Prove retained casts require unsafe

src/lib.rs

@@ -152,6 +152,13 @@ impl TypeErasedVec {
     /// Matching size alone is insufficient because the allocation must also have
     /// the alignment required by `T`. The caller must additionally ensure that
     /// every existing element is valid as `T` and can be soundly dropped as `T`.
+    ///
+    /// ```compile_fail
+    /// use type_erased_vec_capacity::TypeErasedVec;
+    ///
+    /// let mut erased = TypeErasedVec::new(vec![0_u32]);
+    /// let _guard = erased.cast_type::<i32>();
+    /// ```
     pub unsafe fn cast_type<T: 'static>(&mut self) -> ContentGuard<'_, T> {
         debug_assert_eq!(self.layout, Layout::new::<T>());