Add compile fail tests for custom where

crates/bevy_reflect_compile_fail_tests/tests/reflect_derive/custom_where.fail.rs

@@ -0,0 +1,21 @@
+use bevy_reflect::Reflect;
+use std::marker::PhantomData;
+
+#[derive(Clone)]
+struct ReflectMyTrait;
+
+#[derive(Reflect)]
+#[reflect(MyTrait, where)]
+pub struct Foo<T> {
+    value: String,
+    _marker: PhantomData<T>,
+}
+
+#[derive(Reflect)]
+#[reflect(where, MyTrait)]
+pub struct Bar<T> {
+    value: String,
+    _marker: PhantomData<T>,
+}
+
+fn main() {}

crates/bevy_reflect_compile_fail_tests/tests/reflect_derive/custom_where.fail.stderr

@@ -0,0 +1,11 @@
+error: expected identifier, found keyword `where`
+ --> tests/reflect_derive/custom_where.fail.rs:8:20
+  |
+8 | #[reflect(MyTrait, where)]
+  |                    ^^^^^
+
+error: unexpected token
+  --> tests/reflect_derive/custom_where.fail.rs:15:16
+   |
+15 | #[reflect(where, MyTrait)]
+   |                ^

crates/bevy_reflect_compile_fail_tests/tests/reflect_derive/custom_where.pass.rs

@@ -0,0 +1,23 @@
+use bevy_reflect::Reflect;
+use std::marker::PhantomData;
+
+#[derive(Clone)]
+struct ReflectMyTrait;
+
+#[derive(Reflect)]
+#[reflect(MyTrait)]
+#[reflect(where)]
+pub struct Foo<T> {
+    value: String,
+    _marker: PhantomData<T>,
+}
+
+#[derive(Reflect)]
+#[reflect(where)]
+#[reflect(MyTrait)]
+pub struct Bar<T> {
+    value: String,
+    _marker: PhantomData<T>,
+}
+
+fn main() {}