reintroduce commit_if_ok call into higher_ranked_sub

Author: nikomatsakis

src/librustc/infer/higher_ranked/mod.rs

@@ -29,27 +29,29 @@ impl<'a, 'gcx, 'tcx> CombineFields<'a, 'gcx, 'tcx> {
 
         let span = self.trace.cause.span;
 
-        // First, we instantiate each bound region in the supertype with a
-        // fresh placeholder region.
-        let (b_prime, _) = self.infcx.replace_bound_vars_with_placeholders(b);
+        return self.infcx.commit_if_ok(|_snapshot| {
+            // First, we instantiate each bound region in the supertype with a
+            // fresh placeholder region.
+            let (b_prime, _) = self.infcx.replace_bound_vars_with_placeholders(b);
 
-        // Next, we instantiate each bound region in the subtype
-        // with a fresh region variable. These region variables --
-        // but no other pre-existing region variables -- can name
-        // the placeholders.
-        let (a_prime, _) =
-            self.infcx
-                .replace_bound_vars_with_fresh_vars(span, HigherRankedType, a);
+            // Next, we instantiate each bound region in the subtype
+            // with a fresh region variable. These region variables --
+            // but no other pre-existing region variables -- can name
+            // the placeholders.
+            let (a_prime, _) =
+                self.infcx
+                    .replace_bound_vars_with_fresh_vars(span, HigherRankedType, a);
 
-        debug!("a_prime={:?}", a_prime);
-        debug!("b_prime={:?}", b_prime);
+            debug!("a_prime={:?}", a_prime);
+            debug!("b_prime={:?}", b_prime);
 
-        // Compare types now that bound regions have been replaced.
-        let result = self.sub(a_is_expected).relate(&a_prime, &b_prime)?;
+            // Compare types now that bound regions have been replaced.
+            let result = self.sub(a_is_expected).relate(&a_prime, &b_prime)?;
 
-        debug!("higher_ranked_sub: OK result={:?}", result);
+            debug!("higher_ranked_sub: OK result={:?}", result);
 
-        Ok(ty::Binder::bind(result))
+            Ok(ty::Binder::bind(result))
+        });
     }
 }
 
@@ -72,10 +74,10 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
     /// [rustc guide]: https://rust-lang.github.io/rustc-guide/traits/hrtb.html
     pub fn replace_bound_vars_with_placeholders<T>(
         &self,
-        binder: &ty::Binder<T>
+        binder: &ty::Binder<T>,
     ) -> (T, PlaceholderMap<'tcx>)
     where
-        T: TypeFoldable<'tcx>
+        T: TypeFoldable<'tcx>,
     {
         let next_universe = self.create_next_universe();
 
@@ -96,10 +98,15 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         let (result, map) = self.tcx.replace_bound_vars(binder, fld_r, fld_t);
 
         debug!(
-            "replace_bound_vars_with_placeholders(binder={:?}, result={:?}, map={:?})",
+            "replace_bound_vars_with_placeholders(\
+             next_universe={:?}, \
+             binder={:?}, \
+             result={:?}, \
+             map={:?})",
+            next_universe,
             binder,
             result,
-            map
+            map,
         );
 
         (result, map)