clarify safety documentation of ptr::swap and ptr::copy

library/core/src/intrinsics.rs

@@ -2705,9 +2705,13 @@ pub const unsafe fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: us
 ///
 /// Behavior is undefined if any of the following conditions are violated:
 ///
-/// * `src` must be [valid] for reads of `count * size_of::<T>()` bytes.
+/// * `src` must be [valid] for reads of `count * size_of::<T>()` bytes, and must remain valid even
+///   if `dst` is written for `count * size_of::<T>()` bytes. (This means if the memory ranges
+///   overlap, the two pointers must not be subject to aliasing restrictions relative to each
+///   other.)
 ///
-/// * `dst` must be [valid] for writes of `count * size_of::<T>()` bytes.
+/// * `dst` must be [valid] for writes of `count * size_of::<T>()` bytes, and must remain valid even
+///   if `src` is read for `count * size_of::<T>()` bytes.
 ///
 /// * Both `src` and `dst` must be properly aligned.
 ///

library/core/src/ptr/mod.rs

@@ -795,7 +795,9 @@ pub const fn slice_from_raw_parts_mut<T>(data: *mut T, len: usize) -> *mut [T] {
 ///
 /// Behavior is undefined if any of the following conditions are violated:
 ///
-/// * Both `x` and `y` must be [valid] for both reads and writes.
+/// * Both `x` and `y` must be [valid] for both reads and writes. They must remain valid even if the
+///   other pointer is written. (This means if the memory ranges overlap, the two pointers must not
+///   be subject to aliasing restrictions relative to each other.)
 ///
 /// * Both `x` and `y` must be properly aligned.
 ///