Fix shrink and capacity_from_bytes

Author: a1phyr

library/alloc/src/raw_vec.rs

@@ -402,7 +402,8 @@ impl<T, A: Allocator> RawVec<T, A> {
 
     fn capacity_from_bytes(excess: usize) -> usize {
         debug_assert_ne!(mem::size_of::<T>(), 0);
-        excess / mem::size_of::<T>()
+        let size_of_item = Layout::new::<T>().pad_to_align().size();
+        excess / size_of_item
     }
 
     fn set_ptr(&mut self, ptr: NonNull<[u8]>) {
@@ -466,10 +467,11 @@ impl<T, A: Allocator> RawVec<T, A> {
         assert!(amount <= self.capacity(), "Tried to shrink to a larger capacity");
 
         let (ptr, layout) = if let Some(mem) = self.current_memory() { mem } else { return Ok(()) };
-        let new_size = amount * mem::size_of::<T>();
 
         let ptr = unsafe {
-            let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
+            // `Layout::array` cannot overflow here because it would have
+            // owerflown earlier when capacity was larger.
+            let new_layout = Layout::array::<T>(amount).unwrap_unchecked();
             self.alloc
                 .shrink(ptr, layout, new_layout)
                 .map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })?