use serde_with::apply within macro

[ibotty]

serde_with::apply fails to apply attributes to data attributes added in macros. See below error.

use serde_with;

macro_rules! t {
($($param:ident : $ty:ty),*) => {
#[derive(Default)]
#[serde_with::apply(
    Option => #[serde(skip_serializing_if = "Option::is_none")],
)]
#[derive(serde::Serialize)]
struct Data {
    a: Option<String>,
    b: Option<u64>,
    c: Option<String>,
    $(
        pub $param: $ty),*
}
}}

t!(d: Option<bool>);

#[test]
fn t() {
    let golden = r#"{}"#;
    let data: Data = Default::default();
    assert_eq!(serde_json::to_string(&data).unwrap(), golden);
}

This fails with

---- t stdout ----
thread 't' panicked at 'assertion failed: `(left == right)`
  left: `"{\"d\":null}"`,
 right: `"{}"`', doveadm_types/src/lib.rs:34:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[jonasbb]

Hi, thanks for reporting the issue and providing a good reproduction. I can confirm this is indeed a bug. The issue here is that the type of field d is wrapped in an invisible Group on the token level, since it stems from a macro parameter. This doesn't match the Option since a Group is not an Ident.

Normal field

Ident {
    ident: "pub",
    span: #4 bytes(306..309),
},
Ident {
    ident: "_d",
    span: #4 bytes(310..312),
},
Punct {
    ch: ':',
    spacing: Alone,
    span: #4 bytes(312..313),
},
Ident {
    ident: "Option",
    span: #4 bytes(314..320),
},
Punct {
    ch: '<',
    spacing: Alone,
    span: #4 bytes(320..321),
},
Ident {
    ident: "bool",
    span: #4 bytes(321..325),
},
Punct {
    ch: '>',
    spacing: Alone,
    span: #4 bytes(325..326),
},

Field with type from macro_rules argument

Ident {
    ident: "pub",
    span: #4 bytes(282..285),
},
Ident {
    ident: "d",
    span: #0 bytes(337..338),
},
Punct {
    ch: ':',
    spacing: Alone,
    span: #4 bytes(292..293),
},
Group {
    delimiter: None,
    stream: TokenStream [
        Ident {
            ident: "Option",
            span: #0 bytes(340..346),
        },
        Punct {
            ch: '<',
            spacing: Alone,
            span: #0 bytes(346..347),
        },
        Ident {
            ident: "bool",
            span: #0 bytes(347..351),
        },
        Punct {
            ch: '>',
            spacing: Alone,
            span: #0 bytes(351..352),
        },
    ],
    span: #4 bytes(294..297),
},

The solution is simple enough. The type matching must be relaxed such that Groups are treated fully transparent, i.e., if one side (the type or the pattern) has a group then it needs to be thrown away first before continuing. The match is here:

serde_with/serde_with_macros/src/apply.rs

Lines 129 to 134 in d76f283

	(
	Type::Group(TypeGroup {
	elem: ty_pattern, ..
	}),
	Type::Group(TypeGroup { elem: ty, .. }),
	) => ty_pattern_matches_ty(ty_pattern, ty),

I will fix that later.

[jonasbb]

I just published a new version fixing this bug. I hope it works now for you but please report back if you still have issues.

[ibotty]

Thank you very much! Very appreciated.