Edition 2015 DeriveInput with edition 2018 keyword as ident fails to parse in syn 2.x

[matthias-stemmler]

Thanks for your work on the 2.x release. I came across this issue when trying to update.

Starting from syn 2.x, the Parse impl of Ident rejects edition 2018 keywords such as async, so one has to use Ident::parse_any instead.
This is unfortunate when the ident is part of a more complex structure that's actually valid Rust code in edition 2015. Say you're implementing a derive macro and want to support edition 2015 code. Then parsing the macro input as a DeriveInput fails e.g. when the name of a struct field is a 2018 keyword.

Minimal example:

#[test]
fn test() {
    let _: syn::DeriveInput = syn::parse_quote! {
        struct Foo {
            async: (),
        }
    };
}

This panics with the message expected identifier. In syn 1.x it succeeds.

Other than implementing a custom parser for DeriveInput that uses Ident::parse_any, is there a recommended approach in this case?

[dtolnay]

Parsing arbitrary 2015-edition identifiers interchangeably with identifiers in newer edition code is not a thing I would want to do, but if there are specific unambiguous identifiers that are valuable to support in specific locations in the grammar (like async as a field of a struct) then I am open to PRs which make that parse.

The way this issue was filed, I am not sure if async in a field name of struct is specifically the case that you care about.