Fix FZS stacked borrow error (needs Rust 1.64) (#3513)

utils/zerovec/src/flexzerovec/slice.rs

@@ -134,18 +134,18 @@ impl FlexZeroSlice {
         // equal to the length of the `data` field, which will be one less than the length of the
         // overall array.
         #[allow(clippy::panic)] // panic is documented in function contract
-        let (_, remainder) = match bytes.split_last() {
-            Some(v) => v,
-            None => panic!("slice should be non-empty"),
-        };
-        &*(remainder as *const [u8] as *const Self)
+        if bytes.is_empty() {
+            panic!("from_byte_slice_unchecked called with empty slice")
+        }
+        let slice = core::ptr::slice_from_raw_parts(bytes.as_ptr(), bytes.len() - 1);
+        &*(slice as *const Self)
     }
 
     #[inline]
     pub(crate) unsafe fn from_byte_slice_mut_unchecked(bytes: &mut [u8]) -> &mut Self {
         // Safety: See comments in `from_byte_slice_unchecked`
-        let remainder = core::slice::from_raw_parts_mut(bytes.as_mut_ptr(), bytes.len() - 1);
-        &mut *(remainder as *mut [u8] as *mut Self)
+        let remainder = core::ptr::slice_from_raw_parts_mut(bytes.as_mut_ptr(), bytes.len() - 1);
+        &mut *(remainder as *mut Self)
     }
 
     /// Returns this slice as its underlying `&[u8]` byte buffer representation.

utils/zerovec/src/ule/option.rs

@@ -197,9 +197,8 @@ unsafe impl<U: VarULE + ?Sized> VarULE for OptionVarULE<U> {
 
     #[inline]
     unsafe fn from_byte_slice_unchecked(bytes: &[u8]) -> &Self {
-        let metadata = bytes.len() - 1;
         let entire_struct_as_slice: *const [u8] =
-            ::core::slice::from_raw_parts(bytes.as_ptr(), metadata);
+            ::core::ptr::slice_from_raw_parts(bytes.as_ptr(), bytes.len() - 1);
         &*(entire_struct_as_slice as *const Self)
     }
 }