Using PartialEq with empty c-like struct or tuple struct enum variant causes compiler parse error

[DavidAntliff]

Using subenum 1.1.2 with Rust 1.77.2 (stable).

Please consider this, which compiles OK:

#[subenum(Fruit)]
pub enum Food {
    #[subenum(Fruit)]
    Apple {},
}

The addition of the PartialEq trait to the derive clause results in a compiler error:

#[subenum(Fruit)]
#[derive(PartialEq)]
pub enum Food {
    #[subenum(Fruit)]
    Apple {},
}

The error is:

error: expected expression, found `,`
  --> src/problem.rs:19:1
   |
19 | #[subenum(Fruit)]
   | ^^^^^^^^^^^^^^^^^
   | |
   | expected expression
   | while parsing the `match` arm starting here

The same error occurs for an empty tuple struct syntax as well:

#[subenum(Fruit)]
#[derive(PartialEq)]
pub enum Food {
    #[subenum(Fruit)]
    Apple (),  // <--- empty tuple struct variant
}
// Compile Error: expected expression, found `,`

Adding a dummy struct field, or removing the struct variant or tuple struct syntax, makes the code compile again:

#[subenum(Fruit)]
#[derive(PartialEq)]
pub enum Food {
    #[subenum(Fruit)]
    Apple {
        dummy: i32,   // <--- at least one field
    },
}
// Compiles OK

Or:

#[subenum(Fruit)]
#[derive(PartialEq)]
pub enum Food {
    #[subenum(Fruit)]
    Apple (()),
}
// Compiles OK

Or:

#[subenum(Fruit)]
#[derive(PartialEq)]
pub enum Food {
    #[subenum(Fruit)]
    Apple,  // <--- not an empty c-like struct or tuple struct variant
}
// Compiles OK

Changing the order of the macros also resolves the compiler error:

#[derive(PartialEq)]  // <--- swapped
#[subenum(Fruit)]     // <--- swapped
pub enum Food {
    #[subenum(Fruit)]
    Apple {},
}
// Compiles OK

But this seems to cause other problems, like the PartialEq not actually being implemented for the subenums.

I don't think it's too uncommon to use empty struct or tuple variants, and it's certainly part of the language to do so.