reallocate_ptr comment

Co-authored-by: Ralf Jung <[email protected]>

compiler/rustc_const_eval/src/interpret/memory.rs

@@ -291,6 +291,9 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
 
         // For simplicities' sake, we implement reallocate as "alloc, copy, dealloc".
         // This happens so rarely, the perf advantage is outweighed by the maintenance cost.
+        // If requested, we zero-init the entire allocation, to ensure that a growing
+        // allocation has its new bytes properly set. For the part that is copied,
+        // `mem_copy` below will de-initialize things as necessary.
         let new_ptr = self.allocate_ptr(new_size, new_align, kind, init)?;
         let old_size = match old_size_and_align {
             Some((size, _align)) => size,