Function expand_fields 

pub fn expand_fields<'ident, 'fields, EN: Fn(TokenStream, &'fields FieldsNamed) -> R, EU: Fn(TokenStream, &'fields FieldsUnnamed) -> R, U: Fn(TokenStream) -> R, R>(
    ident: TokenStream,
    fields: &'fields Fields,
    expand_named: EN,
    expand_unnamed: EU,
    expand_unit: U,
) -> R

Formats the fields of any structure or enum variant.

Empty lists of named or unnamed fields are treated as unit structs, as they are semantically identical, and the expand_unit codepath can usually generate better code.