diff --git a/src/librustc/middle/subst.rs b/src/librustc/middle/subst.rs
index 5e7284dbfd1de..b9b427a9691b5 100644
--- a/src/librustc/middle/subst.rs
+++ b/src/librustc/middle/subst.rs
@@ -271,6 +271,12 @@ impl<T> VecPerParamSpace<T> {
         }
     }
 
+    pub fn shrink_to_fit(&mut self) {
+        self.vecs.mut0().shrink_to_fit();
+        self.vecs.mut1().shrink_to_fit();
+        self.vecs.mut2().shrink_to_fit();
+    }
+
     pub fn params_from_type(types: Vec<T>) -> VecPerParamSpace<T> {
         VecPerParamSpace::empty().with_vec(TypeSpace, types)
     }
diff --git a/src/librustc/middle/ty.rs b/src/librustc/middle/ty.rs
index e3d94c73bb4e9..c59c2913ad5ac 100644
--- a/src/librustc/middle/ty.rs
+++ b/src/librustc/middle/ty.rs
@@ -4615,6 +4615,7 @@ pub fn construct_parameter_environment(
         push_types_from_defs(tcx, &mut types, space,
                              generics.types.get_vec(space));
     }
+    types.shrink_to_fit();
 
     // map bound 'a => free 'a
     let mut regions = VecPerParamSpace::empty();
@@ -4622,6 +4623,7 @@ pub fn construct_parameter_environment(
         push_region_params(&mut regions, space, free_id,
                            generics.regions.get_vec(space));
     }
+    regions.shrink_to_fit();
 
     let free_substs = Substs {
         types: types,
@@ -4637,6 +4639,7 @@ pub fn construct_parameter_environment(
         push_bounds_from_defs(tcx, &mut bounds, space, &free_substs,
                               generics.types.get_vec(space));
     }
+    bounds.shrink_to_fit();
 
     debug!("construct_parameter_environment: free_id={} \
            free_subst={} \