Add attribute to ignore fields of derived labels (#5366)

# Objective

Fixes #5362 

## Solution

Add the attribute `#[label(ignore_fields)]` for `*Label` types.

```rust
#[derive(SystemLabel)]
pub enum MyLabel {
    One,

    // Previously this was not allowed since labels cannot contain data.
    #[system_label(ignore_fields)]
    Two(PhantomData<usize>),
}
```

## Notes

This label makes it possible for equality to behave differently depending on whether or not you are treating the type as a label. For example:

```rust
#[derive(SystemLabel, PartialEq, Eq)]
#[system_label(ignore_fields)]
pub struct Foo(usize);
```

If you compare it as a label, it will ignore the wrapped fields as the user requested. But if you compare it as a `Foo`, the derive will incorrectly compare the inner fields. I see a few solutions

1. Do nothing. This is technically intended behavior, but I think we should do our best to prevent footguns.
2. Generate impls of `PartialEq` and `Eq` along with the `#[derive(Label)]` macros. This is a breaking change as it requires all users to remove these derives from their types.
3. Only allow `PhantomData` to be used with `ignore_fields` -- seems needlessly prescriptive.

---

## Changelog

* Added the `ignore_fields` attribute to the derive macros for `*Label` types.
* Added an example showing off different forms of the derive macro.

<!--
## Migration Guide

> This section is optional. If there are no breaking changes, you can delete this section.

- If this PR is a breaking change (relative to the last release of Bevy), describe how a user might need to migrate their code to support these changes
- Simply adding new functionality is not a breaking change.
- Fixing behavior that was definitely a bug, rather than a questionable design choice is not a breaking change.
-->

crates/bevy_derive/src/lib.rs

@@ -80,10 +80,14 @@ pub fn derive_enum_variant_meta(input: TokenStream) -> TokenStream {
     enum_variant_meta::derive_enum_variant_meta(input)
 }
 
-#[proc_macro_derive(AppLabel)]
+/// Generates an impl of the `AppLabel` trait.
+///
+/// This works only for unit structs, or enums with only unit variants.
+/// You may force a struct or variant to behave as if it were fieldless with `#[app_label(ignore_fields)]`.
+#[proc_macro_derive(AppLabel, attributes(app_label))]
 pub fn derive_app_label(input: TokenStream) -> TokenStream {
     let input = syn::parse_macro_input!(input as syn::DeriveInput);
     let mut trait_path = BevyManifest::default().get_path("bevy_app");
     trait_path.segments.push(format_ident!("AppLabel").into());
-    derive_label(input, &trait_path)
+    derive_label(input, &trait_path, "app_label")
 }

crates/bevy_ecs/Cargo.toml

@@ -41,3 +41,7 @@ path = "examples/resources.rs"
 [[example]]
 name = "change_detection"
 path = "examples/change_detection.rs"
+
+[[example]]
+name = "derive_label"
+path = "examples/derive_label.rs"

crates/bevy_ecs/examples/derive_label.rs

@@ -0,0 +1,62 @@
+use std::marker::PhantomData;
+
+use bevy_ecs::prelude::*;
+
+fn main() {
+    // Unit labels are always equal.
+    assert_eq!(UnitLabel.as_label(), UnitLabel.as_label());
+
+    // Enum labels depend on the variant.
+    assert_eq!(EnumLabel::One.as_label(), EnumLabel::One.as_label());
+    assert_ne!(EnumLabel::One.as_label(), EnumLabel::Two.as_label());
+
+    // Labels annotated with `ignore_fields` ignore their fields.
+    assert_eq!(WeirdLabel(1).as_label(), WeirdLabel(2).as_label());
+
+    // Labels don't depend only on the variant name but on the full type
+    assert_ne!(
+        GenericLabel::<f64>::One.as_label(),
+        GenericLabel::<char>::One.as_label(),
+    );
+}
+
+#[derive(SystemLabel)]
+pub struct UnitLabel;
+
+#[derive(SystemLabel)]
+pub enum EnumLabel {
+    One,
+    Two,
+}
+
+#[derive(SystemLabel)]
+#[system_label(ignore_fields)]
+pub struct WeirdLabel(i32);
+
+#[derive(SystemLabel)]
+pub enum GenericLabel<T> {
+    One,
+    #[system_label(ignore_fields)]
+    Two(PhantomData<T>),
+}
+
+// FIXME: this should be a compile_fail test
+/*#[derive(SystemLabel)]
+pub union Foo {
+    x: i32,
+}*/
+
+// FIXME: this should be a compile_fail test
+/*#[derive(SystemLabel)]
+#[system_label(ignore_fields)]
+pub enum BadLabel {
+    One,
+    Two,
+}*/
+
+// FIXME: this should be a compile_fail test
+/*#[derive(SystemLabel)]
+pub struct BadLabel2 {
+    #[system_label(ignore_fields)]
+    x: (),
+}*/

crates/bevy_ecs/macros/src/lib.rs

@@ -434,46 +434,62 @@ pub fn derive_world_query(input: TokenStream) -> TokenStream {
     derive_world_query_impl(ast)
 }
 
-#[proc_macro_derive(SystemLabel)]
+/// Generates an impl of the `SystemLabel` trait.
+///
+/// This works only for unit structs, or enums with only unit variants.
+/// You may force a struct or variant to behave as if it were fieldless with `#[system_label(ignore_fields)]`.
+#[proc_macro_derive(SystemLabel, attributes(system_label))]
 pub fn derive_system_label(input: TokenStream) -> TokenStream {
     let input = parse_macro_input!(input as DeriveInput);
     let mut trait_path = bevy_ecs_path();
     trait_path.segments.push(format_ident!("schedule").into());
     trait_path
         .segments
         .push(format_ident!("SystemLabel").into());
-    derive_label(input, &trait_path)
+    derive_label(input, &trait_path, "system_label")
 }
 
-#[proc_macro_derive(StageLabel)]
+/// Generates an impl of the `StageLabel` trait.
+///
+/// This works only for unit structs, or enums with only unit variants.
+/// You may force a struct or variant to behave as if it were fieldless with `#[stage_label(ignore_fields)]`.
+#[proc_macro_derive(StageLabel, attributes(stage_label))]
 pub fn derive_stage_label(input: TokenStream) -> TokenStream {
     let input = parse_macro_input!(input as DeriveInput);
     let mut trait_path = bevy_ecs_path();
     trait_path.segments.push(format_ident!("schedule").into());
     trait_path.segments.push(format_ident!("StageLabel").into());
-    derive_label(input, &trait_path)
+    derive_label(input, &trait_path, "stage_label")
 }
 
-#[proc_macro_derive(AmbiguitySetLabel)]
+/// Generates an impl of the `AmbiguitySetLabel` trait.
+///
+/// This works only for unit structs, or enums with only unit variants.
+/// You may force a struct or variant to behave as if it were fieldless with `#[ambiguity_set_label(ignore_fields)]`.
+#[proc_macro_derive(AmbiguitySetLabel, attributes(ambiguity_set_label))]
 pub fn derive_ambiguity_set_label(input: TokenStream) -> TokenStream {
     let input = parse_macro_input!(input as DeriveInput);
     let mut trait_path = bevy_ecs_path();
     trait_path.segments.push(format_ident!("schedule").into());
     trait_path
         .segments
         .push(format_ident!("AmbiguitySetLabel").into());
-    derive_label(input, &trait_path)
+    derive_label(input, &trait_path, "ambiguity_set_label")
 }
 
-#[proc_macro_derive(RunCriteriaLabel)]
+/// Generates an impl of the `RunCriteriaLabel` trait.
+///
+/// This works only for unit structs, or enums with only unit variants.
+/// You may force a struct or variant to behave as if it were fieldless with `#[run_criteria_label(ignore_fields)]`.
+#[proc_macro_derive(RunCriteriaLabel, attributes(run_criteria_label))]
 pub fn derive_run_criteria_label(input: TokenStream) -> TokenStream {
     let input = parse_macro_input!(input as DeriveInput);
     let mut trait_path = bevy_ecs_path();
     trait_path.segments.push(format_ident!("schedule").into());
     trait_path
         .segments
         .push(format_ident!("RunCriteriaLabel").into());
-    derive_label(input, &trait_path)
+    derive_label(input, &trait_path, "run_criteria_label")
 }
 
 pub(crate) fn bevy_ecs_path() -> syn::Path {

crates/bevy_macro_utils/src/lib.rs

@@ -9,8 +9,9 @@ pub use shape::*;
 pub use symbol::*;
 
 use proc_macro::TokenStream;
-use quote::quote;
+use quote::{quote, quote_spanned};
 use std::{env, path::PathBuf};
+use syn::spanned::Spanned;
 use toml::{map::Map, Value};
 
 pub struct BevyManifest {
@@ -110,8 +111,26 @@ impl BevyManifest {
 ///
 /// - `input`: The [`syn::DeriveInput`] for struct that is deriving the label trait
 /// - `trait_path`: The path [`syn::Path`] to the label trait
-pub fn derive_label(input: syn::DeriveInput, trait_path: &syn::Path) -> TokenStream {
-    let ident = input.ident;
+pub fn derive_label(
+    input: syn::DeriveInput,
+    trait_path: &syn::Path,
+    attr_name: &str,
+) -> TokenStream {
+    // return true if the variant specified is an `ignore_fields` attribute
+    fn is_ignore(attr: &syn::Attribute, attr_name: &str) -> bool {
+        if attr.path.get_ident().as_ref().unwrap() != &attr_name {
+            return false;
+        }
+
+        syn::custom_keyword!(ignore_fields);
+        attr.parse_args_with(|input: syn::parse::ParseStream| {
+            let ignore = input.parse::<Option<ignore_fields>>()?.is_some();
+            Ok(ignore)
+        })
+        .unwrap()
+    }
+
+    let ident = input.ident.clone();
 
     let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
     let mut where_clause = where_clause.cloned().unwrap_or_else(|| syn::WhereClause {
@@ -123,30 +142,69 @@ pub fn derive_label(input: syn::DeriveInput, trait_path: &syn::Path) -> TokenStr
         .push(syn::parse2(quote! { Self: 'static }).unwrap());
 
     let as_str = match input.data {
-        syn::Data::Struct(d) => match d.fields {
-            syn::Fields::Unit => {
+        syn::Data::Struct(d) => {
+            // see if the user tried to ignore fields incorrectly
+            if let Some(attr) = d
+                .fields
+                .iter()
+                .flat_map(|f| &f.attrs)
+                .find(|a| is_ignore(a, attr_name))
+            {
+                let err_msg = format!("`#[{attr_name}(ignore_fields)]` cannot be applied to fields individually: add it to the struct declaration");
+                return quote_spanned! {
+                    attr.span() => compile_error!(#err_msg);
+                }
+                .into();
+            }
+            // Structs must either be fieldless, or explicitly ignore the fields.
+            let ignore_fields = input.attrs.iter().any(|a| is_ignore(a, attr_name));
+            if matches!(d.fields, syn::Fields::Unit) || ignore_fields {
                 let lit = ident.to_string();
                 quote! { #lit }
+            } else {
+                let err_msg = format!("Labels cannot contain data, unless explicitly ignored with `#[{attr_name}(ignore_fields)]`");
+                return quote_spanned! {
+                    d.fields.span() => compile_error!(#err_msg);
+                }
+                .into();
             }
-            _ => panic!("Labels cannot contain data."),
-        },
+        }
         syn::Data::Enum(d) => {
-            let arms = d.variants.iter().map(|v| match v.fields {
-                syn::Fields::Unit => {
+            // check if the user put #[label(ignore_fields)] in the wrong place
+            if let Some(attr) = input.attrs.iter().find(|a| is_ignore(a, attr_name)) {
+                let err_msg = format!("`#[{attr_name}(ignore_fields)]` can only be applied to enum variants or struct declarations");
+                return quote_spanned! {
+                    attr.span() => compile_error!(#err_msg);
+                }
+                .into();
+            }
+            let arms = d.variants.iter().map(|v| {
+                // Variants must either be fieldless, or explicitly ignore the fields.
+                let ignore_fields = v.attrs.iter().any(|a| is_ignore(a, attr_name));
+                if matches!(v.fields, syn::Fields::Unit) | ignore_fields {
                     let mut path = syn::Path::from(ident.clone());
                     path.segments.push(v.ident.clone().into());
                     let lit = format!("{ident}::{}", v.ident.clone());
-                    quote! { #path => #lit }
+                    quote! { #path { .. } => #lit }
+                } else {
+                    let err_msg = format!("Label variants cannot contain data, unless explicitly ignored with `#[{attr_name}(ignore_fields)]`");
+                    quote_spanned! {
+                        v.fields.span() => _ => { compile_error!(#err_msg); }
+                    }
                 }
-                _ => panic!("Label variants cannot contain data."),
             });
             quote! {
                 match self {
                     #(#arms),*
                 }
             }
         }
-        syn::Data::Union(_) => panic!("Unions cannot be used as labels."),
+        syn::Data::Union(_) => {
+            return quote_spanned! {
+                input.span() => compile_error!("Unions cannot be used as labels.");
+            }
+            .into();
+        }
     };
 
     (quote! {