Return Relation tuples in JoinInput::recent instead of stable

This is a bit unintuitive, but we don't have another choice given the
implementation of `Relation::from_antijoin` and the `transmute` in the
next commit, although we could probably assume that `Relation<Vec<T>>`
and `Relation<Vec<(T, ())>>` are layout compatible, it is not
guaranteed.

Perhaps these should be renamed to `old`/`new` to make things clearer?

Author: ecstatic-morse

src/join.rs

@@ -136,14 +136,15 @@ pub(crate) fn gallop<T>(mut slice: &[T], mut cmp: impl FnMut(&T) -> bool) -> &[T
 
 /// An input that can be used with `from_join`; either a `Variable` or a `Relation`.
 pub trait JoinInput<'me, Tuple: Ord>: Copy {
-    /// If we are on iteration N of the loop, these are the tuples
-    /// added on iteration N-1. (For a `Relation`, this is always an
-    /// empty slice.)
+    /// For a `Variable` on iteration N of the loop, these are the tuples added on iteration N-1.
+    ///
+    /// For a `Relation`, this is `self.elements`.
     type RecentTuples: Deref<Target = [Tuple]>;
 
-    /// If we are on iteration N of the loop, these are the tuples
-    /// added on iteration N - 2 or before. (For a `Relation`, this is
-    /// just `self`.)
+    /// For a `Variable` on iteration N of the loop, these are the tuples added on iteration N-2 or
+    /// before.
+    ///
+    /// For a `Relation`, this will be empty.
     type StableTuples: Deref<Target = [Relation<Tuple>]>;
 
     /// Get the set of recent tuples.
@@ -171,10 +172,10 @@ impl<'me, Tuple: Ord> JoinInput<'me, Tuple> for &'me Relation<Tuple> {
     type StableTuples = &'me [Relation<Tuple>];
 
     fn recent(self) -> Self::RecentTuples {
-        &[]
+        &self.elements
     }
 
     fn stable(self) -> Self::StableTuples {
-        std::slice::from_ref(self)
+        &[]
     }
 }