From 6c01c0e9b5b069ee2a61b6078058c69880a14f19 Mon Sep 17 00:00:00 2001
From: Simon Sapin <simon.sapin@exyr.org>
Date: Sun, 29 Sep 2019 11:14:59 +0200
Subject: [PATCH] Zero-initialize `vec![None; n]` for `Option<&T>`,
 `Option<&mut T>` and `Option<Box<T>>`

---
 src/liballoc/vec.rs | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/src/liballoc/vec.rs b/src/liballoc/vec.rs
index e5672f8542ff6..d5089c5e2eba9 100644
--- a/src/liballoc/vec.rs
+++ b/src/liballoc/vec.rs
@@ -1748,6 +1748,31 @@ unsafe impl<T: ?Sized> IsZero for *mut T {
     }
 }
 
+// `Option<&T>`, `Option<&mut T>` and `Option<Box<T>>` are guaranteed to represent `None` as null.
+// For fat pointers, the bytes that would be the pointer metadata in the `Some` variant
+// are padding in the `None` variant, so ignoring them and zero-initializing instead is ok.
+
+unsafe impl<T: ?Sized> IsZero for Option<&T> {
+    #[inline]
+    fn is_zero(&self) -> bool {
+        self.is_none()
+    }
+}
+
+unsafe impl<T: ?Sized> IsZero for Option<&mut T> {
+    #[inline]
+    fn is_zero(&self) -> bool {
+        self.is_none()
+    }
+}
+
+unsafe impl<T: ?Sized> IsZero for Option<Box<T>> {
+    #[inline]
+    fn is_zero(&self) -> bool {
+        self.is_none()
+    }
+}
+
 
 ////////////////////////////////////////////////////////////////////////////////
 // Common trait implementations for Vec