Add a compile_fail test

crates/bevy_ecs_compile_fail_tests/tests/ui/system_param_derive_readonly.rs

@@ -0,0 +1,25 @@
+use bevy_ecs::prelude::*;
+use bevy_ecs::system::{ReadOnlySystemParamFetch, SystemParam, SystemState};
+
+#[derive(Component)]
+struct Foo;
+
+#[derive(SystemParam)]
+struct Mutable<'w, 's> {
+    a: Query<'w, 's, &'static mut Foo>,
+}
+
+fn main() {
+    // Ideally we'd use:
+    // let mut world = World::default();
+    // let state = SystemState::<Mutable>::new(&mut world);
+    // state.get(&world);
+    // But that makes the test show absolute paths
+    assert_readonly::<Mutable>();
+}
+
+fn assert_readonly<P: SystemParam>()
+where
+    <P as SystemParam>::Fetch: ReadOnlySystemParamFetch,
+{
+}

crates/bevy_ecs_compile_fail_tests/tests/ui/system_param_derive_readonly.stderr

@@ -0,0 +1,25 @@
+warning: unused import: `SystemState`
+ --> tests/ui/system_param_derive_readonly.rs:2:63
+  |
+2 | use bevy_ecs::system::{ReadOnlySystemParamFetch, SystemParam, SystemState};
+  |                                                               ^^^^^^^^^^^
+  |
+  = note: `#[warn(unused_imports)]` on by default
+
+error[E0277]: the trait bound `for<'x> WriteFetch<'x, Foo>: ReadOnlyFetch` is not satisfied
+  --> tests/ui/system_param_derive_readonly.rs:18:5
+   |
+18 |     assert_readonly::<Mutable>();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `for<'x> ReadOnlyFetch` is not implemented for `WriteFetch<'x, Foo>`
+   |
+   = note: required because of the requirements on the impl of `ReadOnlySystemParamFetch` for `QueryState<&'static mut Foo>`
+   = note: 2 redundant requirements hidden
+   = note: required because of the requirements on the impl of `ReadOnlySystemParamFetch` for `_::FetchState<(QueryState<&'static mut Foo>,)>`
+note: required by a bound in `assert_readonly`
+  --> tests/ui/system_param_derive_readonly.rs:23:32
+   |
+21 | fn assert_readonly<P: SystemParam>()
+   |    --------------- required by a bound in this
+22 | where
+23 |     <P as SystemParam>::Fetch: ReadOnlySystemParamFetch,
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `assert_readonly`