Extend the WorldQuery macro to tuple structs (#8119)

# Objective

The `#[derive(WorldQuery)]` macro currently only supports structs with
named fields.

Same motivation as #6957. Remove sharp edges from the derive macro, make
it just work more often.

## Solution

Support tuple structs.

---

## Changelog

+ Added support for tuple structs to the `#[derive(WorldQuery)]` macro.

crates/bevy_ecs/macros/src/fetch.rs

@@ -1,12 +1,12 @@
 use bevy_macro_utils::ensure_no_collision;
 use proc_macro::TokenStream;
 use proc_macro2::{Ident, Span};
-use quote::{quote, ToTokens};
+use quote::{format_ident, quote, ToTokens};
 use syn::{
     parse::{Parse, ParseStream},
     parse_macro_input, parse_quote,
     punctuated::Punctuated,
-    Attribute, Data, DataStruct, DeriveInput, Field, Fields,
+    Attribute, Data, DataStruct, DeriveInput, Field, Index,
 };
 
 use crate::bevy_ecs_path;
@@ -116,34 +116,49 @@ pub fn derive_world_query_impl(input: TokenStream) -> TokenStream {
         fetch_struct_name.clone()
     };
 
+    let marker_name =
+        ensure_no_collision(format_ident!("_world_query_derive_marker"), tokens.clone());
+
     // Generate a name for the state struct that doesn't conflict
     // with the struct definition.
     let state_struct_name = Ident::new(&format!("{struct_name}State"), Span::call_site());
     let state_struct_name = ensure_no_collision(state_struct_name, tokens);
 
-    let fields = match &ast.data {
-        Data::Struct(DataStruct {
-            fields: Fields::Named(fields),
-            ..
-        }) => &fields.named,
-        _ => panic!("Expected a struct with named fields"),
+    let Data::Struct(DataStruct { fields, .. }) = &ast.data else {
+        return syn::Error::new(
+            Span::call_site(),
+            "#[derive(WorldQuery)]` only supports structs",
+        )
+        .into_compile_error()
+        .into()
     };
 
     let mut field_attrs = Vec::new();
     let mut field_visibilities = Vec::new();
     let mut field_idents = Vec::new();
+    let mut named_field_idents = Vec::new();
     let mut field_types = Vec::new();
     let mut read_only_field_types = Vec::new();
-
-    for field in fields {
+    for (i, field) in fields.iter().enumerate() {
         let attrs = match read_world_query_field_info(field) {
             Ok(WorldQueryFieldInfo { attrs }) => attrs,
             Err(e) => return e.into_compile_error().into(),
         };
 
+        let named_field_ident = field
+            .ident
+            .as_ref()
+            .cloned()
+            .unwrap_or_else(|| format_ident!("f{i}"));
+        let i = Index::from(i);
+        let field_ident = field
+            .ident
+            .as_ref()
+            .map_or(quote! { #i }, |i| quote! { #i });
+        field_idents.push(field_ident);
+        named_field_idents.push(named_field_ident);
         field_attrs.push(attrs);
         field_visibilities.push(field.vis.clone());
-        field_idents.push(field.ident.as_ref().unwrap().clone());
         let field_ty = field.ty.clone();
         field_types.push(quote!(#field_ty));
         read_only_field_types.push(quote!(<#field_ty as #path::query::WorldQuery>::ReadOnly));
@@ -176,15 +191,34 @@ pub fn derive_world_query_impl(input: TokenStream) -> TokenStream {
             &field_types
         };
 
-        let item_struct = quote! {
-            #derive_macro_call
-            #[doc = "Automatically generated [`WorldQuery`] item type for [`"]
-            #[doc = stringify!(#struct_name)]
-            #[doc = "`], returned when iterating over query results."]
-            #[automatically_derived]
-            #visibility struct #item_struct_name #user_impl_generics_with_world #user_where_clauses_with_world {
-                #(#(#field_attrs)* #field_visibilities #field_idents: <#field_types as #path::query::WorldQuery>::Item<'__w>,)*
-            }
+        let item_struct = match fields {
+            syn::Fields::Named(_) => quote! {
+                #derive_macro_call
+                #[doc = "Automatically generated [`WorldQuery`] item type for [`"]
+                #[doc = stringify!(#struct_name)]
+                #[doc = "`], returned when iterating over query results."]
+                #[automatically_derived]
+                #visibility struct #item_struct_name #user_impl_generics_with_world #user_where_clauses_with_world {
+                    #(#(#field_attrs)* #field_visibilities #field_idents: <#field_types as #path::query::WorldQuery>::Item<'__w>,)*
+                }
+            },
+            syn::Fields::Unnamed(_) => quote! {
+                #derive_macro_call
+                #[doc = "Automatically generated [`WorldQuery`] item type for [`"]
+                #[doc = stringify!(#struct_name)]
+                #[doc = "`], returned when iterating over query results."]
+                #[automatically_derived]
+                #visibility struct #item_struct_name #user_impl_generics_with_world #user_where_clauses_with_world(
+                    #( #field_visibilities <#field_types as #path::query::WorldQuery>::Item<'__w>, )*
+                );
+            },
+            syn::Fields::Unit => quote! {
+                #[doc = "Automatically generated [`WorldQuery`] item type for [`"]
+                #[doc = stringify!(#struct_name)]
+                #[doc = "`], returned when iterating over query results."]
+                #[automatically_derived]
+                #visibility type #item_struct_name #user_ty_generics_with_world = #struct_name #user_ty_generics;
+            },
         };
 
         let query_impl = quote! {
@@ -194,7 +228,8 @@ pub fn derive_world_query_impl(input: TokenStream) -> TokenStream {
             #[doc = "`], used to define the world data accessed by this query."]
             #[automatically_derived]
             #visibility struct #fetch_struct_name #user_impl_generics_with_world #user_where_clauses_with_world {
-                #(#field_idents: <#field_types as #path::query::WorldQuery>::Fetch<'__w>,)*
+                #(#named_field_idents: <#field_types as #path::query::WorldQuery>::Fetch<'__w>,)*
+                #marker_name: &'__w (),
             }
 
             // SAFETY: `update_component_access` and `update_archetype_component_access` are called on every field
@@ -223,14 +258,15 @@ pub fn derive_world_query_impl(input: TokenStream) -> TokenStream {
                     _this_run: #path::component::Tick,
                 ) -> <Self as #path::query::WorldQuery>::Fetch<'__w> {
                     #fetch_struct_name {
-                        #(#field_idents:
+                        #(#named_field_idents:
                             <#field_types>::init_fetch(
                                 _world,
-                                &state.#field_idents,
+                                &state.#named_field_idents,
                                 _last_run,
                                 _this_run,
                             ),
                         )*
+                        #marker_name: &(),
                     }
                 }
 
@@ -239,8 +275,9 @@ pub fn derive_world_query_impl(input: TokenStream) -> TokenStream {
                 ) -> <Self as #path::query::WorldQuery>::Fetch<'__w> {
                     #fetch_struct_name {
                         #(
-                            #field_idents: <#field_types>::clone_fetch(& _fetch. #field_idents),
+                            #named_field_idents: <#field_types>::clone_fetch(& _fetch. #named_field_idents),
                         )*
+                        #marker_name: &(),
                     }
                 }
 
@@ -256,7 +293,7 @@ pub fn derive_world_query_impl(input: TokenStream) -> TokenStream {
                     _archetype: &'__w #path::archetype::Archetype,
                     _table: &'__w #path::storage::Table
                 ) {
-                    #(<#field_types>::set_archetype(&mut _fetch.#field_idents, &_state.#field_idents, _archetype, _table);)*
+                    #(<#field_types>::set_archetype(&mut _fetch.#named_field_idents, &_state.#named_field_idents, _archetype, _table);)*
                 }
 
                 /// SAFETY: we call `set_table` for each member that implements `Fetch`
@@ -266,7 +303,7 @@ pub fn derive_world_query_impl(input: TokenStream) -> TokenStream {
                     _state: &Self::State,
                     _table: &'__w #path::storage::Table
                 ) {
-                    #(<#field_types>::set_table(&mut _fetch.#field_idents, &_state.#field_idents, _table);)*
+                    #(<#field_types>::set_table(&mut _fetch.#named_field_idents, &_state.#named_field_idents, _table);)*
                 }
 
                 /// SAFETY: we call `fetch` for each member that implements `Fetch`.
@@ -277,7 +314,7 @@ pub fn derive_world_query_impl(input: TokenStream) -> TokenStream {
                     _table_row: #path::storage::TableRow,
                 ) -> <Self as #path::query::WorldQuery>::Item<'__w> {
                     Self::Item {
-                        #(#field_idents: <#field_types>::fetch(&mut _fetch.#field_idents, _entity, _table_row),)*
+                        #(#field_idents: <#field_types>::fetch(&mut _fetch.#named_field_idents, _entity, _table_row),)*
                     }
                 }
 
@@ -288,11 +325,11 @@ pub fn derive_world_query_impl(input: TokenStream) -> TokenStream {
                     _entity: #path::entity::Entity,
                     _table_row: #path::storage::TableRow,
                 ) -> bool {
-                    true #(&& <#field_types>::filter_fetch(&mut _fetch.#field_idents, _entity, _table_row))*
+                    true #(&& <#field_types>::filter_fetch(&mut _fetch.#named_field_idents, _entity, _table_row))*
                 }
 
                 fn update_component_access(state: &Self::State, _access: &mut #path::query::FilteredAccess<#path::component::ComponentId>) {
-                    #( <#field_types>::update_component_access(&state.#field_idents, _access); )*
+                    #( <#field_types>::update_component_access(&state.#named_field_idents, _access); )*
                 }
 
                 fn update_archetype_component_access(
@@ -301,18 +338,18 @@ pub fn derive_world_query_impl(input: TokenStream) -> TokenStream {
                     _access: &mut #path::query::Access<#path::archetype::ArchetypeComponentId>
                 ) {
                     #(
-                        <#field_types>::update_archetype_component_access(&state.#field_idents, _archetype, _access);
+                        <#field_types>::update_archetype_component_access(&state.#named_field_idents, _archetype, _access);
                     )*
                 }
 
                 fn init_state(world: &mut #path::world::World) -> #state_struct_name #user_ty_generics {
                     #state_struct_name {
-                        #(#field_idents: <#field_types>::init_state(world),)*
+                        #(#named_field_idents: <#field_types>::init_state(world),)*
                     }
                 }
 
                 fn matches_component_set(state: &Self::State, _set_contains_id: &impl Fn(#path::component::ComponentId) -> bool) -> bool {
-                    true #(&& <#field_types>::matches_component_set(&state.#field_idents, _set_contains_id))*
+                    true #(&& <#field_types>::matches_component_set(&state.#named_field_idents, _set_contains_id))*
                 }
             }
         };
@@ -328,7 +365,7 @@ pub fn derive_world_query_impl(input: TokenStream) -> TokenStream {
             #[doc = "`]."]
             #[automatically_derived]
             #visibility struct #read_only_struct_name #user_impl_generics #user_where_clauses {
-                #( #field_visibilities #field_idents: #read_only_field_types, )*
+                #( #field_visibilities #named_field_idents: #read_only_field_types, )*
             }
 
             #readonly_state
@@ -374,7 +411,7 @@ pub fn derive_world_query_impl(input: TokenStream) -> TokenStream {
             #[doc = "`], used for caching."]
             #[automatically_derived]
             #visibility struct #state_struct_name #user_impl_generics #user_where_clauses {
-                #(#field_idents: <#field_types as #path::query::WorldQuery>::State,)*
+                #(#named_field_idents: <#field_types as #path::query::WorldQuery>::State,)*
             }
 
             #mutable_impl

crates/bevy_ecs/src/query/fetch.rs

@@ -55,8 +55,7 @@ use std::{cell::UnsafeCell, marker::PhantomData};
 /// - Methods can be implemented for the query items.
 /// - There is no hardcoded limit on the number of elements.
 ///
-/// This trait can only be derived if each field also implements `WorldQuery`.
-/// The derive macro only supports regular structs (structs with named fields).
+/// This trait can only be derived for structs, if each field also implements `WorldQuery`.
 ///
 /// ```
 /// # use bevy_ecs::prelude::*;
@@ -1468,11 +1467,37 @@ unsafe impl<T: ?Sized> ReadOnlyWorldQuery for PhantomData<T> {}
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::{self as bevy_ecs, system::Query};
+    use crate::{
+        self as bevy_ecs,
+        system::{assert_is_system, Query},
+    };
 
     #[derive(Component)]
     pub struct A;
 
+    #[derive(Component)]
+    pub struct B;
+
+    // Tests that each variant of struct can be used as a `WorldQuery`.
+    #[test]
+    fn world_query_struct_variants() {
+        #[derive(WorldQuery)]
+        pub struct NamedQuery {
+            id: Entity,
+            a: &'static A,
+        }
+
+        #[derive(WorldQuery)]
+        pub struct TupleQuery(&'static A, &'static B);
+
+        #[derive(WorldQuery)]
+        pub struct UnitQuery;
+
+        fn my_system(_: Query<(NamedQuery, TupleQuery, UnitQuery)>) {}
+
+        assert_is_system(my_system);
+    }
+
     // Compile test for https://github.com/bevyengine/bevy/pull/8030.
     #[test]
     fn world_query_phantom_data() {