Allows conversion of mutable queries to immutable queries (#5376)

# Objective

- Allows conversion of mutable queries to immutable queries.
- Fixes #4606

## Solution

- Add `to_readonly` method on `Query`, which uses `QueryState::as_readonly`
- `AsRef` is not feasible because creation of new queries is needed.

---

## Changelog

### Added

- Allows conversion of mutable queries to immutable queries using `Query::to_readonly`.

crates/bevy_ecs/src/system/mod.rs

@@ -927,6 +927,129 @@ mod tests {
         }
     }
 
+    #[test]
+    fn convert_mut_to_immut() {
+        {
+            let mut world = World::new();
+
+            fn mutable_query(mut query: Query<&mut A>) {
+                for _ in &mut query {}
+
+                immutable_query(query.to_readonly());
+            }
+
+            fn immutable_query(_: Query<&A>) {}
+
+            let mut sys = IntoSystem::into_system(mutable_query);
+            sys.initialize(&mut world);
+        }
+
+        {
+            let mut world = World::new();
+
+            fn mutable_query(mut query: Query<Option<&mut A>>) {
+                for _ in &mut query {}
+
+                immutable_query(query.to_readonly());
+            }
+
+            fn immutable_query(_: Query<Option<&A>>) {}
+
+            let mut sys = IntoSystem::into_system(mutable_query);
+            sys.initialize(&mut world);
+        }
+
+        {
+            let mut world = World::new();
+
+            fn mutable_query(mut query: Query<(&mut A, &B)>) {
+                for _ in &mut query {}
+
+                immutable_query(query.to_readonly());
+            }
+
+            fn immutable_query(_: Query<(&A, &B)>) {}
+
+            let mut sys = IntoSystem::into_system(mutable_query);
+            sys.initialize(&mut world);
+        }
+
+        {
+            let mut world = World::new();
+
+            fn mutable_query(mut query: Query<(&mut A, &mut B)>) {
+                for _ in &mut query {}
+
+                immutable_query(query.to_readonly());
+            }
+
+            fn immutable_query(_: Query<(&A, &B)>) {}
+
+            let mut sys = IntoSystem::into_system(mutable_query);
+            sys.initialize(&mut world);
+        }
+
+        {
+            let mut world = World::new();
+
+            fn mutable_query(mut query: Query<(&mut A, &mut B), With<C>>) {
+                for _ in &mut query {}
+
+                immutable_query(query.to_readonly());
+            }
+
+            fn immutable_query(_: Query<(&A, &B), With<C>>) {}
+
+            let mut sys = IntoSystem::into_system(mutable_query);
+            sys.initialize(&mut world);
+        }
+
+        {
+            let mut world = World::new();
+
+            fn mutable_query(mut query: Query<(&mut A, &mut B), Without<C>>) {
+                for _ in &mut query {}
+
+                immutable_query(query.to_readonly());
+            }
+
+            fn immutable_query(_: Query<(&A, &B), Without<C>>) {}
+
+            let mut sys = IntoSystem::into_system(mutable_query);
+            sys.initialize(&mut world);
+        }
+
+        {
+            let mut world = World::new();
+
+            fn mutable_query(mut query: Query<(&mut A, &mut B), Added<C>>) {
+                for _ in &mut query {}
+
+                immutable_query(query.to_readonly());
+            }
+
+            fn immutable_query(_: Query<(&A, &B), Added<C>>) {}
+
+            let mut sys = IntoSystem::into_system(mutable_query);
+            sys.initialize(&mut world);
+        }
+
+        {
+            let mut world = World::new();
+
+            fn mutable_query(mut query: Query<(&mut A, &mut B), Changed<C>>) {
+                for _ in &mut query {}
+
+                immutable_query(query.to_readonly());
+            }
+
+            fn immutable_query(_: Query<(&A, &B), Changed<C>>) {}
+
+            let mut sys = IntoSystem::into_system(mutable_query);
+            sys.initialize(&mut world);
+        }
+    }
+
     #[test]
     fn update_archetype_component_access_works() {
         use std::collections::HashSet;

crates/bevy_ecs/src/system/query.rs

@@ -267,6 +267,24 @@ impl<'w, 's, Q: WorldQuery, F: WorldQuery> Query<'w, 's, Q, F> {
         }
     }
 
+    /// Downgrades all data accessed in this query to a read-only form.
+    ///
+    /// For example, `Query<(&mut A, &B, &mut C), With<D>>` will become `Query<(&A, &B, &C), With<D>>`.
+    /// This can be useful when working around the borrow checker,
+    /// or reusing functionality between systems via functions that accept query types.
+    pub fn to_readonly(&self) -> Query<'_, '_, Q::ReadOnly, F::ReadOnly> {
+        let new_state = self.state.as_readonly();
+        // SAFETY: This is memory safe because it turns the query immutable.
+        unsafe {
+            Query::new(
+                self.world,
+                new_state,
+                self.last_change_tick,
+                self.change_tick,
+            )
+        }
+    }
+
     /// Returns an [`Iterator`] over the query results.
     ///
     /// This can only return immutable data (mutable data will be cast to an immutable form).

crates/bevy_ecs_compile_fail_tests/tests/ui/query_to_readonly.rs

@@ -0,0 +1,75 @@
+use bevy_ecs::prelude::*;
+
+#[derive(Component, Debug)]
+struct Foo;
+
+fn for_loops(mut query: Query<&mut Foo>) {
+    // this should fail to compile
+    for _ in query.iter_mut() {
+        for _ in query.to_readonly().iter() {}
+    }
+
+    // this should fail to compile
+    for _ in query.to_readonly().iter() {
+        for _ in query.iter_mut() {}
+    }
+
+    // this should *not* fail to compile
+    for _ in query.to_readonly().iter() {
+        for _ in query.to_readonly().iter() {}
+    }
+
+    // this should *not* fail to compile
+    for _ in query.to_readonly().iter() {
+        for _ in query.iter() {}
+    }
+
+    // this should *not* fail to compile
+    for _ in query.iter() {
+        for _ in query.to_readonly().iter() {}
+    }
+}
+
+fn single_mut_query(mut query: Query<&mut Foo>) {
+    // this should fail to compile
+    {
+        let mut mut_foo = query.single_mut();
+
+        // This solves "temporary value dropped while borrowed"
+        let readonly_query = query.to_readonly();
+        
+        let ref_foo = readonly_query.single();
+    
+        *mut_foo = Foo;
+
+        println!("{ref_foo:?}");
+    }
+
+    // this should fail to compile
+    {
+        // This solves "temporary value dropped while borrowed"
+        let readonly_query = query.to_readonly();
+
+        let ref_foo = readonly_query.single();
+
+        let mut mut_foo = query.single_mut();
+
+        println!("{ref_foo:?}");
+
+        *mut_foo = Foo;
+    }
+
+    // this should *not* fail to compile
+    {
+        // This solves "temporary value dropped while borrowed"
+        let readonly_query = query.to_readonly();
+
+        let readonly_foo = readonly_query.single();
+
+        let query_foo = query.single();
+
+        println!("{readonly_foo:?}, {query_foo:?}");
+    }
+}
+
+fn main() {}

crates/bevy_ecs_compile_fail_tests/tests/ui/query_to_readonly.stderr

@@ -0,0 +1,45 @@
+error[E0502]: cannot borrow `query` as immutable because it is also borrowed as mutable
+ --> tests/ui/query_to_readonly.rs:9:18
+  |
+8 |     for _ in query.iter_mut() {
+  |              ----------------
+  |              |
+  |              mutable borrow occurs here
+  |              mutable borrow later used here
+9 |         for _ in query.to_readonly().iter() {}
+  |                  ^^^^^^^^^^^^^^^^^^^ immutable borrow occurs here
+
+error[E0502]: cannot borrow `query` as mutable because it is also borrowed as immutable
+  --> tests/ui/query_to_readonly.rs:14:18
+   |
+13 |     for _ in query.to_readonly().iter() {
+   |              --------------------------
+   |              |
+   |              immutable borrow occurs here
+   |              immutable borrow later used here
+14 |         for _ in query.iter_mut() {}
+   |                  ^^^^^^^^^^^^^^^^ mutable borrow occurs here
+
+error[E0502]: cannot borrow `query` as immutable because it is also borrowed as mutable
+  --> tests/ui/query_to_readonly.rs:39:30
+   |
+36 |         let mut mut_foo = query.single_mut();
+   |                           ------------------ mutable borrow occurs here
+...
+39 |         let readonly_query = query.to_readonly();
+   |                              ^^^^^^^^^^^^^^^^^^^ immutable borrow occurs here
+...
+43 |         *mut_foo = Foo;
+   |          ------- mutable borrow later used here
+
+error[E0502]: cannot borrow `query` as mutable because it is also borrowed as immutable
+  --> tests/ui/query_to_readonly.rs:55:27
+   |
+51 |         let readonly_query = query.to_readonly();
+   |                              ------------------- immutable borrow occurs here
+...
+55 |         let mut mut_foo = query.single_mut();
+   |                           ^^^^^^^^^^^^^^^^^^ mutable borrow occurs here
+56 |
+57 |         println!("{ref_foo:?}");
+   |                    ------- immutable borrow later used here