Fix ICE when ADT tail has type error

compiler/rustc_middle/src/ty/layout.rs

@@ -331,6 +331,12 @@ impl<'tcx> SizeSkeleton<'tcx> {
             ty::Ref(_, pointee, _) | ty::RawPtr(pointee, _) => {
                 let non_zero = !ty.is_unsafe_ptr();
                 let tail = tcx.struct_tail_erasing_lifetimes(pointee, param_env);
+
+                // Check if tail has type error. Fixes ICE #124031
+                if let Err(guar) = tail.error_reported() {
+                    return Err(tcx.arena.alloc(LayoutError::ReferencesError(guar)));
+                }
+
                 match tail.kind() {
                     ty::Param(_) | ty::Alias(ty::Projection | ty::Inherent, _) => {
                         debug_assert!(tail.has_non_region_param());

tests/ui/layout/ice-type-error-in-tail-124031.rs

@@ -0,0 +1,20 @@
+// Regression test for issue #124031
+// Checks that we don't ICE when the tail
+// of an ADT has a type error
+
+trait Trait {
+    type RefTarget;
+}
+
+impl Trait for () {}
+//~^ ERROR not all trait items implemented, missing: `RefTarget`
+
+struct Other {
+    data: <() as Trait>::RefTarget,
+}
+
+fn main() {
+    unsafe {
+        std::mem::transmute::<Option<()>, Option<&Other>>(None);
+    }
+}

tests/ui/layout/ice-type-error-in-tail-124031.stderr

@@ -0,0 +1,12 @@
+error[E0046]: not all trait items implemented, missing: `RefTarget`
+  --> $DIR/ice-type-error-in-tail-124031.rs:9:1
+   |
+LL |     type RefTarget;
+   |     -------------- `RefTarget` from trait
+...
+LL | impl Trait for () {}
+   | ^^^^^^^^^^^^^^^^^ missing `RefTarget` in implementation
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0046`.