generalize the Cons upcast impls

Author: nikomatsakis

crates/formality-core/src/collections.rs

@@ -187,13 +187,15 @@ tuple_upcast!(Vec: A, B, C, D);
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct Cons<T, C>(pub T, pub C);
 
-impl<T, U> UpcastFrom<Cons<U, Set<T>>> for Set<T>
+impl<T, U, V> UpcastFrom<Cons<U, Set<V>>> for Set<T>
 where
     T: Ord + Clone,
     U: Upcast<T>,
+    V: Ord + Upcast<T>,
 {
-    fn upcast_from(term: Cons<U, Set<T>>) -> Self {
-        let Cons(elem, mut set) = term;
+    fn upcast_from(term: Cons<U, Set<V>>) -> Self {
+        let Cons(elem, set) = term;
+        let mut set: Set<T> = set.upcast();
         set.insert(elem.upcast());
         set
     }
@@ -214,15 +216,16 @@ where
     }
 }
 
-impl<T, U> UpcastFrom<Cons<U, Vec<T>>> for Vec<T>
+impl<T, U, V> UpcastFrom<Cons<U, Vec<V>>> for Vec<T>
 where
-    T: Ord + Clone,
     U: Upcast<T>,
+    V: Upcast<T>,
 {
-    fn upcast_from(term: Cons<U, Vec<T>>) -> Self {
-        let Cons(elem, mut vec) = term;
-        vec.insert(0, elem.upcast());
-        vec
+    fn upcast_from(term: Cons<U, Vec<V>>) -> Self {
+        let Cons(elem, vec) = term;
+        std::iter::once(elem.upcast())
+            .chain(vec.into_iter().upcasted())
+            .collect()
     }
 }