Support tuple structs with #[derive(SystemParam)] (#6957)

# Objective

Currently, only named structs can be used with the `SystemParam` derive macro.

## Solution

Remove the restriction. Tuple structs and unit structs are now supported.

---

## Changelog

+ Added support for tuple structs and unit structs to the `SystemParam` derive macro.

crates/bevy_ecs/macros/src/lib.rs

@@ -327,13 +327,14 @@ static SYSTEM_PARAM_ATTRIBUTE_NAME: &str = "system_param";
 #[proc_macro_derive(SystemParam, attributes(system_param))]
 pub fn derive_system_param(input: TokenStream) -> TokenStream {
     let ast = parse_macro_input!(input as DeriveInput);
-    let fields = match get_named_struct_fields(&ast.data) {
+    let syn::Data::Struct(syn::DataStruct { fields: field_definitions, ..}) = ast.data else {
-        Ok(fields) => &fields.named,
+        return syn::Error::new(ast.span(), "Invalid `SystemParam` type: expected a `struct`")
-        Err(e) => return e.into_compile_error().into(),
+            .into_compile_error()
+            .into();
     };
     let path = bevy_ecs_path();
 
-    let field_attributes = fields
+    let field_attributes = field_definitions
         .iter()
         .map(|field| {
             (
@@ -368,9 +369,16 @@ pub fn derive_system_param(input: TokenStream) -> TokenStream {
             ignored_fields.push(field.ident.as_ref().unwrap());
             ignored_field_types.push(&field.ty);
         } else {
-            fields.push(field.ident.as_ref().unwrap());
+            let i = Index::from(i);
+            fields.push(
+                field
+                    .ident
+                    .as_ref()
+                    .map(|f| quote! { #f })
+                    .unwrap_or_else(|| quote! { #i }),
+            );
             field_types.push(&field.ty);
-            field_indices.push(Index::from(i));
+            field_indices.push(i);
         }
     }
 

crates/bevy_ecs/src/system/system_param.rs

@@ -1662,5 +1662,11 @@ mod tests {
     }
 
     #[derive(SystemParam)]
-    pub struct UnitParam {}
+    pub struct UnitParam;
+
+    #[derive(SystemParam)]
+    pub struct TupleParam<'w, 's, R: Resource, L: FromWorld + Send + 'static>(
+        Res<'w, R>,
+        Local<'s, L>,
+    );
 }