From cd26d10edfa8df69266e85b5ab7b991de0a10b38 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Thu, 6 Jul 2023 03:10:11 +0000
Subject: [PATCH] Dont ICE for `dyn* Trait: Trait` goals during selection in
 new trait solver

---
 .../src/solve/eval_ctxt/select.rs                      | 10 ++++++----
 tests/ui/dyn-star/box.rs                               |  4 +++-
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/compiler/rustc_trait_selection/src/solve/eval_ctxt/select.rs b/compiler/rustc_trait_selection/src/solve/eval_ctxt/select.rs
index 18332d6811d..366e9aa793d 100644
--- a/compiler/rustc_trait_selection/src/solve/eval_ctxt/select.rs
+++ b/compiler/rustc_trait_selection/src/solve/eval_ctxt/select.rs
@@ -213,15 +213,16 @@ fn rematch_object<'tcx>(
     mut nested: Vec<PredicateObligation<'tcx>>,
 ) -> SelectionResult<'tcx, Selection<'tcx>> {
     let self_ty = goal.predicate.self_ty();
-    let source_trait_ref = if let ty::Dynamic(data, _, ty::Dyn) = self_ty.kind() {
-        data.principal().unwrap().with_self_ty(infcx.tcx, self_ty)
-    } else {
+    let ty::Dynamic(data, _, source_kind) = *self_ty.kind()
+    else {
         bug!()
     };
+    let source_trait_ref = data.principal().unwrap().with_self_ty(infcx.tcx, self_ty);
 
     let (is_upcasting, target_trait_ref_unnormalized) = if Some(goal.predicate.def_id())
         == infcx.tcx.lang_items().unsize_trait()
     {
+        assert_eq!(source_kind, ty::Dyn, "cannot upcast dyn*");
         if let ty::Dynamic(data, _, ty::Dyn) = goal.predicate.trait_ref.substs.type_at(1).kind() {
             (true, data.principal().unwrap().with_self_ty(infcx.tcx, self_ty))
         } else {
@@ -288,7 +289,8 @@ fn rematch_object<'tcx>(
         bug!();
     };
 
-    // If we're upcasting, get the offset of the vtable pointer, which is
+    // If we're upcasting, get the offset of the vtable pointer, otherwise get
+    // the base of the vtable.
     Ok(Some(if is_upcasting {
         ImplSource::TraitUpcasting(ImplSourceTraitUpcastingData { vtable_vptr_slot, nested })
     } else {
diff --git a/tests/ui/dyn-star/box.rs b/tests/ui/dyn-star/box.rs
index d1f1819d9f3..87c8356a174 100644
--- a/tests/ui/dyn-star/box.rs
+++ b/tests/ui/dyn-star/box.rs
@@ -1,5 +1,7 @@
 // run-pass
-// compile-flags: -C opt-level=0
+// revisions: current next
+//[current] compile-flags: -C opt-level=0
+//[next] compile-flags: -Ztrait-solver=next -C opt-level=0
 
 #![feature(dyn_star)]
 #![allow(incomplete_features)]