introduce a dummy leak check and invoke it in all the right places

This set of diffs was produced by combing through
b68fad6 and seeing where the
`leak_check` used to be invoked and how.

src/librustc/infer/higher_ranked/mod.rs

@@ -4,6 +4,7 @@
 use super::combine::CombineFields;
 use super::{HigherRankedType, InferCtxt, PlaceholderMap};
 
+use crate::infer::CombinedSnapshot;
 use crate::ty::relate::{Relate, RelateResult, TypeRelation};
 use crate::ty::{self, Binder, TypeFoldable};
 
@@ -29,10 +30,10 @@ impl<'a, 'gcx, 'tcx> CombineFields<'a, 'gcx, 'tcx> {
 
         let span = self.trace.cause.span;
 
-        return self.infcx.commit_if_ok(|_snapshot| {
+        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);
+            let (b_prime, placeholder_map) = 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 --
@@ -48,6 +49,9 @@ impl<'a, 'gcx, 'tcx> CombineFields<'a, 'gcx, '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)?;
+
             debug!("higher_ranked_sub: OK result={:?}", result);
 
             Ok(ty::Binder::bind(result))
@@ -108,4 +112,22 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
 
         (result, map)
     }
+
+    /// Searches region constraints created since `snapshot` that
+    /// affect one of the placeholders in `placeholder_map`, returning
+    /// an error if any of the placeholders are related to another
+    /// placeholder or would have to escape into some parent universe
+    /// that cannot name them.
+    ///
+    /// This is a temporary backwards compatibility measure to try and
+    /// retain the older (arguably incorrect) behavior of the
+    /// compiler.
+    pub fn leak_check(
+        &self,
+        _overly_polymorphic: bool,
+        _placeholder_map: &PlaceholderMap<'tcx>,
+        _snapshot: &CombinedSnapshot<'_, 'tcx>,
+    ) -> RelateResult<'tcx, ()> {
+        Ok(())
+    }
 }

src/librustc/infer/mod.rs

@@ -937,34 +937,41 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             return None;
         }
 
-        Some(self.commit_if_ok(|_snapshot| {
+        Some(self.commit_if_ok(|snapshot| {
             let (
                 ty::SubtypePredicate {
                     a_is_expected,
                     a,
                     b,
                 },
-                _,
+                placeholder_map,
             ) = self.replace_bound_vars_with_placeholders(predicate);
 
-            Ok(
-                self.at(cause, param_env)
-                    .sub_exp(a_is_expected, a, b)?
-                    .unit(),
-            )
+            let ok = self.at(cause, param_env)
+                .sub_exp(a_is_expected, a, b)?;
+
+            self.leak_check(false, &placeholder_map, snapshot)?;
+
+            Ok(ok.unit())
         }))
     }
 
     pub fn region_outlives_predicate(
         &self,
         cause: &traits::ObligationCause<'tcx>,
         predicate: &ty::PolyRegionOutlivesPredicate<'tcx>,
-    ) {
-        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`
+    ) -> UnitResult<'tcx> {
+        self.commit_if_ok(|snapshot| {
+            let (ty::OutlivesPredicate(r_a, r_b), placeholder_map) =
+                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)?;
+            Ok(())
+        })
     }
 
     pub fn next_ty_var_id(&self, diverging: bool, origin: TypeVariableOrigin) -> TyVid {

src/librustc/traits/auto_trait.rs

@@ -771,7 +771,13 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
                     }
                 }
                 &ty::Predicate::RegionOutlives(ref binder) => {
-                    let () = select.infcx().region_outlives_predicate(&dummy_cause, binder);
+                    if select
+                        .infcx()
+                        .region_outlives_predicate(&dummy_cause, binder)
+                        .is_err()
+                    {
+                        return false;
+                    }
                 }
                 &ty::Predicate::TypeOutlives(ref binder) => {
                     match (

src/librustc/traits/error_reporting.rs

@@ -730,9 +730,14 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                     }
 
                     ty::Predicate::RegionOutlives(ref predicate) => {
-                        // These errors should show up as region
-                        // inference failures.
-                        panic!("region outlives {:?} failed", predicate);
+                        let predicate = self.resolve_type_vars_if_possible(predicate);
+                        let err = self.region_outlives_predicate(&obligation.cause,
+                                                                 &predicate).err().unwrap();
+                        struct_span_err!(
+                            self.tcx.sess, span, E0279,
+                            "the requirement `{}` is not satisfied (`{}`)",
+                            predicate, err,
+                        )
                     }
 
                     ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {

src/librustc/traits/fulfill.rs

@@ -331,8 +331,10 @@ impl<'a, 'b, 'gcx, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'gcx,
             }
 
             ty::Predicate::RegionOutlives(ref binder) => {
-                let () = self.selcx.infcx().region_outlives_predicate(&obligation.cause, binder);
-                ProcessResult::Changed(vec![])
+                match self.selcx.infcx().region_outlives_predicate(&obligation.cause, binder) {
+                    Ok(()) => ProcessResult::Changed(vec![]),
+                    Err(_) => ProcessResult::Error(CodeSelectionError(Unimplemented)),
+                }
             }
 
             ty::Predicate::TypeOutlives(ref binder) => {

src/librustc/traits/project.rs

@@ -191,12 +191,15 @@ pub fn poly_project_and_unify_type<'cx, 'gcx, 'tcx>(
            obligation);
 
     let infcx = selcx.infcx();
-    infcx.commit_if_ok(|_| {
-        let (placeholder_predicate, _) =
+    infcx.commit_if_ok(|snapshot| {
+        let (placeholder_predicate, placeholder_map) =
             infcx.replace_bound_vars_with_placeholders(&obligation.predicate);
 
         let placeholder_obligation = obligation.with(placeholder_predicate);
-        project_and_unify_type(selcx, &placeholder_obligation)
+        let result = project_and_unify_type(selcx, &placeholder_obligation)?;
+        infcx.leak_check(false, &placeholder_map, snapshot)
+            .map_err(|err| MismatchedProjectionTypes { err })?;
+        Ok(result)
     })
 }
 
@@ -1427,9 +1430,8 @@ fn confirm_callable_candidate<'cx, 'gcx, 'tcx>(
 fn confirm_param_env_candidate<'cx, 'gcx, 'tcx>(
     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
     obligation: &ProjectionTyObligation<'tcx>,
-    poly_cache_entry: ty::PolyProjectionPredicate<'tcx>)
-    -> Progress<'tcx>
-{
+    poly_cache_entry: ty::PolyProjectionPredicate<'tcx>,
+) -> Progress<'tcx> {
     let infcx = selcx.infcx();
     let cause = &obligation.cause;
     let param_env = obligation.param_env;

src/librustc/traits/select.rs

@@ -29,7 +29,7 @@ use super::{
 
 use crate::dep_graph::{DepKind, DepNodeIndex};
 use crate::hir::def_id::DefId;
-use crate::infer::{InferCtxt, InferOk, TypeFreshener};
+use crate::infer::{CombinedSnapshot, InferCtxt, InferOk, PlaceholderMap, TypeFreshener};
 use crate::middle::lang_items;
 use crate::mir::interpret::GlobalId;
 use crate::ty::fast_reject;
@@ -1667,8 +1667,11 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
             _ => return,
         }
 
-        let result = self.infcx.probe(|_| {
-            self.match_projection_obligation_against_definition_bounds(obligation)
+        let result = self.infcx.probe(|snapshot| {
+            self.match_projection_obligation_against_definition_bounds(
+                obligation,
+                snapshot,
+            )
         });
 
         if result {
@@ -1679,10 +1682,11 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
     fn match_projection_obligation_against_definition_bounds(
         &mut self,
         obligation: &TraitObligation<'tcx>,
+        snapshot: &CombinedSnapshot<'_, 'tcx>,
     ) -> bool {
         let poly_trait_predicate = self.infcx()
             .resolve_type_vars_if_possible(&obligation.predicate);
-        let (placeholder_trait_predicate, _) = self.infcx()
+        let (placeholder_trait_predicate, placeholder_map) = self.infcx()
             .replace_bound_vars_with_placeholders(&poly_trait_predicate);
         debug!(
             "match_projection_obligation_against_definition_bounds: \
@@ -1724,6 +1728,8 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                         obligation,
                         bound.clone(),
                         placeholder_trait_predicate.trait_ref.clone(),
+                        &placeholder_map,
+                        snapshot,
                     )
                 })
             });
@@ -1741,6 +1747,8 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                     obligation,
                     bound,
                     placeholder_trait_predicate.trait_ref.clone(),
+                    &placeholder_map,
+                    snapshot,
                 );
 
                 assert!(result);
@@ -1754,12 +1762,16 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
         obligation: &TraitObligation<'tcx>,
         trait_bound: ty::PolyTraitRef<'tcx>,
         placeholder_trait_ref: ty::TraitRef<'tcx>,
+        placeholder_map: &PlaceholderMap<'tcx>,
+        snapshot: &CombinedSnapshot<'_, 'tcx>,
     ) -> bool {
         debug_assert!(!placeholder_trait_ref.has_escaping_bound_vars());
         self.infcx
             .at(&obligation.cause, obligation.param_env)
             .sup(ty::Binder::dummy(placeholder_trait_ref), trait_bound)
             .is_ok()
+            &&
+            self.infcx.leak_check(false, placeholder_map, snapshot).is_ok()
     }
 
     /// Given an obligation like `<SomeTrait for T>`, search the obligations that the caller
@@ -1960,8 +1972,8 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
             obligation.predicate.def_id(),
             obligation.predicate.skip_binder().trait_ref.self_ty(),
             |impl_def_id| {
-                self.infcx.probe(|_| {
-                    if let Ok(_substs) = self.match_impl(impl_def_id, obligation)
+                self.infcx.probe(|snapshot| {
+                    if let Ok(_substs) = self.match_impl(impl_def_id, obligation, snapshot)
                     {
                         candidates.vec.push(ImplCandidate(impl_def_id));
                     }
@@ -2758,9 +2770,12 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
     }
 
     fn confirm_projection_candidate(&mut self, obligation: &TraitObligation<'tcx>) {
-        self.infcx.in_snapshot(|_| {
+        self.infcx.in_snapshot(|snapshot| {
             let result =
-                self.match_projection_obligation_against_definition_bounds(obligation);
+                self.match_projection_obligation_against_definition_bounds(
+                    obligation,
+                    snapshot,
+                );
             assert!(result);
         })
     }
@@ -2912,8 +2927,8 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
 
         // First, create the substitutions by matching the impl again,
         // this time not in a probe.
-        self.infcx.in_snapshot(|_| {
-            let substs = self.rematch_impl(impl_def_id, obligation);
+        self.infcx.in_snapshot(|snapshot| {
+            let substs = self.rematch_impl(impl_def_id, obligation, snapshot);
             debug!("confirm_impl_candidate: substs={:?}", substs);
             let cause = obligation.derived_cause(ImplDerivedObligation);
             self.vtable_impl(
@@ -3504,8 +3519,9 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
         &mut self,
         impl_def_id: DefId,
         obligation: &TraitObligation<'tcx>,
+        snapshot: &CombinedSnapshot<'_, 'tcx>,
     ) -> Normalized<'tcx, &'tcx Substs<'tcx>> {
-        match self.match_impl(impl_def_id, obligation) {
+        match self.match_impl(impl_def_id, obligation, snapshot) {
             Ok(substs) => substs,
             Err(()) => {
                 bug!(
@@ -3521,6 +3537,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
         &mut self,
         impl_def_id: DefId,
         obligation: &TraitObligation<'tcx>,
+        snapshot: &CombinedSnapshot<'_, 'tcx>,
     ) -> Result<Normalized<'tcx, &'tcx Substs<'tcx>>, ()> {
         let impl_trait_ref = self.tcx().impl_trait_ref(impl_def_id).unwrap();
 
@@ -3531,7 +3548,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
             return Err(());
         }
 
-        let (skol_obligation, _) = self.infcx()
+        let (skol_obligation, placeholder_map) = self.infcx()
             .replace_bound_vars_with_placeholders(&obligation.predicate);
         let skol_obligation_trait_ref = skol_obligation.trait_ref;
 
@@ -3563,6 +3580,11 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
             .map_err(|e| debug!("match_impl: failed eq_trait_refs due to `{}`", e))?;
         nested_obligations.extend(obligations);
 
+        if let Err(e) = self.infcx.leak_check(false, &placeholder_map, snapshot) {
+            debug!("match_impl: failed leak check due to `{}`", e);
+            return Err(());
+        }
+
         debug!("match_impl: success impl_substs={:?}", impl_substs);
         Ok(Normalized {
             value: impl_substs,