Ommiting "C" in `extern "C" fn` in derive input causes spans to get lost.

[rodrimati1992]

For some reason ommiting the "C" in extern "C" fn causes the spans to get lost in error messages.

I've created this repository to demonstrate it,simply git clone it and try to build the using_proc_macro crate.

The proc_macro_crate crate:

extern crate proc_macro;

use proc_macro::TokenStream;

use syn::{
    visit::{self, Visit},
    Lifetime, Type, TypeBareFn, TypeReference,
};


#[proc_macro_derive(ProcMacro)]
pub fn stable_abi(input: TokenStream) -> TokenStream {
    syn::parse::<syn::DeriveInput>(input)
        .and_then(|di|{
            let mut res=Ok(());
            Visitor { err:&mut res }.visit_derive_input(&di);
            res.map(|_| quote::quote!() )
        })
        .unwrap_or_else(|e| e.to_compile_error() )
        .into()
}

pub(crate) struct Visitor<'a> {
    err:&'a mut Result<(),syn::Error>,
}

/////////////

impl<'a> Visit<'_> for Visitor<'a> {
    fn visit_type_reference(&mut self, ref_: &TypeReference) {
        *self.err=Err(syn::Error::new_spanned(ref_,format!("hello from proc macro")));
    }
    fn visit_lifetime(&mut self, lt: &Lifetime) {   
        *self.err=Err(syn::Error::new_spanned(lt,format!("hello from proc macro")));
    }
}

The using_proc_macro crate:

#[derive(proc_macro_crate::ProcMacro)]
struct Hello(
    &'a (),
    extern fn(),
);


#[derive(proc_macro_crate::ProcMacro)]
struct World(
    &'a (),
    extern "C" fn(),
);

The error message

error: hello from proc macro

error: hello from proc macro
  --> src/lib.rs:10:5
   |
10 |     &'a (),
   |     ^^^^^^

The first error is for Hello and the second is for World.

[dtolnay]

Thanks! This is one example of a general problem of the compiler losing span information that is being tracked in #43081.

[petrochenkov]

This one is very easily fixable though, by changing Abi to Option<Abi> in AST.
(I'm going to reopen and mark as easy/mentor.)

[petrochenkov]

ast.rs before:

pub struct BareFnTy {
    pub abi: Abi,
    // ...
}
pub struct ForeignMod {
    pub abi: Abi,
    // ...
}
pub struct FnHeader {
    pub abi: Abi,
    ...
}

ast.rs after:

pub enum Extern {
    None,
    Implicit,
    Explicit(Abi),
}

pub struct BareFnTy {
    pub ext: Extern,
    // ...
}
pub struct ForeignMod {
    pub abi: Option<Abi>,
    // ...
}
pub struct FnHeader {
    pub ext: Extern,
    ...
}

[jaredforth]

If no one else is taking this issue, I am up to work on it

[jaredforth]

@petrochenkov

I added the pub enum Extern to ast.rs without any issues, but when I added pub ext: Extern to pub struct BareFnTy the compiler complains because the struct signature has changed.

The errors are below:

error[E0308]: mismatched types
   --> src/libsyntax/feature_gate/check.rs:437:32
    |
437 |                 self.check_abi(bare_fn_ty.abi, ty.span);
    |                                ^^^^^^^^^^^^^^ expected enum `rustc_target::spec::abi::Abi`, found enum `std::option::Option`
    |
    = note: expected type `rustc_target::spec::abi::Abi`
               found type `std::option::Option<rustc_target::spec::abi::Abi>`

error[E0308]: mismatched types
   --> src/libsyntax/parse/parser/ty.rs:303:13
    |
303 |             abi,
    |             ^^^
    |             |
    |             expected enum `std::option::Option`, found enum `rustc_target::spec::abi::Abi`
    |             help: try using a variant of the expected type: `Some(abi)`
    |
    = note: expected type `std::option::Option<rustc_target::spec::abi::Abi>`
               found type `rustc_target::spec::abi::Abi`

error[E0308]: mismatched types
    --> src/libsyntax/print/pprust.rs:1006:34
     |
1006 |                 self.print_ty_fn(f.abi,
     |                                  ^^^^^ expected enum `rustc_target::spec::abi::Abi`, found enum `std::option::Option`
     |
     = note: expected type `rustc_target::spec::abi::Abi`
                found type `std::option::Option<rustc_target::spec::abi::Abi>`

Is the solution to change code in check.rs, ty.rs, and pprust.rs or is there an alternate solution that I am just missing?

[petrochenkov]

@jaredforth
Yes, ABI uses outside of ast.rs need to be adjusted to use the new Option<Abi> and Extern.