Inform tidy about the reason for the ignored rust code

Author: oli-obk

library/core/src/alloc/global.rs

@@ -61,7 +61,7 @@ use crate::ptr;
 ///   More concretely, the following code example is unsound, irrespective of whether your
 ///   custom allocator allows counting how many allocations have happened.
 ///
-///   ```text
+///   ```rust,ignore (unsound and has placeholders)
 ///   drop(Box::new(42));
 ///   let number_of_heap_allocs = /* call private allocator API */;
 ///   unsafe { std::intrinsics::assume(number_of_heap_allocs > 0); }

library/core/src/alloc/mod.rs

@@ -100,7 +100,7 @@ pub unsafe trait AllocRef {
     /// More concretely, the following code example is unsound, irrespective of whether your
     /// custom allocator allows counting how many allocations have happened.
     ///
-    /// ```text
+    /// ```rust,ignore (unsound and has placeholders)
     /// Global::dealloc(Global::alloc(some_layout));
     /// let number_of_heap_allocs = /* call private allocator API */;
     /// unsafe { std::intrinsics::assume(number_of_heap_allocs > 0); }