Declarative macro to parse an enum item completely including generic parameters, where clauses and bounds, and attributes.

parse_macro.rs

macro_rules! parse_enum_variant {
    {
        $(#[$enum_meta:meta])*,
        $vis:vis,
        $name:ident,
        $(<$($generic_param:tt),*>)?,
        @where_clauses {$($where:tt)*},
        @parse {$($eout:tt)*},
        #[$variant_meta:meta]
        $($rest:tt)*
    } => {
        parse_enum_variant! {
            $(#[$enum_meta])*,
            $vis,
            $name,
            $(<$($generic_param),*>)?,
            @where_clauses {$($where)*},
            @parse {
                $($eout)*
                #[$variant_meta]
            },
            $($rest)*
        }
    };
    {
        $(#[$enum_meta:meta])*,
        $vis:vis,
        $name:ident,
        $(<$($generic_param:tt),*>)?,
        @where_clauses {$($where:tt)*},
        @parse {$($eout:tt)*},
        $variant:ident,
        $($rest:tt)*
    } => {
        parse_enum_variant! {
            $(#[$enum_meta])*,
            $vis,
            $name,
            $(<$($generic_param),*>)?,
            @where_clauses {$($where)*},
            @parse {
                $($eout)*
                $variant,
            },
            $($rest)*
        }
    };
    {
        $(#[$enum_meta:meta])*,
        $vis:vis,
        $name:ident,
        $(<$($generic_param:tt),*>)?,
        @where_clauses {$($where:tt)*},
        @parse {$($eout:tt)*},
        $variant:ident = $value:expr,
        $($rest:tt)*
    } => {
        parse_enum_variant!{
            $(#[$enum_meta])*,
            $vis,
            $name,
            $(<$($generic_param),*>)?,
            @where_clauses {$($where)*},
            @parse {
                $($eout)*
                $variant = $value,
            },
            $($rest)*
        }
    };
    {
        $(#[$enum_meta:meta])*,
        $vis:vis,
        $name:ident,
        $(<$($generic_param:tt),*>)?,
        @where_clauses {$($where:tt)*},
        @parse {$($eout:tt)*},
        $variant:ident($param:ty),
        $($rest:tt)*
    } => {
        parse_enum_variant!{
            $(#[$enum_meta])*,
            $vis,
            $name,
            $(<$($generic_param),*>)?,
            @where_clauses {$($where)*},
            @parse {
                $($eout)*
                $variant($param),
            },
            $($rest)*
        }
    };
    {
        $(#[$enum_meta:meta])*,
        $vis:vis,
        $name:ident,
        $(<$($generic_param:tt),*>)?,
        @where_clauses {$($where:tt)*},
        @parse {$($eout:tt)*},
        $variant:ident { $($fields:tt)* },
        $($rest:tt)*
    } => {
        parse_enum_variant!{
            $(#[$enum_meta])*,
            $vis,
            $name,
            $(<$($generic_param),*>)?,
            @where_clauses {$($where)*},
            @parse {
                $($eout)*
                $variant { $($fields)* },
            },
            $($rest)*
        }
    };
    {
        $(#[$enum_meta:meta])*,
        $vis:vis,
        $name:ident,
        $(<$($generic_param:tt),*>)?,
        @where_clauses {$($where:tt)*},
        @parse {$($eout:tt)*} $(,)*
    } => {
        $(#[$enum_meta])*
        $vis enum $name
        $(<$($generic_param),*>)?
        $($where)*
        {
            $($eout)*
        }
    };
    {
        $(#[$enum_meta:meta])*,
        $vis:vis,
        $name:ident,
        $(<$($generic_param:tt),*>)?,
        @where_clauses {$($where:tt)*},
        @parse {$($eout:tt)*} $(,)*
    } => {
        $(#[$enum_meta])*
        $vis enum $name
        $(<$($generic_param),*>)?
        $($where)*
        {
            $($eout)*
        }
    };
    {
        $(#[$enum_meta:meta])*,
        $vis:vis,
        $name:ident,
        $(<$($generic_param:tt),*>)?,
        @where_clauses {$($where:tt)*},
        @parse {$($eout:tt)*} $(,)*
    } => {
        $(#[$enum_meta])*
        $vis enum $name
        $(<$($generic_param),*>)?
        $($where)*
        {
            $($eout)*
        }
    };
    {
        $(#[$enum_meta:meta])*,
        $vis:vis,
        $name:ident,
        $(<$($generic_param:tt),*>)?,
        @where_clauses {$($where:tt)*},
        @parse {$($eout:tt)*} $(,)*
    } => {
        $(#[$enum_meta])*
        $vis enum $name
        $(<$($generic_param),*>)?
        $($where)*
        {
            $($eout)*
        }
    };
}

macro_rules! parse_enum {
    (
         $(#[$enum_meta:meta])*
         $vis:vis enum $enum_name:ident
         $(<$($generic_param:tt),*>)?
         {
            $($variants:tt)*
         }
    ) => {
        parse_enum_variant! {
            $(#[$enum_meta])*,
            $vis,
            $enum_name,
            $(<$($generic_param),*>)?,
            @where_clauses {},
            @parse {}, $($variants)*
        }
    };
    (
         $(#[$enum_meta:meta])*
         $vis:vis enum $enum_name:ident
         $(<$($generic_param:tt),*>)?
         where
           $where_clause:tt $(<$($where_generic_param:tt),*>)?:
               $where_clause_bound:tt $(<$($where_clause_bound_generic_param:tt),*>)?
               $(+ $where_clause_bound_plus:tt $(<$($where_clause_bound_generic_param_plus:tt),*>)?)*

           $(, $more_where_clause:tt $(<$($more_where_generic_param:tt),*>)?:
                   $more_where_clause_bound:tt $(<$($more_where_clause_bound_generic_param:tt),*>)?
                   $(+ $more_where_clause_bound_plus:tt $(<$($more_where_clause_bounds_generic_param_plus:tt),*>)?)* )*
         {
            $($variants:tt)*
         }
    ) => {
        parse_enum_variant! {
            $(#[$enum_meta])*,
            $vis,
            $enum_name,
            $(<$($generic_param),*>)?,
            @where_clauses {
                where
                $where_clause $(<$($where_generic_param),*>)?:
                    $where_clause_bound $(<$($where_clause_bound_generic_param),*>)?
                    $(+ $where_clause_bound_plus $(<$($where_clause_bound_generic_param_plus),*>)?)*

                $(, $more_where_clause $(<$($more_where_generic_param),*>)?:
                        $more_where_clause_bound $(<$($more_where_clause_bound_generic_param),*>)?
                        $(+ $more_where_clause_bound_plus $(<$($more_where_clause_bounds_generic_param_plus),*>)?)* )*
            },
            @parse {}, $($variants)*
        }
    };
}

Inline generic bounds enum X<T: Foo> { Y } are left as an exercise for the reader, I only use where clauses ;) same with impl Trait because I just don't care about it.

FAQ:

• Q: How do I make something private? A visibility is required!
• A: pub(self) https://doc.rust-lang.org/reference/visibility-and-privacy.html