diff --git a/src/base.rs b/src/base.rs
index dc342e151f33..1ce0aacab499 100644
--- a/src/base.rs
+++ b/src/base.rs
@@ -652,7 +652,7 @@ fn codegen_stmt<'tcx>(
                     lval.write_cvalue(fx, res);
                 }
                 Rvalue::Cast(
-                    CastKind::PointerCoercion(PointerCoercion::ReifyFnPointer),
+                    CastKind::PointerCoercion(PointerCoercion::ReifyFnPointer, _),
                     ref operand,
                     to_ty,
                 ) => {
@@ -677,7 +677,7 @@ fn codegen_stmt<'tcx>(
                     }
                 }
                 Rvalue::Cast(
-                    CastKind::PointerCoercion(PointerCoercion::UnsafeFnPointer),
+                    CastKind::PointerCoercion(PointerCoercion::UnsafeFnPointer, _),
                     ref operand,
                     to_ty,
                 ) => {
@@ -688,6 +688,7 @@ fn codegen_stmt<'tcx>(
                 Rvalue::Cast(
                     CastKind::PointerCoercion(
                         PointerCoercion::MutToConstPointer | PointerCoercion::ArrayToPointer,
+                        _,
                     ),
                     ..,
                 ) => {
@@ -741,7 +742,7 @@ fn codegen_stmt<'tcx>(
                     }
                 }
                 Rvalue::Cast(
-                    CastKind::PointerCoercion(PointerCoercion::ClosureFnPointer(_)),
+                    CastKind::PointerCoercion(PointerCoercion::ClosureFnPointer(_), _),
                     ref operand,
                     _to_ty,
                 ) => {
@@ -763,14 +764,18 @@ fn codegen_stmt<'tcx>(
                     }
                 }
                 Rvalue::Cast(
-                    CastKind::PointerCoercion(PointerCoercion::Unsize),
+                    CastKind::PointerCoercion(PointerCoercion::Unsize, _),
                     ref operand,
                     _to_ty,
                 ) => {
                     let operand = codegen_operand(fx, operand);
                     crate::unsize::coerce_unsized_into(fx, operand, lval);
                 }
-                Rvalue::Cast(CastKind::DynStar, ref operand, _) => {
+                Rvalue::Cast(
+                    CastKind::PointerCoercion(PointerCoercion::DynStar, _),
+                    ref operand,
+                    _,
+                ) => {
                     let operand = codegen_operand(fx, operand);
                     crate::unsize::coerce_dyn_star(fx, operand, lval);
                 }
diff --git a/src/constant.rs b/src/constant.rs
index 6ba6a64c5448..ab78584332a0 100644
--- a/src/constant.rs
+++ b/src/constant.rs
@@ -161,13 +161,13 @@ pub(crate) fn codegen_const_value<'tcx>(
                             fx.module.declare_func_in_func(func_id, &mut fx.bcx.func);
                         fx.bcx.ins().func_addr(fx.pointer_type, local_func_id)
                     }
-                    GlobalAlloc::VTable(ty, trait_ref) => {
+                    GlobalAlloc::VTable(ty, dyn_ty) => {
                         let data_id = data_id_for_vtable(
                             fx.tcx,
                             &mut fx.constants_cx,
                             fx.module,
                             ty,
-                            trait_ref,
+                            dyn_ty.principal(),
                         );
                         let local_data_id =
                             fx.module.declare_data_in_func(data_id, &mut fx.bcx.func);
@@ -456,8 +456,8 @@ fn define_all_allocs(tcx: TyCtxt<'_>, module: &mut dyn Module, cx: &mut Constant
                 GlobalAlloc::Memory(target_alloc) => {
                     data_id_for_alloc_id(cx, module, alloc_id, target_alloc.inner().mutability)
                 }
-                GlobalAlloc::VTable(ty, trait_ref) => {
-                    data_id_for_vtable(tcx, cx, module, ty, trait_ref)
+                GlobalAlloc::VTable(ty, dyn_ty) => {
+                    data_id_for_vtable(tcx, cx, module, ty, dyn_ty.principal())
                 }
                 GlobalAlloc::Static(def_id) => {
                     if tcx.codegen_fn_attrs(def_id).flags.contains(CodegenFnAttrFlags::THREAD_LOCAL)
diff --git a/src/unsize.rs b/src/unsize.rs
index 8cfe93b4d9c9..339628053a95 100644
--- a/src/unsize.rs
+++ b/src/unsize.rs
@@ -34,7 +34,22 @@ pub(crate) fn unsized_info<'tcx>(
             let old_info =
                 old_info.expect("unsized_info: missing old info for trait upcasting coercion");
             if data_a.principal_def_id() == data_b.principal_def_id() {
-                // A NOP cast that doesn't actually change anything, should be allowed even with invalid vtables.
+                // Codegen takes advantage of the additional assumption, where if the
+                // principal trait def id of what's being casted doesn't change,
+                // then we don't need to adjust the vtable at all. This
+                // corresponds to the fact that `dyn Tr<A>: Unsize<dyn Tr<B>>`
+                // requires that `A = B`; we don't allow *upcasting* objects
+                // between the same trait with different args. If we, for
+                // some reason, were to relax the `Unsize` trait, it could become
+                // unsound, so let's assert here that the trait refs are *equal*.
+                //
+                // We can use `assert_eq` because the binders should have been anonymized,
+                // and because higher-ranked equality now requires the binders are equal.
+                debug_assert_eq!(
+                    data_a.principal(),
+                    data_b.principal(),
+                    "NOP unsize vtable changed principal trait ref: {data_a} -> {data_b}"
+                );
                 return old_info;
             }