Show offsets and derefs in padding, too

Author: oli-obk

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -121,6 +121,7 @@ pub enum PathElem {
     ArrayElem(usize),
     TupleElem(usize),
     Deref,
+    Offset(Size),
     EnumTag,
     CoroutineTag,
     DynDowncast,
@@ -199,6 +200,7 @@ fn write_path(out: &mut String, path: &[PathElem]) {
             // even use the usual syntax because we are just showing the projections,
             // not the root.
             Deref => write!(out, ".<deref>"),
+            Offset(offset) => write!(out, ".<offset({:x})>", offset.bytes()),
             DynDowncast => write!(out, ".<dyn-downcast>"),
         }
         .unwrap()
@@ -1047,18 +1049,26 @@ impl<'mir, 'tcx> InterpCx<'mir, 'tcx, crate::const_eval::CompileTimeInterpreter<
         if let Some(prov) = prov
             && let Some((_, alloc)) = self.memory.alloc_map().get(&prov.alloc_id())
         {
-            for (_, prov) in alloc.provenance().ptrs().iter() {
+            for &(offset, prov) in alloc.provenance().ptrs().iter() {
                 if let AllocKind::Dead = self.get_alloc_info(prov.alloc_id()).2 {
                     throw_validation_failure!(
                         path,
                         DanglingPtrUseAfterFree { ptr_kind: PointerKind::Ref(Mutability::Not) }
                     )
                 } else {
-                    let ptr = Pointer::new(Some(*prov), Size::ZERO);
+                    let ptr = Pointer::new(Some(prov), Size::ZERO);
                     let ty = self.tcx.types.unit;
                     let layout = self.layout_of(ty).unwrap();
                     let op = self.ptr_to_mplace(ptr, layout);
-                    ref_tracking.track(op, || path.clone())
+                    ref_tracking.track(op, || {
+                        // 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.extend(path.iter().copied());
+                        new_path.push(PathElem::Offset(offset));
+                        new_path.push(PathElem::Deref);
+                        new_path
+                    })
                 }
             }
         }

tests/ui/consts/dangling_raw_ptr.stderr

@@ -24,7 +24,7 @@ error[E0080]: it is undefined behavior to use this value
   --> $DIR/dangling_raw_ptr.rs:20:1
    |
 LL | const BAZ: Union = {
-   | ^^^^^^^^^^^^^^^^ constructing invalid value: encountered a dangling reference (use-after-free)
+   | ^^^^^^^^^^^^^^^^ constructing invalid value at .<offset(0)>.<deref>: encountered a dangling reference (use-after-free)
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
    = note: the raw bytes of the constant (size: $SIZE, align: $ALIGN) {