Parsing keywords in Meta-path positions fails with syn v2

[jonasbb]

I have an attribute like #[serde_as(as = "FooBar")]. While trying to use syn v2 in serde_with, I see many errors like this (log):

error: expected path
  --> serde_with_test/tests/serde_as_crate_path.rs:14:16
   |
14 |     #[serde_as(as = "_")]
   |                ^^

The issue seems to be that the Path types in MetaNameValue does not allow keywords.
This does prevent me from upgrading syn in serde_with, which I tried TedDriggs/darling#226 and jonasbb/serde_with#578.

This parses with syn v1, but panics with expected path for v2.

fn main() {
    let attr: syn::MetaNameValue = syn::parse_quote!{
        as = "FooBar"
    };
    dbg!(attr);
}

[dtolnay]

I think this is behaving correctly, as Rust macro_rules also do not consider as = "FooBar" to be a valid "meta" (which MetaNameValue corresponds to a variant of).

macro_rules! meta {
    ($m:meta) => {};
}

meta!(as = "FooBar");

error: expected identifier, found keyword `as`
 --> src/lib.rs:5:7
  |
5 | meta!(as = "FooBar");
  |       ^^ expected identifier, found keyword
  |
help: escape `as` to use it as an identifier
  |
5 | meta!(r#as = "FooBar");
  |       ++

[JelteF]

I don't think the example you gives proves that this is behaving correctly, because it's not actually parsing the same thing. The following is accepted by macro_rules:

macro_rules! meta {
    ($m:meta) => {};
}

meta!(serde_as(as = "FooBar"));

[dtolnay]

Yes, that is accepted by syn::Meta as well.

[JelteF]

Okay, so the problem I guess is that we're trying to parse the content of the element as syn::Meta again. And because it can contain keyword arguments that fails.