Rollup merge of rust-lang#95222 - RalfJung:validity, r=Dylan-DPC

interpret/validity: improve clarity

I was confused by my own (ancient) comment in `validity.rs` so I figured I'd clarify. (And I don't think ZST-ness is relevant at all inside that branch, no idea where that comment comes from.)

Also `extend` seems more clear than `clone_from`.

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -432,9 +432,8 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
         if let Some(ref mut ref_tracking) = self.ref_tracking {
             // Proceed recursively even for ZST, no reason to skip them!
             // `!` is a ZST and we want to validate it.
-            // Skip validation entirely for some external statics
             if let Ok((alloc_id, _offset, _ptr)) = self.ecx.memory.ptr_try_get_alloc(place.ptr) {
-                // not a ZST
+                // Special handling for pointers to statics (irrespective of their type).
                 let alloc_kind = self.ecx.tcx.get_global_alloc(alloc_id);
                 if let Some(GlobalAlloc::Static(did)) = alloc_kind {
                     assert!(!self.ecx.tcx.is_thread_local_static(did));
@@ -469,7 +468,7 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
                 // We need to clone the path anyway, make sure it gets created
                 // with enough space for the additional `Deref`.
                 let mut new_path = Vec::with_capacity(path.len() + 1);
-                new_path.clone_from(path);
+                new_path.extend(path);
                 new_path.push(PathElem::Deref);
                 new_path
             });