Allow only parentheses in #[reflect(...)]

Fixes #8906

Author: dloukadakis

crates/bevy_reflect/derive/src/derive_data.rs

@@ -14,7 +14,7 @@ use crate::{
 };
 use bevy_macro_utils::ResultSifter;
 use quote::{format_ident, quote, ToTokens};
-use syn::token::Comma;
+use syn::{token::Comma, MacroDelimiter};
 
 use crate::enum_utility::{EnumVariantOutputData, ReflectCloneVariantBuilder, VariantBuilder};
 use crate::field_attributes::CloneBehavior;
@@ -197,7 +197,16 @@ impl<'a> ReflectDerive<'a> {
         for attribute in &input.attrs {
             match &attribute.meta {
                 Meta::List(meta_list) if meta_list.path.is_ident(REFLECT_ATTRIBUTE_NAME) => {
-                    container_attributes.parse_meta_list(meta_list, provenance.trait_)?;
+                    if let MacroDelimiter::Paren(_) = meta_list.delimiter {
+                        container_attributes.parse_meta_list(meta_list, provenance.trait_)?;
+                    } else {
+                        return Err(syn::Error::new(
+                            meta_list.delimiter.span().join(),
+                            format_args!(
+                                "`#[{REFLECT_ATTRIBUTE_NAME}(\"...\")]` must use parentheses `(` and `)`"
+                            ),
+                        ));
+                    }
                 }
                 Meta::NameValue(pair) if pair.path.is_ident(TYPE_PATH_ATTRIBUTE_NAME) => {
                     let syn::Expr::Lit(syn::ExprLit {

crates/bevy_render/src/sync_world.rs

@@ -119,7 +119,7 @@ impl Plugin for SyncWorldPlugin {
 /// [`ExtractComponentPlugin`]: crate::extract_component::ExtractComponentPlugin
 /// [`SyncComponentPlugin`]: crate::sync_component::SyncComponentPlugin
 #[derive(Component, Copy, Clone, Debug, Default, Reflect)]
-#[reflect[Component, Default, Clone]]
+#[reflect(Component, Default, Clone)]
 #[component(storage = "SparseSet")]
 pub struct SyncToRenderWorld;
 

release-content/migration-guides/reflect_parentheses.md

@@ -0,0 +1,26 @@
+---
+title: "`#[reflect(...)]` now supports only parentheses" 
+pull_requests: [21400]
+---
+
+Previously, the `#[reflect(...)]` attribute of the `Reflect` derive macro
+supported parentheses, braces, or brackets, to standardize the syntax going
+forward, it now supports only parentheses.
+
+```rust
+/// before
+#[derive(Clone, Reflect)
+#[reflect[Clone]]
+
+/// after
+#[derive(Clone, Reflect)
+#[reflect(Clone)]
+
+/// before
+#[derive(Clone, Reflect)
+#[reflect{Clone}]
+
+/// after
+#[derive(Clone, Reflect)
+#[reflect(Clone)]
+```