Span::unstable to expose the proc_macro::Span

[dtolnay]

This is a way to expose the diagnostics API without a large surface area in proc_macro2. Sergio needed this for a demo.

let a = parser.parse::<ExprTuple>()?;
parser.parse::<token::Eq>()?;
let b = parser.parse::<ExprTuple>()?;
parser.eof()?;

let (a_len, b_len) = (a.elems.len(), b.elems.len());
if a_len != b_len {
    return Err(b.span().unstable()
        .error(format!("expected {} element(s), got {}", a_len, b_len))
        .span_note(a.span().unstable(), "because of this"));
}

[alexcrichton]

👍

[antoyo]

Thanks for adding this feature.
I wonder where the method span() from the example in the first post comes from, though.
Is it a method that is not yet implemented?

[dtolnay]

It's from Sergio's fork, SergioBenitez/syn@b49a4b4. We are working to get it upstreamed.

[JeanMertz]

I can across this issue from one of the dyn examples as I was researching how to show accurate errors to the user of my macro.

Am I correct in assuming that this is (one of) the feature(s) that is behind the unstable feature flag as described in the readme?

Is there any roadmap to when this might become stable, or at least behind a regular cargo feature flag instead of requiring every user of a crate that uses this to also set the unstable flag?