diff --git a/src/librustc/mir/mod.rs b/src/librustc/mir/mod.rs
index 8a32e8f46c29..b8ccec4fec72 100644
--- a/src/librustc/mir/mod.rs
+++ b/src/librustc/mir/mod.rs
@@ -1939,7 +1939,7 @@ pub enum ProjectionElem<'tcx, V, T> {
     /// "Downcast" to a variant of an ADT. Currently, we only introduce
     /// this for ADTs with more than one variant. It may be better to
     /// just introduce it always, or always for enums.
-    Downcast(&'tcx AdtDef, usize),
+    Downcast(&'tcx AdtDef, u32),
 }
 
 /// Alias for projections as they appear in places, where the base is a place
@@ -1950,6 +1950,11 @@ pub type PlaceProjection<'tcx> = Projection<'tcx, Place<'tcx>, Local, Ty<'tcx>>;
 /// and the index is a local.
 pub type PlaceElem<'tcx> = ProjectionElem<'tcx, Local, Ty<'tcx>>;
 
+// at least on 64 bit systems, `PlaceElem` should not be larger than two pointers
+static_assert!(PROJECTION_ELEM_IS_2_PTRS_LARGE:
+    mem::size_of::<PlaceElem<'_>>() <= 16
+);
+
 /// Alias for projections as they appear in `UserTypeProjection`, where we
 /// need neither the `V` parameter for `Index` nor the `T` for `Field`.
 pub type ProjectionKind<'tcx> = ProjectionElem<'tcx, (), ()>;
@@ -1970,7 +1975,7 @@ impl<'tcx> Place<'tcx> {
     }
 
     pub fn downcast(self, adt_def: &'tcx AdtDef, variant_index: usize) -> Place<'tcx> {
-        self.elem(ProjectionElem::Downcast(adt_def, variant_index))
+        self.elem(ProjectionElem::Downcast(adt_def, variant_index as u32))
     }
 
     pub fn index(self, index: Local) -> Place<'tcx> {
@@ -2021,7 +2026,7 @@ impl<'tcx> Debug for Place<'tcx> {
             Promoted(ref promoted) => write!(fmt, "({:?}: {:?})", promoted.0, promoted.1),
             Projection(ref data) => match data.elem {
                 ProjectionElem::Downcast(ref adt_def, index) => {
-                    write!(fmt, "({:?} as {})", data.base, adt_def.variants[index].name)
+                    write!(fmt, "({:?} as {})", data.base, adt_def.variants[index as usize].name)
                 }
                 ProjectionElem::Deref => write!(fmt, "(*{:?})", data.base),
                 ProjectionElem::Field(field, ty) => {
diff --git a/src/librustc/mir/tcx.rs b/src/librustc/mir/tcx.rs
index f773f46b6f58..c9a396ff8c59 100644
--- a/src/librustc/mir/tcx.rs
+++ b/src/librustc/mir/tcx.rs
@@ -27,9 +27,13 @@ pub enum PlaceTy<'tcx> {
     /// Downcast to a particular variant of an enum.
     Downcast { adt_def: &'tcx AdtDef,
                substs: &'tcx Substs<'tcx>,
-               variant_index: usize },
+               variant_index: u32 },
 }
 
+static_assert!(PLACE_TY_IS_3_PTRS_LARGE:
+    mem::size_of::<PlaceTy<'_>>() <= 24
+);
+
 impl<'a, 'gcx, 'tcx> PlaceTy<'tcx> {
     pub fn from_ty(ty: Ty<'tcx>) -> PlaceTy<'tcx> {
         PlaceTy::Ty { ty }
@@ -58,7 +62,7 @@ impl<'a, 'gcx, 'tcx> PlaceTy<'tcx> {
             (PlaceTy::Ty {
                 ty: &ty::TyS { sty: ty::TyKind::Adt(adt_def, substs), .. } }, variant_index) |
             (PlaceTy::Downcast { adt_def, substs, variant_index }, _) => {
-                let variant_def = &adt_def.variants[variant_index];
+                let variant_def = &adt_def.variants[variant_index as usize];
                 let field_def = &variant_def.fields[f.index()];
                 field_def.ty(tcx, substs)
             }
@@ -134,7 +138,7 @@ impl<'a, 'gcx, 'tcx> PlaceTy<'tcx> {
                 match self.to_ty(tcx).sty {
                     ty::Adt(adt_def, substs) => {
                         assert!(adt_def.is_enum());
-                        assert!(index < adt_def.variants.len());
+                        assert!(index < adt_def.variants.len() as u32);
                         assert_eq!(adt_def, adt_def1);
                         PlaceTy::Downcast { adt_def,
                                             substs,
diff --git a/src/librustc/ty/mod.rs b/src/librustc/ty/mod.rs
index 9a63bb374c66..0ba28811a90a 100644
--- a/src/librustc/ty/mod.rs
+++ b/src/librustc/ty/mod.rs
@@ -1801,6 +1801,7 @@ pub struct FieldDef {
 /// table.
 pub struct AdtDef {
     pub did: DefId,
+    // TODO: make this an IndexVec
     pub variants: Vec<VariantDef>,
     flags: AdtFlags,
     pub repr: ReprOptions,
diff --git a/src/librustc/ty/structural_impls.rs b/src/librustc/ty/structural_impls.rs
index 59a66513eef0..17a5f0999e7b 100644
--- a/src/librustc/ty/structural_impls.rs
+++ b/src/librustc/ty/structural_impls.rs
@@ -33,6 +33,7 @@ CloneTypeFoldableAndLiftImpls! {
     (),
     bool,
     usize,
+    u32,
     u64,
     ::middle::region::Scope,
     ::syntax::ast::FloatTy,