How can I parse an attribute with nested arguments with darling?

Question

I'm trying to parse an attribute with darling, and I want to support the following usages:

// att not specified
#[derive(MyTrait)]
struct Foo(u64);

// att specified without an argument
#[derive(MyTrait)]
#[myderive(att)]
struct Foo(u64);

// att specified with an argument
#[derive(MyTrait)]
#[myderive(att(value = "String"))]
struct Foo(u64);

These are my types:

#[derive(FromDeriveInput)]
#[darling(attributes(myderive))]
struct MyDeriveInput {
    #[darling(default)]
    att: Option<MyAttr>,
}

#[derive(FromMeta, Default)]
struct MyAttr {
    #[darling(default)]
    value: Option<Path>,
}

And a test:

#[test]
fn test() {
    let derive_input = syn::parse_str(
        r#"
        #[derive(MyTrait)]
        #[myderive(att)]
        struct Foo(u64);
    "#,
    )
    .unwrap();

    let parsed: MyDeriveInput = FromDeriveInput::from_derive_input(&derive_input).unwrap();
    assert!(parsed.att.is_some());
}

I get this error:

thread 'test' panicked at 'called `Result::unwrap()` on an `Err` value:
Error { kind: UnexpectedFormat("word"), locations: ["att"], span: Some(Span) }'

I get the same error if I specify att, regardless of whether value is specified.

Is this possible? If so, what structure does darling expect to parse this into?

Answer 1

The syntax for the derive doesn't exactly work this way for attributes values which are structs.

If you want to specify there's an att but a default one, you should set it as att().

Here's a fixed complete code and test units:

extern crate proc_macro;
extern crate syn;

use {
    darling::*,
    std::path::*,
};

#[derive(FromMeta)]
struct MyAttr {
    #[darling(default)]
    value: Option<String>, // I dunno what was your "Path" so I've put String
}

#[derive(FromDeriveInput, Default)]
#[darling(attributes(myderive))]
struct MyTrait {
    #[darling(default)]
    att: Option<MyAttr>,
}


#[test]
fn test() {

    // with specified MyAttr:
    let derive_input = syn::parse_str(
        r#"
        #[derive(MyTrait)]
        #[myderive(att(value = "test"))]
        struct Foo(u64);
    "#,
    )
    .unwrap();
    let parsed: MyTrait = FromDeriveInput::from_derive_input(&derive_input).unwrap();
    assert!(parsed.att.is_some());

    // with default MyAttr:
    let derive_input = syn::parse_str(
        r#"
        #[derive(MyTrait)]
        #[myderive(att())]
        struct Foo(u64);
    "#,
    )
    .unwrap();
    let parsed: MyTrait = FromDeriveInput::from_derive_input(&derive_input).unwrap();
    assert!(parsed.att.is_some());

    // with no MyAttr:
    let derive_input = syn::parse_str(
        r#"
        #[derive(MyTrait)]
        #[myderive()]
        struct Foo(u64);
    "#,
    )
    .unwrap();
    let parsed: MyTrait = FromDeriveInput::from_derive_input(&derive_input).unwrap();
    assert!(parsed.att.is_none());


    // with no myderive
    let derive_input = syn::parse_str(
        r#"
        #[derive(MyTrait)]
        struct Foo(u64);
    "#,
    )
    .unwrap();
    let parsed: MyTrait = FromDeriveInput::from_derive_input(&derive_input).unwrap();
    assert!(parsed.att.is_none());
}