Do not pass nullptr to slice::from_raw_parts

Pass dangling, but well-aligned pointer instead as per
https://doc.rust-lang.org/std/slice/fn.from_raw_parts.html#safety.

PiperOrigin-RevId: 539924021

rust/shared.rs

@@ -46,6 +46,7 @@ pub use __runtime::SerializedData;
 
 use std::fmt;
 use std::slice;
+use std::ptr::NonNull;
 
 /// An error that happened during deserialization.
 #[derive(Debug, Clone)]
@@ -73,6 +74,11 @@ pub struct PtrAndLen {
 
 impl PtrAndLen {
     pub unsafe fn as_ref<'a>(self) -> &'a [u8] {
-        slice::from_raw_parts(self.ptr, self.len)
+        assert!(self.len == 0 || !self.ptr.is_null());
+        if self.len == 0 {
+            slice::from_raw_parts(NonNull::dangling().as_ptr(), 0)
+        } else {
+            slice::from_raw_parts(self.ptr, self.len)
+        }
     }
 }

rust/test/shared/accessors_test.rs

@@ -65,4 +65,7 @@ fn test_optional_bytes_accessors() {
 
     msg.optional_bytes_set(None);
     assert_eq!(msg.optional_bytes(), None);
+
+    msg.optional_bytes_set(Some(b""));
+    assert_eq!(msg.optional_bytes().unwrap(), b"");
 }