Support placeholder in #[display] for struct

[yeongjet]

I have a struct which contains many fields and some of these fields (like b) need to display in Hex:

#[derive(Display)]
#[display("{a}, {b:x}, ..., {z}")]
struct A {
    a: u32,
    b: u32,
    ...,
    z: u32
}

but #[display("{a}, {b:x}, ..., {z}")] is long and verbose, how to use placeholder for struct?
I wish to use placeholder like enum:

#[derive(Display)]
#[display("{_variant}")]
struct A {
    a: u32,
    #[display("{:x}")]
    b: u32,
    ...,
    z: u32
}

[tyranron]

@yeongjet I don't understand your example:

I wish to use placeholder like enum:

#[derive(Display)]
#[display("{_variant}")]
struct A {
    a: u32,
    #[display("{:x}")]
    b: u32,
    ...,
    z: u32
}

How it's supposed to work? What's the logic behind {_variant} here? Why you're implying it should be something like #[display("{a}, {b:x}, ..., {z}")]?

We can theoretically support intermediate transformation of the field:

    #[display("{:x}")]
    b: u32,

But you still somehow should refer whether to include this into the final fmt representation.

[yeongjet]

@tyranron Sorry, it's just my rough idea which is somewhat lacking, more detailed example here:

#[derive(Display)]
// The _fields here is reserved like _variant, which is string array like `Vec<&str>` , 
// and the items is formatted by the macro `#[display("{_variant}")]` or `#[display("{:x}")]`
#[display(fmt = "all fields:{}, total:{}", _fields.join(","), _fields.len())
// Default field display
#[display("{_variant}")]
struct A {
    a: u32,
    // Special field display
    #[display("{:x}")]
    b: u32,
    ...,
    z: u32
}

Just my assumption, of course it won't work.