Support parsing #![feature(default_field_values)]

[estebank]

• RFC: [RFC] Default field values rust-lang/rfcs#3681
• Tracking issue: Tracking issue for RFC 3681: Default field values rust-lang/rust#132162
• Feature gate: #![feature(default_field_values)]

#[derive(Default)]
struct Pet {
    name: Option<String>,
    age: i128 = 42,
    //        ^^^^
}

Fix #1774.

[dtolnay]

Thanks!

[dtolnay]

This field cannot be added until 3.x, and even then, not unless the default_field_values feature is closer to stabilization.

For now please send a different PR that parses structs containing field default values as syn::Item::Verbatim instead. I'll close this PR to keep this version of the implementation for 3.x.

[madsmtm]

In the spirit of not blocking this language feature on v3.x, I wonder: Maybe it'd be possible to hackily smuggle it as an #[__syn_internal_default = $expr] attribute? Something like:

// Parse
let eq_token;
let value;

let field = Field {
    attrs: vec![
        Attribute {
            pound_token: Token![#] { span: fake_span!() },
            style: AttrStyle::Outer,
            bracket_token: Bracket { span: fake_span!() },
            meta: Meta::NameValue(MetaNameValue {
                path: fake_path!("__syn_internal_default"),
                eq_token,
                value,
            }),
        },
    ],
    vis,
    mutability,
    ident,
    colon_token,
    ty,
}

I get that that nobody likes this solution, and that it has issues with the serialization and deserialization logic becoming more complex and people possibly accidentally using the fake attribute in unintended ways. But it'd be somewhat more workable than Verbatim.

Another option: Add ItemStruct2, Fields2, FieldsNamed2, FieldsUnnamed2, Field2 etc.

WDYT?