reflect: add insert and remove methods to List (bevyengine#7063)

# Objective

- Fixes bevyengine#7061

## Solution

- Add and implement `insert` and `remove` methods for `List`.

---

## Changelog

- Added `insert` and `remove` methods to `List`.
- Changed the `push` and `pop` methods on `List` to have default implementations.

## Migration Guide

- Manual implementors of `List` need to implement the new methods `insert` and `remove` and 
consider whether to use the new default implementation of `push` and `pop`.

Co-authored-by: radiish <thesethskigamer@gmail.com>

crates/bevy_reflect/src/impls/smallvec.rs

@@ -49,6 +49,22 @@ impl<T: smallvec::Array + Send + Sync + 'static> List for SmallVec<T>
 where
     T::Item: FromReflect,
 {
+    fn insert(&mut self, index: usize, value: Box<dyn Reflect>) {
+        let value = value.take::<T::Item>().unwrap_or_else(|value| {
+            <T as smallvec::Array>::Item::from_reflect(&*value).unwrap_or_else(|| {
+                panic!(
+                    "Attempted to insert invalid value of type {}.",
+                    value.type_name()
+                )
+            })
+        });
+        SmallVec::insert(self, index, value);
+    }
+
+    fn remove(&mut self, index: usize) -> Box<dyn Reflect> {
+        Box::new(self.remove(index))
+    }
+
     fn push(&mut self, value: Box<dyn Reflect>) {
         let value = value.take::<T::Item>().unwrap_or_else(|value| {
             <T as smallvec::Array>::Item::from_reflect(&*value).unwrap_or_else(|| {

crates/bevy_reflect/src/impls/std.rs

@@ -179,7 +179,7 @@ impl_from_reflect_value!(NonZeroU8);
 impl_from_reflect_value!(NonZeroI8);
 
 macro_rules! impl_reflect_for_veclike {
-    ($ty:ty, $push:expr, $pop:expr, $sub:ty) => {
+    ($ty:ty, $insert:expr, $remove:expr, $push:expr, $pop:expr, $sub:ty) => {
         impl<T: FromReflect> Array for $ty {
             #[inline]
             fn get(&self, index: usize) -> Option<&dyn Reflect> {
@@ -213,6 +213,22 @@ macro_rules! impl_reflect_for_veclike {
         }
 
         impl<T: FromReflect> List for $ty {
+            fn insert(&mut self, index: usize, value: Box<dyn Reflect>) {
+                let value = value.take::<T>().unwrap_or_else(|value| {
+                    T::from_reflect(&*value).unwrap_or_else(|| {
+                        panic!(
+                            "Attempted to insert invalid value of type {}.",
+                            value.type_name()
+                        )
+                    })
+                });
+                $insert(self, index, value);
+            }
+
+            fn remove(&mut self, index: usize) -> Box<dyn Reflect> {
+                Box::new($remove(self, index))
+            }
+
             fn push(&mut self, value: Box<dyn Reflect>) {
                 let value = value.take::<T>().unwrap_or_else(|value| {
                     T::from_reflect(&*value).unwrap_or_else(|| {
@@ -328,9 +344,11 @@ macro_rules! impl_reflect_for_veclike {
     };
 }
 
-impl_reflect_for_veclike!(Vec<T>, Vec::push, Vec::pop, [T]);
+impl_reflect_for_veclike!(Vec<T>, Vec::insert, Vec::remove, Vec::push, Vec::pop, [T]);
 impl_reflect_for_veclike!(
     VecDeque<T>,
+    VecDeque::insert,
+    VecDeque::remove,
     VecDeque::push_back,
     VecDeque::pop_back,
     VecDeque::<T>

crates/bevy_reflect/src/list.rs

@@ -9,18 +9,43 @@ use crate::{
 
 /// An ordered, mutable list of [Reflect] items. This corresponds to types like [`std::vec::Vec`].
 ///
-/// This is a sub-trait of [`Array`] as it implements a [`push`](List::push) function, allowing
-/// it's internal size to grow.
+/// This is a sub-trait of [`Array`], however as it implements [insertion](List::insert) and [removal](List::remove),
+/// it's internal size may change.
 ///
 /// This trait expects index 0 to contain the _front_ element.
 /// The _back_ element must refer to the element with the largest index.
 /// These two rules above should be upheld by manual implementors.
+///
+/// [`push`](List::push) and [`pop`](List::pop) have default implementations,
+/// however it may be faster to implement them manually.
 pub trait List: Reflect + Array {
+    /// Inserts an element at position `index` within the list,
+    /// shifting all elements after it towards the back of the list.
+    ///
+    /// # Panics
+    /// Panics if `index > len`.
+    fn insert(&mut self, index: usize, element: Box<dyn Reflect>);
+
+    /// Removes and returns the element at position `index` within the list,
+    /// shifting all elements before it towards the front of the list.
+    ///
+    /// # Panics
+    /// Panics if `index` is out of bounds.
+    fn remove(&mut self, index: usize) -> Box<dyn Reflect>;
+
     /// Appends an element to the _back_ of the list.
-    fn push(&mut self, value: Box<dyn Reflect>);
+    fn push(&mut self, value: Box<dyn Reflect>) {
+        self.insert(self.len(), value);
+    }
 
     /// Removes the _back_ element from the list and returns it, or [`None`] if it is empty.
-    fn pop(&mut self) -> Option<Box<dyn Reflect>>;
+    fn pop(&mut self) -> Option<Box<dyn Reflect>> {
+        if self.is_empty() {
+            None
+        } else {
+            Some(self.remove(self.len() - 1))
+        }
+    }
 
     /// Clones the list, producing a [`DynamicList`].
     fn clone_dynamic(&self) -> DynamicList {
@@ -174,6 +199,14 @@ impl Array for DynamicList {
 }
 
 impl List for DynamicList {
+    fn insert(&mut self, index: usize, element: Box<dyn Reflect>) {
+        self.values.insert(index, element);
+    }
+
+    fn remove(&mut self, index: usize) -> Box<dyn Reflect> {
+        self.values.remove(index)
+    }
+
     fn push(&mut self, value: Box<dyn Reflect>) {
         DynamicList::push_box(self, value);
     }