delay_span_bug in wfcheck's ty.lift_to_tcx unwrap

src/librustc_typeck/check/wfcheck.rs

@@ -250,11 +250,14 @@ fn check_type_defn<'a, 'tcx, F>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
             let needs_drop_copy = || {
                 packed && {
                     let ty = variant.fields.last().unwrap().ty;
-                    let ty = fcx.tcx.erase_regions(&ty).lift_to_tcx(fcx_tcx)
+                    fcx.tcx.erase_regions(&ty).lift_to_tcx(fcx_tcx)
+                        .map(|ty| ty.needs_drop(fcx_tcx, fcx_tcx.param_env(def_id)))
                         .unwrap_or_else(|| {
-                            span_bug!(item.span, "inference variables in {:?}", ty)
-                        });
-                    ty.needs_drop(fcx_tcx, fcx_tcx.param_env(def_id))
+                            fcx_tcx.sess.delay_span_bug(
+                                item.span, &format!("inference variables in {:?}", ty));
+                            // Just treat unresolved type expression as if it needs drop.
+                            true
+                        })
                 }
             };
             let all_sized =

src/test/ui/wf/wf-packed-on-proj-of-type-as-unimpl-trait.rs

@@ -0,0 +1,31 @@
+// rust-lang/rust#58158: We have special-case code to deal with case
+// when a type is both packed and needs drop glue, (we move the fields
+// out of their potentially unaligned locations before dropping them,
+// which requires they be Sized; see PR #44884).
+//
+// So, we need to check if a given type needs drop-glue. That requires
+// that we actually know that the concrete type, and we guard against
+// the type having unknown parts (i.e. type variables) by ICE'ing in
+// that scenario.
+//
+// But in a case where we have a projection (`Type as Trait::Assoc`)
+// where `Type` does not actually implement `Trait`, we of course
+// cannot have a concrete type, because there is no impl to look up
+// the concrete type for the associated type `Assoc`.
+//
+// So, this test is just making sure that in such a case that we do
+// not immediately ICE, and instead allow the underlying type error to
+// surface.
+
+pub struct Matrix<S>(S);
+pub struct DefaultAllocator;
+
+pub trait Allocator { type Buffer; }
+
+// impl Allocator for DefaultAllocator { type Buffer = (); }
+
+#[repr(packed)]
+struct Foo(Matrix<<DefaultAllocator as Allocator>::Buffer>);
+//~^ ERROR the trait bound `DefaultAllocator: Allocator` is not satisfied
+
+fn main() { }

src/test/ui/wf/wf-packed-on-proj-of-type-as-unimpl-trait.stderr

@@ -0,0 +1,9 @@
+error[E0277]: the trait bound `DefaultAllocator: Allocator` is not satisfied
+  --> $DIR/wf-packed-on-proj-of-type-as-unimpl-trait.rs:28:12
+   |
+LL | struct Foo(Matrix<<DefaultAllocator as Allocator>::Buffer>);
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Allocator` is not implemented for `DefaultAllocator`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.