Contract and Harnesses for <*const T>::offset_from (#154)

Towards #76 

### Changes

* Adds contracts for `<*const T>::offset_from`.
* Adds harnesses for the function verifying:
    * All integer types
    * Tuples (composite types)
    * Unit Type
* Accomplishes this using a macro.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.

---------

Co-authored-by: Carolyn Zech <cmzech@amazon.com>

library/core/src/ptr/const_ptr.rs

@@ -3,6 +3,10 @@ use crate::cmp::Ordering::{Equal, Greater, Less};
 use crate::intrinsics::const_eval_select;
 use crate::mem::SizedTypeProperties;
 use crate::slice::{self, SliceIndex};
+use safety::{ensures, requires};
+
+#[cfg(kani)]
+use crate::kani;
 
 impl<T: ?Sized> *const T {
     /// Returns `true` if the pointer is null.
@@ -667,6 +671,16 @@ impl<T: ?Sized> *const T {
     #[rustc_const_stable(feature = "const_ptr_offset_from", since = "1.65.0")]
     #[inline]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
+    #[requires(
+        // Ensures subtracting `origin` from `self` doesn't overflow
+        (self as isize).checked_sub(origin as isize).is_some() &&
+        // Ensure the distance between `self` and `origin` is aligned to `T`
+        (self as isize - origin as isize) % (mem::size_of::<T>() as isize) == 0 &&
+        // Ensure both pointers are in the same allocation or are pointing to the same address
+        (self as isize == origin as isize || kani::mem::same_allocation(self, origin))
+    )]
+    // The result should equal the distance in terms of elements of type `T` as per the documentation above
+    #[ensures(|result| *result == (self as isize - origin as isize) / (mem::size_of::<T>() as isize))]
     pub const unsafe fn offset_from(self, origin: *const T) -> isize
     where
         T: Sized,
@@ -1899,3 +1913,149 @@ impl<T: ?Sized> PartialOrd for *const T {
         *self >= *other
     }
 }
+
+#[cfg(kani)]
+#[unstable(feature = "kani", issue = "none")]
+mod verify {
+    use crate::kani;
+    use core::mem;
+    use kani::PointerGenerator;
+
+    // Proof for unit size will panic as offset_from needs the pointee size to be greater then 0
+    #[kani::proof_for_contract(<*const ()>::offset_from)]
+    #[kani::should_panic]
+    pub fn check_const_offset_from_unit() {
+        let val: () = ();
+        let src_ptr: *const () = &val;
+        let dest_ptr: *const () = &val;
+        unsafe {
+            dest_ptr.offset_from(src_ptr);
+        }
+    }
+
+    // Array size bound for kani::any_array
+    const ARRAY_LEN: usize = 40;
+
+    macro_rules! generate_offset_from_harness {
+        ($type: ty, $proof_name1: ident, $proof_name2: ident) => {
+            // Proof for a single element
+            #[kani::proof_for_contract(<*const $type>::offset_from)]
+            pub fn $proof_name1() {
+                const gen_size: usize = mem::size_of::<$type>();
+                let mut generator1 = PointerGenerator::<gen_size>::new();
+                let mut generator2 = PointerGenerator::<gen_size>::new();
+                let ptr1: *const $type = generator1.any_in_bounds().ptr;
+                let ptr2: *const $type = if kani::any() {
+                    generator1.any_alloc_status().ptr
+                } else {
+                    generator2.any_alloc_status().ptr
+                };
+
+                unsafe {
+                    ptr1.offset_from(ptr2);
+                }
+            }
+
+            // Proof for large arrays
+            #[kani::proof_for_contract(<*const $type>::offset_from)]
+            pub fn $proof_name2() {
+                const gen_size: usize = mem::size_of::<$type>();
+                let mut generator1 = PointerGenerator::<{ gen_size * ARRAY_LEN }>::new();
+                let mut generator2 = PointerGenerator::<{ gen_size * ARRAY_LEN }>::new();
+                let ptr1: *const $type = generator1.any_in_bounds().ptr;
+                let ptr2: *const $type = if kani::any() {
+                    generator1.any_alloc_status().ptr
+                } else {
+                    generator2.any_alloc_status().ptr
+                };
+
+                unsafe {
+                    ptr1.offset_from(ptr2);
+                }
+            }
+        };
+    }
+
+    generate_offset_from_harness!(
+        u8,
+        check_const_offset_from_u8,
+        check_const_offset_from_u8_arr
+    );
+    generate_offset_from_harness!(
+        u16,
+        check_const_offset_from_u16,
+        check_const_offset_from_u16_arr
+    );
+    generate_offset_from_harness!(
+        u32,
+        check_const_offset_from_u32,
+        check_const_offset_from_u32_arr
+    );
+    generate_offset_from_harness!(
+        u64,
+        check_const_offset_from_u64,
+        check_const_offset_from_u64_arr
+    );
+    generate_offset_from_harness!(
+        u128,
+        check_const_offset_from_u128,
+        check_const_offset_from_u128_arr
+    );
+    generate_offset_from_harness!(
+        usize,
+        check_const_offset_from_usize,
+        check_const_offset_from_usize_arr
+    );
+
+    generate_offset_from_harness!(
+        i8,
+        check_const_offset_from_i8,
+        check_const_offset_from_i8_arr
+    );
+    generate_offset_from_harness!(
+        i16,
+        check_const_offset_from_i16,
+        check_const_offset_from_i16_arr
+    );
+    generate_offset_from_harness!(
+        i32,
+        check_const_offset_from_i32,
+        check_const_offset_from_i32_arr
+    );
+    generate_offset_from_harness!(
+        i64,
+        check_const_offset_from_i64,
+        check_const_offset_from_i64_arr
+    );
+    generate_offset_from_harness!(
+        i128,
+        check_const_offset_from_i128,
+        check_const_offset_from_i128_arr
+    );
+    generate_offset_from_harness!(
+        isize,
+        check_const_offset_from_isize,
+        check_const_offset_from_isize_arr
+    );
+
+    generate_offset_from_harness!(
+        (i8, i8),
+        check_const_offset_from_tuple_1,
+        check_const_offset_from_tuple_1_arr
+    );
+    generate_offset_from_harness!(
+        (f64, bool),
+        check_const_offset_from_tuple_2,
+        check_const_offset_from_tuple_2_arr
+    );
+    generate_offset_from_harness!(
+        (u32, i16, f32),
+        check_const_offset_from_tuple_3,
+        check_const_offset_from_tuple_3_arr
+    );
+    generate_offset_from_harness!(
+        ((), bool, u8, u16, i32, f64, i128, usize),
+        check_const_offset_from_tuple_4,
+        check_const_offset_from_tuple_4_arr
+    );
+}