Does this work with custom derives?

[thomaseizinger]

I am trying to use this with a custom-derive and it doesn't seem to work. Here is a MRE: https://github.com/thomaseizinger/proc-macro-warning-MRE

Any ideas?

[ggwpez]

Thanks for investigating and the MRE!
I actually observed the same thing, but did not look closer since it worked in Substrate.

Looks like it is important to pass in a span - otherwise it wont trigger (was not actively aware of that).
Your MRE can be fixed by putting the span of the parsed input:

#[proc_macro_derive(MyDerive)]
pub fn my_derive(input: TokenStream) -> TokenStream {
    let input = syn::parse_macro_input!(input as syn::DeriveInput);

    let warning = proc_macro_warning::Warning::new_deprecated("test")
        .old("foo").new("bar")
        .span(input.span()) // <- here
        .build();

    quote::quote!{ #warning }.into()
}

produces:

Please let me know if this fixes your issue. I will add an example specifically for derive macros and maybe make the .span call mandatory.

[thomaseizinger]

@tcoratger Mind trying this on libp2p/rust-libp2p#3848?

[thomaseizinger]

Thanks for investigating and the MRE!
I actually observed the same thing, but did not look closer since it worked in Substrate.

Looks like it is important to pass in a span - otherwise it wont trigger (was not actively aware of that).
Your MRE can be fixed by putting the span of the parsed input:

#[proc_macro_derive(MyDerive)]
pub fn my_derive(input: TokenStream) -> TokenStream {
    let input = syn::parse_macro_input!(input as syn::DeriveInput);

    let warning = proc_macro_warning::Warning::new_deprecated("test")
        .old("foo").new("bar")
        .span(input.span()) // <- here
        .build();

    quote::quote!{ #warning }.into()
}

produces:

Please let me know if this fixes your issue. I will add an example specifically for derive macros and maybe make the .span call mandatory.

Awesome, thank you! I hope this works :)

[thomaseizinger]

Thanks! This now works.