Proc_macro attribute in cargo build

help
1

Hi guys,

I was working on a custom proc_macro attribute, and I have code like this:

#[attribute]
mod mod_name {
   pub fn fn_name(args);
}

The syntax here is that the function only has a signature but no body, which should be an error if you compile. The body will later be populated in my attribute, in which I parse the token stream and rewrite the function with a body.

However, when I compile with cargo build, the free body error is still reported.

Is it that the #[attribute] is not resolved and called in cargo build? Therefore I have to give a body in the function? Or is there a workaround so that I can keep the syntax but compile it successfully?

Thank you so much!

2

The input to attribute and derive macros needs to be syntactically valid Rust code, so you can't omit the function body like that. Function-like macros (mac!(...)) do not have this restriction.

3

Actually, fn fun(…) -> …; and thus

mod mod_name {
    pub
    fn fn_name (_: args);
}

is syntactically valid code.

  • (the only issue is that fn fn_name (args)… is not syntactically valid code, even with a function's body, since a function's input needs to be made of self and/or <pat>: <ty> elements)

Btw, for those not knowing about this trick: to know what kind of input is valid to a proc-macro attribute, it suffices to use #[cfg(any())] as a dummy proc-macro placeholder (it "acts" as the empty-expansion / eraser proc-macro attribute). Since the empty expansion is always valid, syntactically, it's an easy way to identify whether a compile error issue lies within the input to the proc-macro being invalid (in which case it would fail even with #[cfg(any())]), or with the output of the proc-macro being invalid (e.g., because it did not amend the code well enough).

  • And for function-like macros (proc_macro! { … }), #[cfg(any())] stringify! { … } does the job. But function-like macros will actually accept any input which can be lexed and which has balanced parenthesis / braces / brackets inside it:

    • some_macro! { ) } :x:

    • some_macro! { ` } :x:

In this instance, thus, the issue necessarily lies within the expansion of your #[attribute]. Try to run cargo expand or println!("{}", ret); (where ret would be the TokenStream about to be returned / emitted by the #[proc_macro_attribute]) to try and identify the culprit:

Since I suspect this to be the problematic step, could you share the code that does this, or the output of such expanded code?

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