Allow Relation<T> to be joined with Variable<(T, S)>

This case occurred in `Polonius`, where we had to create a separate
`Relation<(T, ())>` to make things work.

src/join.rs

@@ -179,3 +179,24 @@ impl<'me, Tuple: Ord> JoinInput<'me, Tuple> for &'me Relation<Tuple> {
         &[]
     }
 }
+
+impl<'me, Tuple: Ord> JoinInput<'me, (Tuple, ())> for &'me Relation<Tuple> {
+    type RecentTuples = &'me [(Tuple, ())];
+    type StableTuples = &'me [Relation<(Tuple, ())>];
+
+    fn recent(self) -> Self::RecentTuples {
+        use std::mem;
+        assert_eq!(mem::size_of::<(Tuple, ())>(), mem::size_of::<Tuple>());
+        assert_eq!(mem::align_of::<(Tuple, ())>(), mem::align_of::<Tuple>());
+
+        // SAFETY: https://rust-lang.github.io/unsafe-code-guidelines/layout/structs-and-tuples.html#structs-with-1-zst-fields
+        // guarantees that `T` is layout compatible with `(T, ())`, since `()` is a 1-ZST.
+        let elements: &'me [Tuple] = self.elements.as_slice();
+        let elements: &'me [(Tuple, ())] = unsafe { std::mem::transmute(elements) };
+        elements
+    }
+
+    fn stable(self) -> Self::StableTuples {
+        &[]
+    }
+}