Fix ICE when ADT tail has type error

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`.