From dfdd6bf0ec98dc0ae78dfa2e5d953116d2d596f7 Mon Sep 17 00:00:00 2001
From: JoJoJet <21144246+JoJoJet@users.noreply.github.com>
Date: Wed, 28 Dec 2022 23:24:09 -0500
Subject: [PATCH] implement unconditionally read-only params

---
 crates/bevy_ecs/macros/src/lib.rs | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/crates/bevy_ecs/macros/src/lib.rs b/crates/bevy_ecs/macros/src/lib.rs
index 8dbadc4393069..5006196d0bb3d 100644
--- a/crates/bevy_ecs/macros/src/lib.rs
+++ b/crates/bevy_ecs/macros/src/lib.rs
@@ -518,7 +518,21 @@ pub fn derive_system_param(input: TokenStream) -> TokenStream {
                 {}
             }
         }
-        ParamAccess::ReadOnly => todo!(),
+        ParamAccess::ReadOnly => {
+            quote! {
+                fn __assert_field_is_read_only_system_param<T: #path::system::ReadOnlySystemParam>() {}
+                fn __assert_each_field_is_read_only_system_param #impl_generics () #where_clause {
+                    #(
+                        __assert_field_is_read_only_system_param::<#field_types>();
+                    )*
+                }
+
+                // SAFETY: The above code will cause a compile error if each field does not implement `ReadOnlySystemParam`.
+                // Since each field is `ReadOnlySystemParam`, this can only read from the `World`
+                unsafe impl #impl_generics #path::system::ReadOnlySystemParam for #struct_name #ty_generics #where_clause
+                {}
+            }
+        }
         ParamAccess::Mutable => quote! {},
     };