diff --git a/src/librustc/infer/equate.rs b/src/librustc/infer/equate.rs
index 5dfa0d29daf..96d40bc81ad 100644
--- a/src/librustc/infer/equate.rs
+++ b/src/librustc/infer/equate.rs
@@ -97,7 +97,7 @@ impl TypeRelation<'tcx> for Equate<'combine, 'infcx, 'tcx> {
                self.tag(),
                a,
                b);
-        let origin = Subtype(self.fields.trace.clone());
+        let origin = Subtype(box self.fields.trace.clone());
         self.fields.infcx.borrow_region_constraints()
                          .make_eqregion(origin, a, b);
         Ok(a)
diff --git a/src/librustc/infer/error_reporting/nice_region_error/placeholder_error.rs b/src/librustc/infer/error_reporting/nice_region_error/placeholder_error.rs
index 19bd38b45b3..bfa8353ca34 100644
--- a/src/librustc/infer/error_reporting/nice_region_error/placeholder_error.rs
+++ b/src/librustc/infer/error_reporting/nice_region_error/placeholder_error.rs
@@ -30,7 +30,7 @@ impl NiceRegionError<'me, 'tcx> {
             Some(RegionResolutionError::SubSupConflict(
                 vid,
                 _,
-                SubregionOrigin::Subtype(TypeTrace {
+                SubregionOrigin::Subtype(box TypeTrace {
                     cause,
                     values: ValuePairs::TraitRefs(ExpectedFound { expected, found }),
                 }),
@@ -50,7 +50,7 @@ impl NiceRegionError<'me, 'tcx> {
             Some(RegionResolutionError::SubSupConflict(
                 vid,
                 _,
-                SubregionOrigin::Subtype(TypeTrace {
+                SubregionOrigin::Subtype(box TypeTrace {
                     cause,
                     values: ValuePairs::TraitRefs(ExpectedFound { expected, found }),
                 }),
@@ -70,7 +70,7 @@ impl NiceRegionError<'me, 'tcx> {
             Some(RegionResolutionError::SubSupConflict(
                 vid,
                 _,
-                SubregionOrigin::Subtype(TypeTrace {
+                SubregionOrigin::Subtype(box TypeTrace {
                     cause,
                     values: ValuePairs::TraitRefs(ExpectedFound { expected, found }),
                 }),
@@ -92,7 +92,7 @@ impl NiceRegionError<'me, 'tcx> {
                 _,
                 _,
                 _,
-                SubregionOrigin::Subtype(TypeTrace {
+                SubregionOrigin::Subtype(box TypeTrace {
                     cause,
                     values: ValuePairs::TraitRefs(ExpectedFound { expected, found }),
                 }),
@@ -108,7 +108,7 @@ impl NiceRegionError<'me, 'tcx> {
             )),
 
             Some(RegionResolutionError::ConcreteFailure(
-                SubregionOrigin::Subtype(TypeTrace {
+                SubregionOrigin::Subtype(box TypeTrace {
                     cause,
                     values: ValuePairs::TraitRefs(ExpectedFound { expected, found }),
                 }),
@@ -125,7 +125,7 @@ impl NiceRegionError<'me, 'tcx> {
             )),
 
             Some(RegionResolutionError::ConcreteFailure(
-                SubregionOrigin::Subtype(TypeTrace {
+                SubregionOrigin::Subtype(box TypeTrace {
                     cause,
                     values: ValuePairs::TraitRefs(ExpectedFound { expected, found }),
                 }),
@@ -142,7 +142,7 @@ impl NiceRegionError<'me, 'tcx> {
             )),
 
             Some(RegionResolutionError::ConcreteFailure(
-                SubregionOrigin::Subtype(TypeTrace {
+                SubregionOrigin::Subtype(box TypeTrace {
                     cause,
                     values: ValuePairs::TraitRefs(ExpectedFound { expected, found }),
                 }),
diff --git a/src/librustc/infer/error_reporting/note.rs b/src/librustc/infer/error_reporting/note.rs
index caed4288892..115ffea97bf 100644
--- a/src/librustc/infer/error_reporting/note.rs
+++ b/src/librustc/infer/error_reporting/note.rs
@@ -138,7 +138,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                                           sup: Region<'tcx>)
                                           -> DiagnosticBuilder<'tcx> {
         match origin {
-            infer::Subtype(trace) => {
+            infer::Subtype(box trace) => {
                 let terr = TypeError::RegionsDoesNotOutlive(sup, sub);
                 let mut err = self.report_and_explain_type_error(trace, &terr);
                 self.tcx.note_and_explain_region(region_scope_tree, &mut err, "", sup, "...");
@@ -450,7 +450,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     ) -> DiagnosticBuilder<'tcx> {
         // I can't think how to do better than this right now. -nikomatsakis
         match placeholder_origin {
-            infer::Subtype(trace) => {
+            infer::Subtype(box trace) => {
                 let terr = TypeError::RegionsPlaceholderMismatch;
                 self.report_and_explain_type_error(trace, &terr)
             }
diff --git a/src/librustc/infer/glb.rs b/src/librustc/infer/glb.rs
index 2cef5211762..10e45321a6d 100644
--- a/src/librustc/infer/glb.rs
+++ b/src/librustc/infer/glb.rs
@@ -57,7 +57,7 @@ impl TypeRelation<'tcx> for Glb<'combine, 'infcx, 'tcx> {
                a,
                b);
 
-        let origin = Subtype(self.fields.trace.clone());
+        let origin = Subtype(box self.fields.trace.clone());
         Ok(self.fields.infcx.borrow_region_constraints().glb_regions(self.tcx(), origin, a, b))
     }
 
diff --git a/src/librustc/infer/lub.rs b/src/librustc/infer/lub.rs
index e20372f1513..8b64cda7bd2 100644
--- a/src/librustc/infer/lub.rs
+++ b/src/librustc/infer/lub.rs
@@ -57,7 +57,7 @@ impl TypeRelation<'tcx> for Lub<'combine, 'infcx, 'tcx> {
                a,
                b);
 
-        let origin = Subtype(self.fields.trace.clone());
+        let origin = Subtype(box self.fields.trace.clone());
         Ok(self.fields.infcx.borrow_region_constraints().lub_regions(self.tcx(), origin, a, b))
     }
 
diff --git a/src/librustc/infer/mod.rs b/src/librustc/infer/mod.rs
index 8638f42976f..a886c44a479 100644
--- a/src/librustc/infer/mod.rs
+++ b/src/librustc/infer/mod.rs
@@ -254,7 +254,7 @@ pub struct TypeTrace<'tcx> {
 #[derive(Clone, Debug)]
 pub enum SubregionOrigin<'tcx> {
     /// Arose from a subtyping relation
-    Subtype(TypeTrace<'tcx>),
+    Subtype(Box<TypeTrace<'tcx>>),
 
     /// Stack-allocated closures cannot outlive innermost loop
     /// or function so as to ensure we only require finite stack
@@ -340,6 +340,10 @@ pub enum SubregionOrigin<'tcx> {
     },
 }
 
+// `SubregionOrigin` is used a lot. Make sure it doesn't unintentionally get bigger.
+#[cfg(target_arch = "x86_64")]
+static_assert_size!(SubregionOrigin<'_>, 32);
+
 /// Places that type/region parameters can appear.
 #[derive(Clone, Copy, Debug)]
 pub enum ParameterOrigin {
diff --git a/src/librustc/infer/sub.rs b/src/librustc/infer/sub.rs
index cd1d206b5fc..76db55ecfa8 100644
--- a/src/librustc/infer/sub.rs
+++ b/src/librustc/infer/sub.rs
@@ -130,7 +130,7 @@ impl TypeRelation<'tcx> for Sub<'combine, 'infcx, 'tcx> {
         // FIXME -- we have more fine-grained information available
         // from the "cause" field, we could perhaps give more tailored
         // error messages.
-        let origin = SubregionOrigin::Subtype(self.fields.trace.clone());
+        let origin = SubregionOrigin::Subtype(box self.fields.trace.clone());
         self.fields.infcx.borrow_region_constraints()
                          .make_subregion(origin, a, b);