From c6b259fa3f561e70eb996994cf4f0d1b1e2c6edd Mon Sep 17 00:00:00 2001
From: Mikolaj Wielgus <wielgusmikolaj@gmail.com>
Date: Tue, 3 Nov 2026 21:42:01 +0100
Subject: [PATCH] Implement `Index<usize>` and `IndexMut<usize>` for `Deque`

This continues f70f19c7946c0d3dd8ac3c121c03975f6e056274.

This is consistent with standard library's `VecDeque`, which also has
`Index<usize>` and `IndexMut<usize>` implemented.
---
 CHANGELOG.md   |  2 +-
 src/deque.rs   | 28 +++++++++++++++++++++++++++-
 src/vec/mod.rs |  4 ++--
 3 files changed, 30 insertions(+), 4 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index f710f83b05..c913283227 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,7 +22,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Limited max size of `IndexMap` to u16::MAX + 1.
   The implementation for sizes higher than u16::MAX were unsound anyway.
 - Added `shift_remove`, `shift_remove_entry` and `shift_remove_full` to `IndexMap` and `IndexSet`.
-- Implemented `Index` and `IndexMut` for `Vec`.
+- Implemented `Index` and `IndexMut` for `Vec`, `Deque`.
 
 ## [v0.9.3] 2025-04-15
 
diff --git a/src/deque.rs b/src/deque.rs
index a1e4ddf9dd..d2b0aba7ee 100644
--- a/src/deque.rs
+++ b/src/deque.rs
@@ -43,7 +43,7 @@ use core::{
     iter::FusedIterator,
     marker::PhantomData,
     mem::{ManuallyDrop, MaybeUninit},
-    ptr, slice,
+    ops, ptr, slice,
 };
 
 #[cfg(feature = "zeroize")]
@@ -1364,6 +1364,22 @@ where
 
 impl<T: Eq, const N: usize> Eq for Deque<T, N> {}
 
+impl<T, S: VecStorage<T> + ?Sized> ops::Index<usize> for DequeInner<T, S> {
+    type Output = T;
+
+    #[inline]
+    fn index(&self, index: usize) -> &Self::Output {
+        self.get(index).expect("out of bounds access")
+    }
+}
+
+impl<T, S: VecStorage<T> + ?Sized> ops::IndexMut<usize> for DequeInner<T, S> {
+    #[inline]
+    fn index_mut(&mut self, index: usize) -> &mut Self::Output {
+        self.get_mut(index).expect("out of bounds access")
+    }
+}
+
 impl<T, const NS: usize, const ND: usize> TryFrom<[T; NS]> for Deque<T, ND> {
     /// Converts a `[T; NS]` into a `Deque<T, ND>`.
     ///
@@ -1871,6 +1887,16 @@ mod tests {
         assert_eq!(q.pop_front(), None);
     }
 
+    #[test]
+    fn index() {
+        let mut q = Deque::<_, 4>::try_from([1, 2, 3]).unwrap();
+        assert_eq!(q[1], 2);
+        q[1] = 9;
+        assert_eq!(q[1], 9);
+        q[2] += 1;
+        assert_eq!(q[2], 4);
+    }
+
     #[test]
     fn swap() {
         let mut q: Deque<i32, 4> = Deque::new();
diff --git a/src/vec/mod.rs b/src/vec/mod.rs
index 1a6a804ca8..6846203446 100644
--- a/src/vec/mod.rs
+++ b/src/vec/mod.rs
@@ -1943,14 +1943,14 @@ mod tests {
         increment(&mut v, 1);
         assert_eq!(v[1], 10);
 
-        fn get<C>(container: &mut C, index: usize) -> &<C as core::ops::Index<usize>>::Output
+        fn get<C>(container: &C, index: usize) -> &<C as core::ops::Index<usize>>::Output
         where
             C: std::ops::Index<usize>,
         {
             &container[index]
         }
 
-        assert_eq!(*get(&mut v, 1), 10);
+        assert_eq!(*get(&v, 1), 10);
     }
 
     #[test]