Auto merge of #107167 - the8472:rawvec-simpler-layout, r=thomcc

simplify layout calculations in rawvec

The use of `Layout::array` was introduced in #83706 which lead to a [perf regression](rust-lang/rust#83706 (comment)).

This PR basically reverts that change since rust currently only supports stride == size types, but to be on the safe side it leaves a const-assert there to make sure this gets caught if those assumptions ever change.

alloc/src/raw_vec.rs

@@ -241,10 +241,15 @@ impl<T, A: Allocator> RawVec<T, A> {
         if T::IS_ZST || self.cap == 0 {
             None
         } else {
-            // We have an allocated chunk of memory, so we can bypass runtime
-            // checks to get our current layout.
+            // We could use Layout::array here which ensures the absence of isize and usize overflows
+            // and could hypothetically handle differences between stride and size, but this memory
+            // has already been allocated so we know it can't overflow and currently rust does not
+            // support such types. So we can do better by skipping some checks and avoid an unwrap.
+            let _: () = const { assert!(mem::size_of::<T>() % mem::align_of::<T>() == 0) };
             unsafe {
-                let layout = Layout::array::<T>(self.cap).unwrap_unchecked();
+                let align = mem::align_of::<T>();
+                let size = mem::size_of::<T>().unchecked_mul(self.cap);
+                let layout = Layout::from_size_align_unchecked(size, align);
                 Some((self.ptr.cast().into(), layout))
             }
         }
@@ -426,11 +431,13 @@ impl<T, A: Allocator> RawVec<T, A> {
         assert!(cap <= self.capacity(), "Tried to shrink to a larger capacity");
 
         let (ptr, layout) = if let Some(mem) = self.current_memory() { mem } else { return Ok(()) };
-
+        // See current_memory() why this assert is here
+        let _: () = const { assert!(mem::size_of::<T>() % mem::align_of::<T>() == 0) };
         let ptr = unsafe {
             // `Layout::array` cannot overflow here because it would have
             // overflowed earlier when capacity was larger.
-            let new_layout = Layout::array::<T>(cap).unwrap_unchecked();
+            let new_size = mem::size_of::<T>().unchecked_mul(cap);
+            let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
             self.alloc
                 .shrink(ptr, layout, new_layout)
                 .map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })?