remove deref impl and add an index impl

The constraint set is going to be more than just a set of outlives
constraints.

src/librustc_mir/borrow_check/nll/constraints/mod.rs

@@ -1,11 +1,10 @@
+use crate::borrow_check::nll::type_check::Locations;
 use rustc::mir::ConstraintCategory;
 use rustc::ty::RegionVid;
 use rustc_data_structures::graph::scc::Sccs;
 use rustc_data_structures::indexed_vec::{Idx, IndexVec};
-use crate::borrow_check::nll::type_check::Locations;
-
 use std::fmt;
-use std::ops::Deref;
+use std::ops::Index;
 
 crate mod graph;
 
@@ -58,13 +57,17 @@ impl ConstraintSet {
         let region_graph = &constraint_graph.region_graph(self, static_region);
         Sccs::new(region_graph)
     }
+
+    crate fn outlives(&self) -> &IndexVec<OutlivesConstraintIndex, OutlivesConstraint> {
+        &self.outlives
+    }
 }
 
-impl Deref for ConstraintSet {
-    type Target = IndexVec<OutlivesConstraintIndex, OutlivesConstraint>;
+impl Index<OutlivesConstraintIndex> for ConstraintSet {
+    type Output = OutlivesConstraint;
 
-    fn deref(&self) -> &Self::Target {
-        &self.outlives
+    fn index(&self, i: OutlivesConstraintIndex) -> &Self::Output {
+        &self.outlives[i]
     }
 }
 

src/librustc_mir/borrow_check/nll/region_infer/dump_mir.rs

@@ -71,7 +71,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             }
         }
 
-        let mut constraints: Vec<_> = self.constraints.iter().collect();
+        let mut constraints: Vec<_> = self.constraints.outlives().iter().collect();
         constraints.sort();
         for constraint in &constraints {
             let OutlivesConstraint {

src/librustc_mir/borrow_check/nll/region_infer/graphviz.rs

@@ -63,7 +63,7 @@ impl<'a, 'this, 'tcx> dot::GraphWalk<'this> for RawConstraints<'a, 'tcx> {
         vids.into()
     }
     fn edges(&'this self) -> dot::Edges<'this, OutlivesConstraint> {
-        (&self.regioncx.constraints.raw[..]).into()
+        (&self.regioncx.constraints.outlives().raw[..]).into()
     }
 
     // Render `a: b` as `a -> b`, indicating the flow

src/librustc_mir/borrow_check/nll/region_infer/mod.rs

@@ -472,7 +472,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         debug!("propagate_constraints()");
 
         debug!("propagate_constraints: constraints={:#?}", {
-            let mut constraints: Vec<_> = self.constraints.iter().collect();
+            let mut constraints: Vec<_> = self.constraints.outlives().iter().collect();
             constraints.sort();
             constraints
                 .into_iter()

src/librustc_mir/borrow_check/nll/type_check/mod.rs

@@ -215,7 +215,7 @@ fn translate_outlives_facts(cx: &mut BorrowCheckContext<'_, '_>) {
         let location_table = cx.location_table;
         facts
             .outlives
-            .extend(cx.constraints.outlives_constraints.iter().flat_map(
+            .extend(cx.constraints.outlives_constraints.outlives().iter().flat_map(
                 |constraint: &OutlivesConstraint| {
                     if let Some(from_location) = constraint.locations.from_location() {
                         Either::Left(iter::once((
@@ -582,7 +582,7 @@ impl<'a, 'b, 'tcx> TypeVerifier<'a, 'b, 'tcx> {
         );
 
         let locations = location.to_locations();
-        for constraint in constraints.iter() {
+        for constraint in constraints.outlives().iter() {
             let mut constraint = *constraint;
             constraint.locations = locations;
             if let ConstraintCategory::Return