From a23a77fb19b24adb22d0a3d3886acac1a7a31f68 Mon Sep 17 00:00:00 2001
From: Aleksey Kladov <aleksey.kladov@gmail.com>
Date: Mon, 27 May 2019 09:43:20 +0300
Subject: [PATCH 1/3] avoid materializing unintialized Boxes in RawVec

---
 src/liballoc/boxed.rs   |  9 ++++++---
 src/liballoc/raw_vec.rs | 10 ++++++----
 2 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index bf8f5b8b91a1e..5affdbdb7e9d1 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -546,9 +546,12 @@ impl<T: Copy> From<&[T]> for Box<[T]> {
     /// println!("{:?}", boxed_slice);
     /// ```
     fn from(slice: &[T]) -> Box<[T]> {
-        let mut boxed = unsafe { RawVec::with_capacity(slice.len()).into_box() };
-        boxed.copy_from_slice(slice);
-        boxed
+        let len = slice.len();
+        let buf = RawVec::with_capacity(len);
+        unsafe {
+            ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
+            buf.into_box()
+        }
     }
 }
 
diff --git a/src/liballoc/raw_vec.rs b/src/liballoc/raw_vec.rs
index d1fc5ac3b30d4..0454a56443579 100644
--- a/src/liballoc/raw_vec.rs
+++ b/src/liballoc/raw_vec.rs
@@ -685,12 +685,14 @@ impl<T, A: Alloc> RawVec<T, A> {
 impl<T> RawVec<T, Global> {
     /// Converts the entire buffer into `Box<[T]>`.
     ///
-    /// While it is not *strictly* Undefined Behavior to call
-    /// this procedure while some of the RawVec is uninitialized,
-    /// it certainly makes it trivial to trigger it.
-    ///
     /// Note that this will correctly reconstitute any `cap` changes
     /// that may have been performed. (see description of type for details)
+    ///
+    /// # Undefined Behavior
+    ///
+    /// All elements of `RawVec<T, Global>` must be initialized. Notice that
+    /// the rules around uninitialized boxed values are not finalized yet,
+    /// but until they are, it is advisable to avoid them.
     pub unsafe fn into_box(self) -> Box<[T]> {
         // NOTE: not calling `cap()` here, actually using the real `cap` field!
         let slice = slice::from_raw_parts_mut(self.ptr(), self.cap);

From 0653e78ae1b35eb50e06423052a34d807f1ac2af Mon Sep 17 00:00:00 2001
From: Aleksey Kladov <aleksey.kladov@gmail.com>
Date: Mon, 27 May 2019 09:43:20 +0300
Subject: [PATCH 2/3] make Box<str>::clone simpler & safer

---
 src/liballoc/boxed.rs | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index 5affdbdb7e9d1..2f45ec7d643f8 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -395,11 +395,9 @@ impl<T: Clone> Clone for Box<T> {
 #[stable(feature = "box_slice_clone", since = "1.3.0")]
 impl Clone for Box<str> {
     fn clone(&self) -> Self {
-        let len = self.len();
-        let buf = RawVec::with_capacity(len);
+        let buf: Box<[u8]> = self.as_bytes().into();
         unsafe {
-            ptr::copy_nonoverlapping(self.as_ptr(), buf.ptr(), len);
-            from_boxed_utf8_unchecked(buf.into_box())
+            from_boxed_utf8_unchecked(buf)
         }
     }
 }

From fe31ad38cb5c6fba5be871ab63082d7cdf430374 Mon Sep 17 00:00:00 2001
From: Aleksey Kladov <aleksey.kladov@gmail.com>
Date: Mon, 27 May 2019 22:42:50 +0300
Subject: [PATCH 3/3] Update src/liballoc/boxed.rs

Co-Authored-By: Ralf Jung <post@ralfj.de>
---
 src/liballoc/boxed.rs | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index 2f45ec7d643f8..76b660fba685c 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -395,6 +395,7 @@ impl<T: Clone> Clone for Box<T> {
 #[stable(feature = "box_slice_clone", since = "1.3.0")]
 impl Clone for Box<str> {
     fn clone(&self) -> Self {
+        // this makes a copy of the data
         let buf: Box<[u8]> = self.as_bytes().into();
         unsafe {
             from_boxed_utf8_unchecked(buf)