Fix ICE when ADT tail has type error

compiler/rustc_middle/src/ty/layout.rs

@@ -338,6 +338,10 @@ impl<'tcx> SizeSkeleton<'tcx> {
                         debug_assert!(tail.has_non_region_param());
                         Ok(SizeSkeleton::Pointer { non_zero, tail: tcx.erase_regions(tail) })
                     }
+                    ty::Error(guar) => {
+                        // Fixes ICE #124031
+                        return Err(tcx.arena.alloc(LayoutError::ReferencesError(*guar)));
+                    }
                     _ => bug!(
                         "SizeSkeleton::compute({ty}): layout errored ({err:?}), yet \
                               tail `{tail}` is not a type parameter or a projection",

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`.