Add contracts for NonZero::from_mut_unchecked (#345)

Contract can now be expressed as
model-checking/kani#4151 has been merged.

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

Author: tautschnig

library/core/src/num/nonzero.rs

@@ -450,6 +450,12 @@ where
     #[must_use]
     #[inline]
     #[track_caller]
+    #[requires({
+        let size = core::mem::size_of::<T>();
+        let ptr = n as *const T as *const u8;
+        let slice = unsafe { core::slice::from_raw_parts(ptr, size) };
+        !slice.iter().all(|&byte| byte == 0)
+    })]
     pub unsafe fn from_mut_unchecked(n: &mut T) -> &mut Self {
         match Self::from_mut(n) {
             Some(n) => n,
@@ -2395,6 +2401,80 @@ mod verify {
     nonzero_check!(u128, core::num::NonZeroU128, nonzero_check_new_unchecked_for_u128);
     nonzero_check!(usize, core::num::NonZeroUsize, nonzero_check_new_unchecked_for_usize);
 
+    macro_rules! nonzero_check_from_mut_unchecked {
+        ($t:ty, $nonzero_type:ty, $harness_name:ident) => {
+            #[kani::proof_for_contract(NonZero::<$t>::from_mut_unchecked)]
+            pub fn $harness_name() {
+                let mut x: $t = kani::any();
+                unsafe {
+                    <$nonzero_type>::from_mut_unchecked(&mut x);
+                }
+            }
+        };
+    }
+
+    // Generate harnesses for multiple types
+    nonzero_check_from_mut_unchecked!(
+        i8,
+        core::num::NonZeroI8,
+        nonzero_check_from_mut_unchecked_i8
+    );
+    nonzero_check_from_mut_unchecked!(
+        i16,
+        core::num::NonZeroI16,
+        nonzero_check_from_mut_unchecked_i16
+    );
+    nonzero_check_from_mut_unchecked!(
+        i32,
+        core::num::NonZeroI32,
+        nonzero_check_from_mut_unchecked_i32
+    );
+    nonzero_check_from_mut_unchecked!(
+        i64,
+        core::num::NonZeroI64,
+        nonzero_check_from_mut_unchecked_i64
+    );
+    nonzero_check_from_mut_unchecked!(
+        i128,
+        core::num::NonZeroI128,
+        nonzero_check_from_mut_unchecked_i128
+    );
+    nonzero_check_from_mut_unchecked!(
+        isize,
+        core::num::NonZeroIsize,
+        nonzero_check_from_mut_unchecked_isize
+    );
+    nonzero_check_from_mut_unchecked!(
+        u8,
+        core::num::NonZeroU8,
+        nonzero_check_from_mut_unchecked_u8
+    );
+    nonzero_check_from_mut_unchecked!(
+        u16,
+        core::num::NonZeroU16,
+        nonzero_check_from_mut_unchecked_u16
+    );
+    nonzero_check_from_mut_unchecked!(
+        u32,
+        core::num::NonZeroU32,
+        nonzero_check_from_mut_unchecked_u32
+    );
+    nonzero_check_from_mut_unchecked!(
+        u64,
+        core::num::NonZeroU64,
+        nonzero_check_from_mut_unchecked_u64
+    );
+    nonzero_check_from_mut_unchecked!(
+        u128,
+        core::num::NonZeroU128,
+        nonzero_check_from_mut_unchecked_u128
+    );
+    nonzero_check_from_mut_unchecked!(
+        usize,
+        core::num::NonZeroUsize,
+        nonzero_check_from_mut_unchecked_usize
+    );
+
     macro_rules! nonzero_check_cmp {
         ($nonzero_type:ty, $nonzero_check_cmp_for:ident) => {
             #[kani::proof]