Fix UserRef<[T]>::copy_to_enclave_vec

It reinterprets uninitialized memory as initialized and does not drop
existing elements of the Vec.

Author: thaliaarchi

library/std/src/sys/pal/sgx/abi/usercalls/alloc.rs

@@ -684,17 +684,12 @@ where
 
     /// Copies the value from user memory and place it into `dest`. Afterwards,
     /// `dest` will contain exactly `self.len()` elements.
-    ///
-    /// # Panics
-    /// This function panics if the destination doesn't have the same size as
-    /// the source. This can happen for dynamically-sized types such as slices.
     pub fn copy_to_enclave_vec(&self, dest: &mut Vec<T>) {
-        if let Some(missing) = self.len().checked_sub(dest.capacity()) {
-            dest.reserve(missing)
-        }
+        dest.clear();
+        dest.reserve(self.len());
+        self.copy_to_enclave(&mut dest.spare_capacity_mut()[..self.len()]);
         // SAFETY: We reserve enough space above.
         unsafe { dest.set_len(self.len()) };
-        self.copy_to_enclave(&mut dest[..]);
     }
 
     /// Copies the value from user memory into a vector in enclave memory.