Meta does not parse paths with leading double-colon

[dbeckwith]

Minimal reproduction:

use syn;

fn main() {
    // correctly parses
    let attr: syn::Attribute = syn::parse_quote!(#[derive(serde::Serialize)]);
    let _ = dbg!(attr.parse_meta());
    
    // error, expected identifier or literal
    let attr: syn::Attribute = syn::parse_quote!(#[derive(::serde::Serialize)]);
    let _ = dbg!(attr.parse_meta());
}

I would expect the second example to parse since it's a valid path. I think the meta parsing code does a lookahead expecting only a literal or an identifier, but paths can start with the non-identifier ::.

I read the previous discussion about not wanting Meta to have to cover all possible attribute syntaxes, but I think this fits solidly within the current expectations of what Meta should parse. Given that it stores Paths, it should be able to parse any kind of Path.

[dtolnay]

Fixed in 1.0.46.