EnumVariantMeta derive (#1972)

There are cases where we want an enum variant name. Right now the only way to do that with rust's std is to derive Debug, but this will also print out the variant's fields. This creates the unfortunate situation where we need to manually write out each variant's string name (ex: in #1963), which is both boilerplate-ey and error-prone. Crates such as `strum` exist for this reason, but it includes a lot of code and complexity that we don't need.

This adds a dead-simple `EnumVariantMeta` derive that exposes `enum_variant_index` and `enum_variant_name` functions. This allows us to make cases like #1963 much cleaner (see the second commit). We might also be able to reuse this logic for `bevy_reflect` enum derives.

crates/bevy_derive/src/enum_variant_meta.rs

@@ -0,0 +1,40 @@
+use crate::modules::{get_modules, get_path};
+use proc_macro::TokenStream;
+use quote::quote;
+use syn::{parse_macro_input, Data, DeriveInput};
+
+pub fn derive_enum_variant_meta(input: TokenStream) -> TokenStream {
+    let ast = parse_macro_input!(input as DeriveInput);
+    let variants = match &ast.data {
+        Data::Enum(v) => &v.variants,
+        _ => panic!("Expected an enum."),
+    };
+
+    let modules = get_modules(&ast.attrs);
+    let bevy_util_path = get_path(&modules.bevy_utils);
+
+    let generics = ast.generics;
+    let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
+
+    let struct_name = &ast.ident;
+    let idents = variants.iter().map(|v| &v.ident);
+    let names = variants.iter().map(|v| v.ident.to_string());
+    let indices = 0..names.len();
+
+    TokenStream::from(quote! {
+        impl #impl_generics #bevy_util_path::EnumVariantMeta for #struct_name#ty_generics #where_clause {
+            fn enum_variant_index(&self) -> usize {
+                match self {
+                    #(#struct_name::#idents {..} => #indices,)*
+                }
+            }
+            fn enum_variant_name(&self) -> &'static str {
+                static variants: &[&str] = &[
+                    #(#names,)*
+                ];
+                let index = self.enum_variant_index();
+                variants[index]
+            }
+        }
+    })
+}

crates/bevy_derive/src/lib.rs

@@ -3,6 +3,7 @@ extern crate proc_macro;
 mod app_plugin;
 mod bevy_main;
 mod bytes;
+mod enum_variant_meta;
 mod modules;
 mod render_resource;
 mod render_resources;
@@ -54,3 +55,8 @@ pub fn derive_dynamic_plugin(input: TokenStream) -> TokenStream {
 pub fn bevy_main(attr: TokenStream, item: TokenStream) -> TokenStream {
     bevy_main::bevy_main(attr, item)
 }
+
+#[proc_macro_derive(EnumVariantMeta, attributes(as_crate))]
+pub fn derive_enum_variant_meta(input: TokenStream) -> TokenStream {
+    enum_variant_meta::derive_enum_variant_meta(input)
+}

crates/bevy_derive/src/modules.rs

@@ -7,6 +7,7 @@ pub struct Modules {
     pub bevy_render: String,
     pub bevy_asset: String,
     pub bevy_core: String,
+    pub bevy_utils: String,
     pub bevy_app: String,
 }
 
@@ -16,6 +17,7 @@ impl Modules {
             bevy_asset: format!("{}::asset", name),
             bevy_render: format!("{}::render", name),
             bevy_core: format!("{}::core", name),
+            bevy_utils: format!("{}::utils", name),
             bevy_app: format!("{}::app", name),
         }
     }
@@ -25,6 +27,7 @@ impl Modules {
             bevy_asset: "bevy_asset".to_string(),
             bevy_render: "bevy_render".to_string(),
             bevy_core: "bevy_core".to_string(),
+            bevy_utils: "bevy_utils".to_string(),
             bevy_app: "bevy_app".to_string(),
         }
     }

crates/bevy_render/src/mesh/mesh.rs

@@ -15,6 +15,7 @@ use bevy_ecs::{
 };
 use bevy_math::*;
 use bevy_reflect::TypeUuid;
+use bevy_utils::EnumVariantMeta;
 use std::{borrow::Cow, collections::BTreeMap};
 
 use crate::pipeline::{InputStepMode, VertexAttribute, VertexBufferLayout};
@@ -24,7 +25,7 @@ pub const INDEX_BUFFER_ASSET_INDEX: u64 = 0;
 pub const VERTEX_ATTRIBUTE_BUFFER_ID: u64 = 10;
 
 /// An array where each entry describes a property of a single vertex.
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, EnumVariantMeta)]
 pub enum VertexAttributeValues {
     Float(Vec<f32>),
     Int(Vec<i32>),