[really good to do] Add a way to map Value into another Value rather then a String

[zhiburt]

So we do have #[tabled(display_with)] which very good, but it does require us to return String.
Which sometimes is not what we are looking for.

Sometimes it's valuable to convert a value into another value so it could be #[tabled(inline)]ed.

Here's a good example from #486

// we have this structure
struct DbEntry {
    index: usize,
    time: u64,
    text: String,
}

// but we wanna to print the structure
pub struct Entry {
    index: usize,
    #[tabled(inline)]
    time: Date,
    text: String,
}

pub struct Date {
    y: usize,
    m: usize,
    d: usize,
}

So the result will be something like

struct DbEntry {
    index: usize,
    #[tabled(map = "Date::from_u64")]
    #[tabled(inline)]    
    time: u64,
    text: String,
}

pub struct Date {
    y: usize,
    m: usize,
    d: usize,
}

I guess we need one more macro helper?

---

And actually it's valuable not only for inline case,
but we could map and the value could impl Display so we would kind be ok with it.

So yessss
Very prominent idea.
I do very like it.

[KekmaTime]

@zhiburt can i take up this issue

[zhiburt]

Hi @KekmaTime sure,

But it's not the easiest issue to cover,
cause tabled_derive is not quite clean 😅 .
It's good but could be better.

You would need to take a look at how display_with implemented, the difference will be in return type of function and that's it I guess?
So maybe not that difficult then?:)

If you'll encounter problems please let me know and I'll try to help.

tabled/tabled_derive/src/lib.rs

Lines 411 to 431 in 965a800

	let value = if let Some(func) = &attr.display_with {
	let args = match &attr.display_with_args {
	None => None,
	Some(args) => match args.is_empty() {
	true => None,
	false => {
	let args = args
	.iter()
	.map(|arg| fnarg_tokens(arg, &Fields::Unit, struct_field_name))
	.collect::<Vec<_>>();
	Some(quote!( #(#args,)* ))
	}
	},
	};
	let result = match args {
	Some(args) => use_function(&args, func),
	None => use_function_no_args(func),
	};
	quote! { ::std::borrow::Cow::from(format!("{}", #result)) }

[KekmaTime]

@zhiburt 👍🏽 for sure