declaration or impl block). A let statement is only valid inside a code block. This is why your first example works (where the macro call is inside main), while your second example doesn't.

This seems like an instance of the XY problem. Check out procedural macros if you want to run code to generate code at compile time.

Upvotes: 7

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