Function require_ident 

pub fn require_ident(ident: Option<Ident>) -> Result<Ident>

Attaches error message to Option<Ident>, turning it into darling::Result<Ident>, so it composes nicely with #[darling(with = ...)] on a struct deriving FromField

Using #[derive(FromField)] requires the ident field, if present, to be Option<Ident>. This is correct if you will use this struct as both part of a tuple struct, and a struct with named fields.

However, if you don’t need to do that, then there’s no need to wrap it in an Option.

#[derive(FromField)]
struct Field {
    // Without #[darling(with)], this must be `ident: Option<syn::Ident>`
    #[darling(with = darling::util::require_ident)]
    ident: syn::Ident,
}

#[derive(FromDeriveInput)]
struct Input {
    data: darling::ast::Data<darling_core::util::Ignored, Field>,
}

let input = Input::from_derive_input(&parse_quote! {
    struct Demo<T> {
        hello: T
    }
})?;

let fields = input.data.take_struct()?;
let first_field = fields.into_iter().next()?;

// Not wrapped in `Some`!
assert_eq!(first_field.ident, Ident::new("hello", Span::call_site()));