senseless line / column ?

[Dushistov]

I build syn with procmacro2_semver_exempt.
And for such wrong code (a: i32, <) -> i32; it reports span: 1:0 - 1:11, in other words
(a: i32, <), but if print ParseStream I can see that it stops on <,
so there is information about exact wrong token,
thus is it possible somehow report where parser stops in case of parenthesized usage?

use quote::quote;
use syn::{
    parenthesized,
    parse::{Parse, ParseStream},
    punctuated::Punctuated,
    Result, Token,
};

struct Item;
impl Parse for Item {
    fn parse(input: ParseStream) -> Result<Self> {
        let args_parser;
        parenthesized!(args_parser in input);
        let args_in: Result<Punctuated<syn::FnArg, Token![,]>> =
            args_parser.parse_terminated(syn::FnArg::parse);
        println!("state, input {}\nargs_parser {}", input, args_parser);
        let args_in = args_in?;
        unimplemented!();
    }
}

fn main() {
    if let Err(err) = syn::parse_str::<Item>("(a: i32, <) -> i32;") {
        println!(
            "err: {}, at {}:{} - {}:{}",
            err,
            err.span().start().line,
            err.span().start().column,
            err.span().end().line,
            err.span().end().column
        );
    }
}

[dtolnay]

The error is being triggered on the close parenthesis. The matched set of parentheses carries only a single span covering the set of parentheses so that is where the error is reported.

[dtolnay]

I fixed this for proc macros in fe89fcc, so that errors can appear on just the opening or just the closing delimiter. When the Group::span_open and Group::span_close methods are stabilized (tracked in rust-lang/rust#54725) we will be able to carry this over to build.rs / main.rs based parsers as well.