Auto merge of #114795 - RalfJung:cell-swap, r=dtolnay

make Cell::swap panic if the Cells partially overlap

The following function ought to be sound:
```rust
fn as_cell_of_array<T, const N: usize>(c: &[Cell<T>; N]) -> &Cell<[T; N]> {
    unsafe { transmute(c) }
}
```
However, due to `Cell::swap`, it currently is not -- safe code can [cause a use-after-free](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=c9415799722d985ff7d2c2c997b724ca). This PR fixes that.

Fixes #80778

Author: bors

library/core/src/cell.rs

@@ -237,6 +237,7 @@
 
 use crate::cmp::Ordering;
 use crate::fmt::{self, Debug, Display};
+use crate::intrinsics::is_nonoverlapping;
 use crate::marker::{PhantomData, Unsize};
 use crate::mem;
 use crate::ops::{CoerceUnsized, Deref, DerefMut, DispatchFromDyn};
@@ -415,6 +416,12 @@ impl<T> Cell<T> {
     /// Swaps the values of two `Cell`s.
     /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
     ///
+    /// # Panics
+    ///
+    /// This function will panic if `self` and `other` are different `Cell`s that partially overlap.
+    /// (Using just standard library methods, it is impossible to create such partially overlapping `Cell`s.
+    /// However, unsafe code is allowed to e.g. create two `&Cell<[i32; 2]>` that partially overlap.)
+    ///
     /// # Examples
     ///
     /// ```
@@ -430,14 +437,20 @@ impl<T> Cell<T> {
     #[stable(feature = "move_cell", since = "1.17.0")]
     pub fn swap(&self, other: &Self) {
         if ptr::eq(self, other) {
+            // Swapping wouldn't change anything.
             return;
         }
+        if !is_nonoverlapping(self, other, 1) {
+            // See <https://github.com/rust-lang/rust/issues/80778> for why we need to stop here.
+            panic!("`Cell::swap` on overlapping non-identical `Cell`s");
+        }
         // SAFETY: This can be risky if called from separate threads, but `Cell`
         // is `!Sync` so this won't happen. This also won't invalidate any
         // pointers since `Cell` makes sure nothing else will be pointing into
-        // either of these `Cell`s.
+        // either of these `Cell`s. We also excluded shenanigans like partially overlapping `Cell`s,
+        // so `swap` will just properly copy two full values of type `T` back and forth.
         unsafe {
-            ptr::swap(self.value.get(), other.value.get());
+            mem::swap(&mut *self.value.get(), &mut *other.value.get());
         }
     }
 

library/core/src/intrinsics.rs

@@ -2567,7 +2567,7 @@ pub(crate) fn is_nonoverlapping<T>(src: *const T, dst: *const T, count: usize) -
     let size = mem::size_of::<T>()
         .checked_mul(count)
         .expect("is_nonoverlapping: `size_of::<T>() * count` overflows a usize");
-    let diff = if src_usize > dst_usize { src_usize - dst_usize } else { dst_usize - src_usize };
+    let diff = src_usize.abs_diff(dst_usize);
     // If the absolute distance between the ptrs is at least as big as the size of the buffer,
     // they do not overlap.
     diff >= size