Rollup merge of rust-lang#65174 - SimonSapin:zero-box, r=alexcrichton

Fix zero-size uninitialized boxes

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

CC rust-lang#63291 (comment)

src/liballoc/boxed.rs

@@ -142,6 +142,9 @@ impl<T> Box<T> {
  #[unstable(feature = "new_uninit", issue = "63291")]
  pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
  let layout = alloc::Layout::new::<mem::MaybeUninit<T>>();
+ if layout.size() == 0 {
+ return Box(NonNull::dangling().into())
+ }
  let ptr = unsafe {
  Global.alloc(layout)
  .unwrap_or_else(|_| alloc::handle_alloc_error(layout))
@@ -182,9 +185,16 @@ 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 layout.size() == 0 {
+ NonNull::dangling()
+ } else {
+ 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))
  }
 }

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(),
+ );
+}
+

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(trusted_len)]
@@ -14,6 +15,7 @@ use std::collections::hash_map::DefaultHasher;
 
 mod arc;
 mod binary_heap;
+mod boxed;
 mod btree;
 mod cow_str;
 mod fmt;