Add comments to examples

Author: traviscross

src/behavior-considered-undefined.md

@@ -222,15 +222,22 @@ r[undefined.validity.const-provenance]
  > [!EXAMPLE]
  > All of the following are UB:
  > ```rust,compile_fail
- > const REINTERPRET_PTR_AS_INT: usize = {
+ > # use core::mem::MaybeUninit;
+ > # use core::ptr;
+ > // We cannot reinterpret a pointer with provenance as an integer,
+ > // as then the bytes of the integer will have provenance.
+ > const _: usize = {
  > let ptr = &0;
  > unsafe { (&raw const ptr as *const usize).read() }
  > };
  >
- > const PTR_BYTES_IN_WRONG_ORDER: &i32 = {
+ > // We cannot rearrange the bytes of a pointer with provenance and
+ > // then interpret them as a reference, as then a value holding
+ > // pointer data will have pointer fragments in the wrong order.
+ > const _: &i32 = {
  > let mut ptr = &0;
- > let ptr_bytes = &raw mut ptr as *mut std::mem::MaybeUninit::<u8>;
- > unsafe { std::ptr::swap(ptr_bytes.add(1), ptr_bytes.add(2)) };
+ > let ptr_bytes = &raw mut ptr as *mut MaybeUninit::<u8>;
+ > unsafe { ptr::swap(ptr_bytes.add(1), ptr_bytes.add(2)) };
  > ptr
  > };
  > ```