Handle multiple leaf goals in LowerGoal

Author: tmandry

src/ir/lowering/mod.rs

@@ -995,10 +995,16 @@ impl<'k> LowerGoal<Env<'k>> for Goal {
             }
             Goal::Not(g) => Ok(Box::new(ir::Goal::Not(g.lower(env)?))),
             Goal::Leaf(wc) => {
-                let leaf = wc.lower(env)?;
-                assert_eq!(1, leaf.len());
-                let leaf = leaf.into_iter().next().unwrap();
-                Ok(Box::new(ir::Goal::Leaf(leaf)))
+                // A where clause can lower to multiple leaf goals; wrap these in Goal::And.
+                let mut leaves = wc.lower(env)?.into_iter();
+                let mut goal = Box::new(ir::Goal::Leaf(leaves.next().unwrap()));
+                for leaf in leaves {
+                    goal = Box::new(ir::Goal::And(
+                        goal,
+                        Box::new(ir::Goal::Leaf(leaf)),
+                    ));
+                }
+                Ok(goal)
             }
         }
     }