From 23d3ff1b9756c768c4412dcd1d80cff0617fd5c5 Mon Sep 17 00:00:00 2001
From: Simon Sapin <simon.sapin@exyr.org>
Date: Sun, 6 Oct 2019 23:45:25 +0200
Subject: [PATCH 1/3] Fix zero-size uninitialized boxes

Requesting a zero-size allocation is not allowed,
return a dangling pointer instead.

CC https://github.com/rust-lang/rust/issues/63291#issuecomment-538692745
---
 src/liballoc/boxed.rs | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index 9b5d9431ae204..2693a64e13ba1 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -141,6 +141,9 @@ impl<T> Box<T> {
     /// ```
     #[unstable(feature = "new_uninit", issue = "63291")]
     pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
+        if mem::size_of::<T>() == 0 {
+            return Box(NonNull::dangling().into())
+        }
         let layout = alloc::Layout::new::<mem::MaybeUninit<T>>();
         let ptr = unsafe {
             Global.alloc(layout)
@@ -181,10 +184,17 @@ impl<T> Box<[T]> {
     /// ```
     #[unstable(feature = "new_uninit", issue = "63291")]
     pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
-        let layout = alloc::Layout::array::<mem::MaybeUninit<T>>(len).unwrap();
-        let ptr = unsafe { alloc::alloc(layout) };
-        let unique = Unique::new(ptr).unwrap_or_else(|| alloc::handle_alloc_error(layout));
-        let slice = unsafe { slice::from_raw_parts_mut(unique.cast().as_ptr(), len) };
+        let ptr = if mem::size_of::<T>() == 0 || len == 0 {
+            NonNull::dangling()
+        } else {
+            let layout = alloc::Layout::array::<mem::MaybeUninit<T>>(len).unwrap();
+            unsafe {
+                Global.alloc(layout)
+                    .unwrap_or_else(|_| alloc::handle_alloc_error(layout))
+                    .cast()
+            }
+        };
+        let slice = unsafe { slice::from_raw_parts_mut(ptr.as_ptr(), len) };
         Box(Unique::from(slice))
     }
 }

From ca1cfdab78d1966efd492cd550a3684f3b95527c Mon Sep 17 00:00:00 2001
From: Simon Sapin <simon.sapin@exyr.org>
Date: Wed, 16 Oct 2019 20:32:58 +0200
Subject: [PATCH 2/3] Uninitialized boxes: check for zero-size allocation based
 on Layout::size

---
 src/liballoc/boxed.rs | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index 2693a64e13ba1..567b8ea722491 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -141,10 +141,10 @@ impl<T> Box<T> {
     /// ```
     #[unstable(feature = "new_uninit", issue = "63291")]
     pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
-        if mem::size_of::<T>() == 0 {
+        let layout = alloc::Layout::new::<mem::MaybeUninit<T>>();
+        if layout.size() == 0 {
             return Box(NonNull::dangling().into())
         }
-        let layout = alloc::Layout::new::<mem::MaybeUninit<T>>();
         let ptr = unsafe {
             Global.alloc(layout)
                 .unwrap_or_else(|_| alloc::handle_alloc_error(layout))
@@ -184,10 +184,10 @@ impl<T> Box<[T]> {
     /// ```
     #[unstable(feature = "new_uninit", issue = "63291")]
     pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
-        let ptr = if mem::size_of::<T>() == 0 || len == 0 {
+        let layout = alloc::Layout::array::<mem::MaybeUninit<T>>(len).unwrap();
+        let ptr = if layout.size() == 0 {
             NonNull::dangling()
         } else {
-            let layout = alloc::Layout::array::<mem::MaybeUninit<T>>(len).unwrap();
             unsafe {
                 Global.alloc(layout)
                     .unwrap_or_else(|_| alloc::handle_alloc_error(layout))

From 352b0265d056aa811384750cb726136f73308ab9 Mon Sep 17 00:00:00 2001
From: Simon Sapin <simon.sapin@exyr.org>
Date: Wed, 16 Oct 2019 20:33:55 +0200
Subject: [PATCH 3/3] Uninitialized boxes: add test for zero-size allocations

---
 src/liballoc/tests/boxed.rs | 19 +++++++++++++++++++
 src/liballoc/tests/lib.rs   |  2 ++
 2 files changed, 21 insertions(+)
 create mode 100644 src/liballoc/tests/boxed.rs

diff --git a/src/liballoc/tests/boxed.rs b/src/liballoc/tests/boxed.rs
new file mode 100644
index 0000000000000..53ec6f3f40110
--- /dev/null
+++ b/src/liballoc/tests/boxed.rs
@@ -0,0 +1,19 @@
+use std::ptr::NonNull;
+use std::mem::MaybeUninit;
+
+#[test]
+fn unitialized_zero_size_box() {
+    assert_eq!(
+        &*Box::<()>::new_uninit() as *const _,
+        NonNull::<MaybeUninit<()>>::dangling().as_ptr(),
+    );
+    assert_eq!(
+        Box::<[()]>::new_uninit_slice(4).as_ptr(),
+        NonNull::<MaybeUninit<()>>::dangling().as_ptr(),
+    );
+    assert_eq!(
+        Box::<[String]>::new_uninit_slice(0).as_ptr(),
+        NonNull::<MaybeUninit<String>>::dangling().as_ptr(),
+    );
+}
+
diff --git a/src/liballoc/tests/lib.rs b/src/liballoc/tests/lib.rs
index 5723a30c0f34f..59f5c8dfb8a87 100644
--- a/src/liballoc/tests/lib.rs
+++ b/src/liballoc/tests/lib.rs
@@ -2,6 +2,7 @@
 #![feature(box_syntax)]
 #![feature(drain_filter)]
 #![feature(exact_size_is_empty)]
+#![feature(new_uninit)]
 #![feature(option_flattening)]
 #![feature(pattern)]
 #![feature(repeat_generic_slice)]
@@ -15,6 +16,7 @@ use std::collections::hash_map::DefaultHasher;
 
 mod arc;
 mod binary_heap;
+mod boxed;
 mod btree;
 mod cow_str;
 mod fmt;