agent-driven: Fix soundness in try_create_array, add safety comments

src/foreign/alloc/ffi/c_str.rs

@@ -7,7 +7,12 @@ impl<'a> Arbitrary<'a> for CString {
     fn arbitrary(u: &mut Unstructured<'a>) -> Result<Self> {
         <Vec<u8> as Arbitrary>::arbitrary(u).map(|mut x| {
             x.retain(|&c| c != 0);
-            // SAFETY: all zero bytes have been removed
+            // SAFETY:
+            // Contract from `CString::from_vec_unchecked`: the vector must not contain
+            // any interior nul (zero) bytes.
+            // Evidence: `x.retain(|&c| c != 0)` removes all bytes equal to `0` from the
+            // vector `x`. Consequently, `x` contains no nul bytes, which guarantees
+            // it has no interior nul bytes.
             unsafe { Self::from_vec_unchecked(x) }
         })
     }

src/foreign/core/array.rs

@@ -18,6 +18,17 @@ impl<T, const N: usize> Drop for ArrayGuard<T, N> {
     fn drop(&mut self) {
         debug_assert!(self.initialized <= N);
         let initialized_part = ptr::slice_from_raw_parts_mut(self.dst, self.initialized);
+        // SAFETY:
+        // Contract from `ptr::drop_in_place`: the pointer must be valid for reads and
+        // writes, and must be properly aligned.
+        // Evidence:
+        // - `self.dst` is derived from `MaybeUninit<[T; N]>` on the stack, which remains
+        //   alive and allocated because the guard is a local variable dropped before
+        //   the stack frame is destroyed.
+        // - The alignment of `self.dst` matches that of `[T; N]`, which is aligned for `T`.
+        // - Only the first `self.initialized` elements are dropped, which have been
+        //   fully initialized by `ptr::write` in `try_create_array`.
+        // Therefore the contract of `drop_in_place` is discharged.
         unsafe {
             ptr::drop_in_place(initialized_part);
         }
@@ -30,17 +41,28 @@ where
 {
     let mut array: MaybeUninit<[T; N]> = MaybeUninit::uninit();
     let array_ptr = array.as_mut_ptr();
-    let dst = array_ptr as _;
+    let dst = array_ptr as *mut T;
     let mut guard: ArrayGuard<T, N> = ArrayGuard {
         dst,
         initialized: 0,
     };
-    unsafe {
-        for (idx, value_ptr) in (*array.as_mut_ptr()).iter_mut().enumerate() {
+    for idx in 0..N {
+        // SAFETY: `dst` is a valid pointer to the start of the `[T; N]` array.
+        // `idx` is within `0..N`, so `dst.add(idx)` is within the bounds of the allocation.
+        // The pointer is properly aligned for `T`.
+        unsafe {
+            let value_ptr = dst.add(idx);
             ptr::write(value_ptr, cb(idx)?);
-            guard.initialized += 1;
         }
+        guard.initialized += 1;
+    }
+    unsafe {
         mem::forget(guard);
+        // SAFETY:
+        // Contract from `MaybeUninit::assume_init`: the value must be fully initialized.
+        // Evidence: the loop executes exactly `N` times, successfully writing a value
+        // to each of the `N` indices in the array. Since the loop completed without
+        // returning early or panicking, all elements of the array are initialized.
         Ok(array.assume_init())
     }
 }

src/foreign/core/str.rs

@@ -12,6 +12,16 @@ fn arbitrary_str<'a>(u: &mut Unstructured<'a>, size: usize) -> Result<&'a str> {
         Err(e) => {
             let i = e.valid_up_to();
             let valid = u.bytes(i).unwrap();
+            // SAFETY:
+            // Contract from `str::from_utf8_unchecked`: the bytes must be valid UTF-8.
+            // Evidence:
+            // - `str::from_utf8` was called on the next `size` peeked bytes from `u`.
+            // - The call failed, returning a `Utf8Error` `e`.
+            // - `e.valid_up_to()` returns the length of the prefix of the peeked bytes
+            //   which is guaranteed to be valid UTF-8.
+            // - `u.bytes(i)` consumes and returns exactly this valid prefix (since `u`
+            //   was not mutated or consumed between peeking and calling `bytes`).
+            // - Therefore, `valid` is guaranteed to contain valid UTF-8.
             let s = unsafe {
                 debug_assert!(str::from_utf8(valid).is_ok());
                 str::from_utf8_unchecked(valid)