change ucanonicalize from folder to visitor

Author: basil-cow

chalk-solve/src/infer/ucanonicalize.rs

@@ -1,12 +1,13 @@
 use chalk_engine::fallible::*;
 use chalk_ir::fold::{Fold, Folder};
+use chalk_ir::visit::{Visit, Visitor};
 use chalk_ir::interner::Interner;
 use chalk_ir::*;
 
 use super::InferenceTable;
 
 impl<I: Interner> InferenceTable<I> {
-    pub(crate) fn u_canonicalize<T: Fold<I>>(
+    pub(crate) fn u_canonicalize<T: Fold<I> + Visit<I>>(
         &mut self,
         interner: &I,
         value0: &Canonical<T>,
@@ -17,14 +18,13 @@ impl<I: Interner> InferenceTable<I> {
         let mut universes = UniverseMap::new();
         value0
             .value
-            .fold_with(
+            .visit_with(
                 &mut UCollector {
                     universes: &mut universes,
                     interner,
                 },
                 DebruijnIndex::INNERMOST,
-            )
-            .unwrap();
+            );
 
         // Now re-map the universes found in value. We have to do this
         // in a second pass because it is only then that we know the
@@ -239,30 +239,30 @@ struct UCollector<'q, 'i, I> {
     interner: &'i I,
 }
 
-impl<'i, I: Interner> Folder<'i, I> for UCollector<'_, 'i, I>
+impl<'i, I: Interner> Visitor<'i, I> for UCollector<'_, 'i, I>
 where
     I: 'i,
 {
-    fn as_dyn(&mut self) -> &mut dyn Folder<'i, I> {
+    type Result = ();
+
+    fn as_dyn(&mut self) -> &mut dyn Visitor<'i, I, Result = ()> {
         self
     }
 
-    fn fold_free_placeholder_ty(
+    fn visit_free_placeholder_ty(
         &mut self,
         universe: PlaceholderIndex,
         _outer_binder: DebruijnIndex,
-    ) -> Fallible<Ty<I>> {
+    ) {
         self.universes.add(universe.ui);
-        Ok(universe.to_ty::<I>(self.interner()))
     }
 
-    fn fold_free_placeholder_lifetime(
+    fn visit_free_placeholder_lifetime(
         &mut self,
         universe: PlaceholderIndex,
         _outer_binder: DebruijnIndex,
-    ) -> Fallible<Lifetime<I>> {
+    ) {
         self.universes.add(universe.ui);
-        Ok(universe.to_lifetime(self.interner()))
     }
 
     fn forbid_inference_vars(&self) -> bool {
@@ -272,10 +272,6 @@ where
     fn interner(&self) -> &'i I {
         self.interner
     }
-
-    fn target_interner(&self) -> &'i I {
-        self.interner()
-    }
 }
 
 struct UMapToCanonical<'q, I> {