From 0f30b7dd87b8555698ca7e7de360b037230e1f23 Mon Sep 17 00:00:00 2001
From: Justus K <justus.k@protonmail.com>
Date: Sun, 13 Dec 2020 10:02:36 +0100
Subject: [PATCH] fix panic if converting ZST Vec to VecDeque

---
 library/alloc/src/collections/vec_deque/mod.rs | 8 ++++++--
 library/alloc/tests/vec_deque.rs               | 7 +++++++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/library/alloc/src/collections/vec_deque/mod.rs b/library/alloc/src/collections/vec_deque/mod.rs
index 57807bc5453..1303b58e31c 100644
--- a/library/alloc/src/collections/vec_deque/mod.rs
+++ b/library/alloc/src/collections/vec_deque/mod.rs
@@ -2793,8 +2793,12 @@ impl<T> From<Vec<T>> for VecDeque<T> {
             let len = other.len();
 
             // We need to extend the buf if it's not a power of two, too small
-            // or doesn't have at least one free space
-            if !buf.capacity().is_power_of_two()
+            // or doesn't have at least one free space.
+            // We check if `T` is a ZST in the first condition,
+            // because `usize::MAX` (the capacity returned by `capacity()` for ZST)
+            // is not a power of zero and thus it'll always try
+            // to reserve more memory which will panic for ZST (rust-lang/rust#78532)
+            if (!buf.capacity().is_power_of_two() && mem::size_of::<T>() != 0)
                 || (buf.capacity() < (MINIMUM_CAPACITY + 1))
                 || (buf.capacity() == len)
             {
diff --git a/library/alloc/tests/vec_deque.rs b/library/alloc/tests/vec_deque.rs
index 705f0d62fbb..a962a5494a9 100644
--- a/library/alloc/tests/vec_deque.rs
+++ b/library/alloc/tests/vec_deque.rs
@@ -1728,3 +1728,10 @@ fn test_zero_sized_push() {
         }
     }
 }
+
+#[test]
+fn test_from_zero_sized_vec() {
+    let v = vec![(); 100];
+    let queue = VecDeque::from(v);
+    assert!(queue.len(), 100);
+}