Rollup merge of #55764 - murarth:fix-rc-alloc, r=RalfJung

Fix Rc/Arc allocation layout

* Rounds allocation layout up to a multiple of alignment
* Adds a convenience method `Layout::pad_to_align` to perform rounding

Closes #55747

cc #55724

1 files changed, 4 insertions, 2 deletions

diff --git a/src/liballoc/sync.rs b/src/liballoc/sync.rs
index 2c396b3b06b..b63b3684964 100644
--- a/src/liballoc/sync.rs
+++ b/src/liballoc/sync.rs
@@ -575,14 +575,16 @@ impl<T: ?Sized> Arc<T> {
         // Previously, layout was calculated on the expression
         // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
         // reference (see #54908).
-        let (layout, _) = Layout::new::<ArcInner<()>>()
-            .extend(Layout::for_value(&*ptr)).unwrap();
+        let layout = Layout::new::<ArcInner<()>>()
+            .extend(Layout::for_value(&*ptr)).unwrap().0
+            .pad_to_align().unwrap();
 
         let mem = Global.alloc(layout)
             .unwrap_or_else(|_| handle_alloc_error(layout));
 
         // Initialize the ArcInner
         let inner = set_data_ptr(ptr as *mut T, mem.as_ptr() as *mut u8) as *mut ArcInner<T>;
+        debug_assert_eq!(Layout::for_value(&*inner), layout);
 
         ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
         ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));