From f8f78b7475b0d0f441decbcdd2cb53653f0f8632 Mon Sep 17 00:00:00 2001
From: JoJoJet <21144246+JoJoJet@users.noreply.github.com>
Date: Mon, 9 Jan 2023 22:20:10 +0000
Subject: [PATCH] Add `Mut::reborrow` (#7114)

# Objective

- In some cases, you need a `Mut<T>` pointer, but you only have a mutable reference to one. There is no easy way of converting `&'a mut Mut<'_, T>` -> `Mut<'a, T>` outside of the engine.

### Example (Before)

```rust
fn do_with_mut<T>(val: Mut<T>) { ... }

for x: Mut<T> in &mut query {
    // The function expects a `Mut<T>`, so `x` gets moved here.
    do_with_mut(x);
    // Error: use of moved value.
    do_a_thing(&x);
}
```

## Solution

- Add the function `reborrow`, which performs the mapping. This is analogous to `PtrMut::reborrow`.

### Example (After)

```rust
fn do_with_mut<T>(val: Mut<T>) { ... }

for x: Mut<T> in &mut query {
    // We reborrow `x`, so the original does not get moved.
    do_with_mut(x.reborrow());
    // Works fine.
    do_a_thing(&x);
}
```

---

## Changelog

- Added the method `reborrow` to `Mut`, `ResMut`, `NonSendMut`, and `MutUntyped`.
---
 crates/bevy_ecs/src/change_detection.rs | 37 +++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/crates/bevy_ecs/src/change_detection.rs b/crates/bevy_ecs/src/change_detection.rs
index d9ce86e60294d..ac28a8c29a11b 100644
--- a/crates/bevy_ecs/src/change_detection.rs
+++ b/crates/bevy_ecs/src/change_detection.rs
@@ -209,6 +209,25 @@ macro_rules! impl_methods {
                 self.value
             }
 
+            /// Returns a `Mut<>` with a smaller lifetime.
+            /// This is useful if you have `&mut
+            #[doc = stringify!($name)]
+            /// <T>`, but you need a `Mut<T>`.
+            ///
+            /// Note that calling [`DetectChanges::set_last_changed`] on the returned value
+            /// will not affect the original.
+            pub fn reborrow(&mut self) -> Mut<'_, $target> {
+                Mut {
+                    value: self.value,
+                    ticks: Ticks {
+                        added: self.ticks.added,
+                        changed: self.ticks.changed,
+                        last_change_tick: self.ticks.last_change_tick,
+                        change_tick: self.ticks.change_tick,
+                    }
+                }
+            }
+
             /// Maps to an inner value by applying a function to the contained reference, without flagging a change.
             ///
             /// You should never modify the argument passed to the closure -- if you want to modify the data
@@ -427,6 +446,24 @@ impl<'a> MutUntyped<'a> {
         self.value
     }
 
+    /// Returns a [`MutUntyped`] with a smaller lifetime.
+    /// This is useful if you have `&mut MutUntyped`, but you need a `MutUntyped`.
+    ///
+    /// Note that calling [`DetectChanges::set_last_changed`] on the returned value
+    /// will not affect the original.
+    #[inline]
+    pub fn reborrow(&mut self) -> MutUntyped {
+        MutUntyped {
+            value: self.value.reborrow(),
+            ticks: Ticks {
+                added: self.ticks.added,
+                changed: self.ticks.changed,
+                last_change_tick: self.ticks.last_change_tick,
+                change_tick: self.ticks.change_tick,
+            },
+        }
+    }
+
     /// Returns a pointer to the value without taking ownership of this smart pointer, marking it as changed.
     ///
     /// In order to avoid marking the value as changed, you need to call [`bypass_change_detection`](DetectChanges::bypass_change_detection).