Listing enum variants

For the the command command in Redis we want to display a list of all the available commands.

Doing so by hand is prone to forgetting to include a newly implemented command and have the list become outdated.

The following proc-macro adds a method to an enum which lists all its variants by deriving VariantName.

extern crate proc_macro;
use proc_macro::TokenStream;
use quote::quote;
use syn::{parse_macro_input, Data, DeriveInput};
 
#[proc_macro_derive(VariantNames)]
pub fn enum_variant_names_derive(input: TokenStream) -> TokenStream {
    let input = parse_macro_input!(input as DeriveInput);
 
    let name = &input.ident;
    let variants = if let Data::Enum(ref data_enum) = input.data {
        data_enum
            .variants
            .iter()
            .map(|v| &v.ident)
            .collect::<Vec<_>>()
    } else {
        panic!("VariantNames can only be derived for enums");
    };
 
    let generated = quote! {
        impl #name {
            pub fn all_variants() -> &'static [&'static str] {
                &[
                    #(stringify!(#variants)),*
                ]
            }
        }
    };
 
    TokenStream::from(generated)
}

Here’s how it’s used:

#[derive(VariantNames)]
enum Foo {
  Bar,
  Baz,
}
 
fn main() {
    let variants = Foo::all_variants();
 
    assert_eq!(
        Foo::all_variants(),
        ["Bar", "Baz"]
    );
}

Using cargo-expand we can see the code generated by the macro, which basically contains a function returning the same list we would’ve written by hand:

impl Foo {
	pub fn all_variants() -> &'static [&'static str] {
		&[
			"Bar",
			"Baz",
		]
	}
}