add an attribute for specifying unconditionally readonly or mutable p…

…arams

crates/bevy_ecs/macros/src/lib.rs

@@ -333,6 +333,13 @@ struct SystemParamFieldAttributes {
 
 static SYSTEM_PARAM_ATTRIBUTE_NAME: &str = "system_param";
 
+#[derive(Debug)]
+enum ParamAccess {
+    Auto,
+    ReadOnly,
+    Mutable,
+}
+
 /// Implement `SystemParam` to use a struct as a parameter in a system
 #[proc_macro_derive(SystemParam, attributes(system_param))]
 pub fn derive_system_param(input: TokenStream) -> TokenStream {
@@ -344,6 +351,28 @@ pub fn derive_system_param(input: TokenStream) -> TokenStream {
     };
     let path = bevy_ecs_path();
 
+    let mut access = ParamAccess::Auto;
+    for attr in &ast.attrs {
+        let expected_attr_name = format_ident!("{SYSTEM_PARAM_ATTRIBUTE_NAME}");
+        if attr.path.get_ident() != Some(&expected_attr_name) {
+            continue;
+        }
+
+        syn::custom_keyword!(read_only);
+        syn::custom_keyword!(mutable);
+
+        attr.parse_args_with(|input: ParseStream| {
+            if input.parse::<Option<read_only>>()?.is_some() {
+                access = ParamAccess::ReadOnly;
+            }
+            if input.parse::<Option<mutable>>()?.is_some() {
+                access = ParamAccess::Mutable;
+            }
+            Ok(())
+        })
+        .expect("Invalid 'system_param' attribute format.");
+    }
+
     let field_attributes = field_definitions
         .iter()
         .map(|field| {

crates/bevy_ecs/src/system/system_param.rs

@@ -1811,6 +1811,7 @@ mod tests {
     );
 
     #[derive(SystemParam)]
+    #[system_param(read_only)]
     pub struct EncapsulatedParam<'s> {
         p: PrivateParam<'s>,
     }