Add ability to strip doc / attributes info from the Span

[Mingun]

I'm working on and PR for fixing serde-rs/serde#2105 and the related stuff and making compiletests for them. For the piece

#[derive(Serialize)]
#[serde(tag = "type")]
enum Serializable {
    /// Error should be reported
    Tuple2(u8, u8),
}

the error span would cover several lines:

error: #[serde(tag = "...")] cannot be used with tuple variants
  --> tests/ui/enum-representation/internal-tuple-variant.rs:10:5
   |
10 | /     /// Error should be reported
11 | |     Tuple2(u8, u8),
   | |__________________^

Because doc comments can be very long, it is impractical to bring them into error messages. I would like to get the following message:

error: #[serde(tag = "...")] cannot be used with tuple variants
  --> tests/ui/enum-representation/internal-tuple-variant.rs:10:5
   |
10 |       /// Error should be reported
11 |       Tuple2(u8, u8),
   |       ^^^^^^^^^^^^^^

That error is generated by the following code in serde (here in action):

// variant.original is a `&syn::Variant`
cx.error_spanned_by(
    variant.original,
    "#[serde(tag = \"...\")] cannot be used with tuple variants",
);

Is there any way to strip span that related to doc comments (and probably to all other attributes) and keep only code span of syn::Variant, syn::Field, etc?

[dtolnay]

The range that the error points to is determined by providing a ToTokens implementation that contains the tokens which are to be highlighted in the error. So for this case, something like:

let error = syn::Error::new_spanned(
    WithoutAttrs(variant.original),
    "...",
);

where the ToTokens impl is:

struct WithoutAttrs<T>(T);

impl ToTokens for WithoutAttrs<&syn::Variant> {
    fn to_tokens(&self, tokens: &mut proc_macro2::TokenStream) {
        self.0.ident.to_tokens(tokens);
        self.0.fields.to_tokens(tokens);
    }
}