Added try_map_unchanged. (#17653)

# Objective

Allow mapping `Mut` to another value while returning a custom error on
failure.

## Solution

Added `try_map_unchanged` to `Mut` which returns a `Result` instead of
`Option` .

Author: mintlu8

crates/bevy_ecs/src/change_detection.rs

@@ -494,6 +494,20 @@ macro_rules! impl_methods {
                 })
             }
 
+            /// Optionally maps to an inner value by applying a function to the contained reference, returns an error on failure.
+            /// This is useful in a situation where you need to convert a `Mut<T>` to a `Mut<U>`, but only if `T` contains `U`.
+            ///
+            /// As with `map_unchanged`, you should never modify the argument passed to the closure.
+            pub fn try_map_unchanged<U: ?Sized, E>(self, f: impl FnOnce(&mut $target) -> Result<&mut U, E>) -> Result<Mut<'w, U>, E> {
+                let value = f(self.value);
+                value.map(|value| Mut {
+                    value,
+                    ticks: self.ticks,
+                    #[cfg(feature = "track_location")]
+                    changed_by: self.changed_by,
+                })
+            }
+
             /// Allows you access to the dereferenced value of this pointer without immediately
             /// triggering change detection.
             pub fn as_deref_mut(&mut self) -> Mut<'_, <$target as Deref>::Target>