Why can I not access a variable declared in a macro unless I pass in the name of the variable?

Question

I have this macro:

macro_rules! set_vars {
    ( $($x:ident),* ) => {
        let outer = 42;
        $( let $x = outer; )*
    }
}                                    


        

Answer 1

Yes, this is macro hygiene. Identifiers declared within the macro are not available outside of the macro (and vice versa). Rust macros are not C macros (that is, Rust macros are more than glorified text replacement).

See also:

• The Little Book of Rust Macros
• A Practical Intro to Macros
• So, what are hygienic macros anyway?