rewrite leak check to be based on universes

In the new leak check, instead of getting a list of placeholders to
track, we look for any placeholder that is part of a universe which
was created during the snapshot.

We are looking for the following error patterns:

* P1: P2, where P1 != P2
* P1: R, where R is in some universe that cannot name P1

This new leak check is more precise than before, in that it accepts
this patterns:

* R: P1, even if R cannot name P1, because R = 'static is a valid
sol'n
* R: P1, R: P2, as above

Note that this leak check, when running during subtyping, is less
efficient than before in some sense because it is going to check and
re-check all the universes created since the snapshot. We're going to
move when the leak check runs to try and correct that.

Author: nikomatsakis

src/librustc_infer/infer/higher_ranked/mod.rs

@@ -33,7 +33,7 @@ impl<'a, 'tcx> CombineFields<'a, 'tcx> {
         self.infcx.commit_if_ok(|snapshot| {
             // First, we instantiate each bound region in the supertype with a
             // fresh placeholder region.
-            let (b_prime, placeholder_map) = self.infcx.replace_bound_vars_with_placeholders(b);
+            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 --
@@ -48,7 +48,7 @@ impl<'a, 'tcx> CombineFields<'a, 'tcx> {
             // Compare types now that bound regions have been replaced.
             let result = self.sub(a_is_expected).relate(&a_prime, &b_prime)?;
 
-            self.infcx.leak_check(!a_is_expected, &placeholder_map, snapshot)?;
+            self.infcx.leak_check(!a_is_expected, snapshot)?;
 
             debug!("higher_ranked_sub: OK result={:?}", result);
 
@@ -119,7 +119,6 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     pub fn leak_check(
         &self,
         overly_polymorphic: bool,
-        placeholder_map: &PlaceholderMap<'tcx>,
         snapshot: &CombinedSnapshot<'_, 'tcx>,
     ) -> RelateResult<'tcx, ()> {
         // If the user gave `-Zno-leak-check`, or we have been
@@ -135,7 +134,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         self.inner.borrow_mut().unwrap_region_constraints().leak_check(
             self.tcx,
             overly_polymorphic,
-            placeholder_map,
+            self.universe(),
             snapshot,
         )
     }

src/librustc_infer/infer/mod.rs

@@ -992,12 +992,12 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         }
 
         Some(self.commit_if_ok(|snapshot| {
-            let (ty::SubtypePredicate { a_is_expected, a, b }, placeholder_map) =
+            let (ty::SubtypePredicate { a_is_expected, a, b }, _) =
                 self.replace_bound_vars_with_placeholders(&predicate);
 
             let ok = self.at(cause, param_env).sub_exp(a_is_expected, a, b)?;
 
-            self.leak_check(false, &placeholder_map, snapshot)?;
+            self.leak_check(false, snapshot)?;
 
             Ok(ok.unit())
         }))
@@ -1009,13 +1009,13 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         predicate: ty::PolyRegionOutlivesPredicate<'tcx>,
     ) -> UnitResult<'tcx> {
         self.commit_if_ok(|snapshot| {
-            let (ty::OutlivesPredicate(r_a, r_b), placeholder_map) =
+            let (ty::OutlivesPredicate(r_a, r_b), _) =
                 self.replace_bound_vars_with_placeholders(&predicate);
             let origin = SubregionOrigin::from_obligation_cause(cause, || {
                 RelateRegionParamBound(cause.span)
             });
             self.sub_regions(origin, r_b, r_a); // `b : a` ==> `a <= b`
-            self.leak_check(false, &placeholder_map, snapshot)?;
+            self.leak_check(false, snapshot)?;
             Ok(())
         })
     }