Add functions to allocate zeroed Arc and Rc (#283)

These require rustc 1.82

Cargo.toml

@@ -47,6 +47,8 @@ transparentwrapper_extra = []
 
 const_zeroed = [] # MSRV 1.75.0: support const `zeroed()`
 
+alloc_uninit = [] # MSRV 1.82.0: support `zeroed_*rc*`
+
 # Do not use if you can avoid it, because this is **unsound**!!!!
 unsound_ptr_pod_impl = []
 
@@ -61,6 +63,7 @@ latest_stable_rust = [
   # Keep this list sorted.
   "aarch64_simd",
   "align_offset",
+  "alloc_uninit",
   "const_zeroed",
   "derive",
   "min_const_generics",

src/allocation.rs

@@ -149,6 +149,38 @@ pub fn zeroed_slice_box<T: Zeroable>(length: usize) -> Box<[T]> {
   try_zeroed_slice_box(length).unwrap()
 }
 
+/// Allocates a `Arc<T>` with all contents being zeroed out.
+#[cfg(all(feature = "alloc_uninit", target_has_atomic = "ptr"))]
+pub fn zeroed_arc<T: Zeroable>() -> Arc<T> {
+  let mut arc = Arc::new_uninit();
+  crate::write_zeroes(Arc::get_mut(&mut arc).unwrap()); // unwrap never fails for a newly allocated Arc
+  unsafe { arc.assume_init() }
+}
+
+/// Allocates a `Arc<[T]>` with all contents being zeroed out.
+#[cfg(all(feature = "alloc_uninit", target_has_atomic = "ptr"))]
+pub fn zeroed_arc_slice<T: Zeroable>(length: usize) -> Arc<[T]> {
+  let mut arc = Arc::new_uninit_slice(length);
+  crate::fill_zeroes(Arc::get_mut(&mut arc).unwrap()); // unwrap never fails for a newly allocated Arc
+  unsafe { arc.assume_init() }
+}
+
+/// Allocates a `Rc<T>` with all contents being zeroed out.
+#[cfg(feature = "alloc_uninit")]
+pub fn zeroed_rc<T: Zeroable>() -> Rc<T> {
+  let mut rc = Rc::new_uninit();
+  crate::write_zeroes(Rc::get_mut(&mut rc).unwrap()); // unwrap never fails for a newly allocated Rc
+  unsafe { rc.assume_init() }
+}
+
+/// Allocates a `Rc<[T]>` with all contents being zeroed out.
+#[cfg(feature = "alloc_uninit")]
+pub fn zeroed_rc_slice<T: Zeroable>(length: usize) -> Rc<[T]> {
+  let mut rc = Rc::new_uninit_slice(length);
+  crate::fill_zeroes(Rc::get_mut(&mut rc).unwrap()); // unwrap never fails for a newly allocated Rc
+  unsafe { rc.assume_init() }
+}
+
 /// As [`try_cast_slice_box`], but unwraps for you.
 #[inline]
 pub fn cast_slice_box<A: NoUninit, B: AnyBitPattern>(