fix clippy

tokio/src/io/util/read_to_end.rs

@@ -146,13 +146,9 @@ fn reserve<R: AsyncRead + ?Sized>(buf: &mut Vec<u8>, read: &R, bytes: usize) {
 fn get_unused_capacity(buf: &mut Vec<u8>) -> &mut [MaybeUninit<u8>] {
     let prepare_from = buf.len();
     let prepare_len = buf.capacity() - prepare_from;
-    // safety: prepare_from is the length of a vector, so it can't overflow isize,
-    // and the pointer will stay inside the allocation.
-    let ptr = unsafe {
-        buf.as_mut_ptr()
-            .offset(prepare_from as isize)
-            .cast::<MaybeUninit<u8>>()
-    };
+    // safety: prepare_from is the length of the vector, so it will stay inside the
+    // allocation.
+    let ptr = unsafe { buf.as_mut_ptr().add(prepare_from).cast::<MaybeUninit<u8>>() };
 
     // safety: The memory is properly allocated due to the invariants provided by
     // Vec<u8>, and since the item type is MaybeUninit<u8>, it is safe for the