diff --git a/src/librustc_mir/interpret/intrinsics.rs b/src/librustc_mir/interpret/intrinsics.rs
index 4c86c53256e9b..334f1ea1a6901 100644
--- a/src/librustc_mir/interpret/intrinsics.rs
+++ b/src/librustc_mir/interpret/intrinsics.rs
@@ -95,7 +95,8 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             | "bitreverse" => {
                 let ty = substs.type_at(0);
                 let layout_of = self.layout_of(ty)?;
-                let bits = self.read_scalar(args[0])?.to_bits(layout_of.size)?;
+                let val = self.read_scalar(args[0])?.not_undef()?;
+                let bits = self.force_bits(val, layout_of.size)?;
                 let kind = match layout_of.abi {
                     ty::layout::Abi::Scalar(ref scalar) => scalar.value,
                     _ => throw_unsup!(TypeNotPrimitive(ty)),
@@ -149,7 +150,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                         // term since the sign of the second term can be inferred from this and
                         // the fact that the operation has overflowed (if either is 0 no
                         // overflow can occur)
-                        let first_term: u128 = l.to_scalar()?.to_bits(l.layout.size)?;
+                        let first_term: u128 = self.force_bits(l.to_scalar()?, l.layout.size)?;
                         let first_term_positive = first_term & (1 << (num_bits-1)) == 0;
                         if first_term_positive {
                             // Negative overflow not possible since the positive first term
@@ -187,7 +188,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 let (val, overflowed, _ty) = self.overflowing_binary_op(bin_op, l, r)?;
                 if overflowed {
                     let layout = self.layout_of(substs.type_at(0))?;
-                    let r_val = r.to_scalar()?.to_bits(layout.size)?;
+                    let r_val = self.force_bits(r.to_scalar()?, layout.size)?;
                     throw_ub_format!("Overflowing shift by {} in `{}`", r_val, intrinsic_name);
                 }
                 self.write_scalar(val, dest)?;
@@ -196,8 +197,10 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 // rotate_left: (X << (S % BW)) | (X >> ((BW - S) % BW))
                 // rotate_right: (X << ((BW - S) % BW)) | (X >> (S % BW))
                 let layout = self.layout_of(substs.type_at(0))?;
-                let val_bits = self.read_scalar(args[0])?.to_bits(layout.size)?;
-                let raw_shift_bits = self.read_scalar(args[1])?.to_bits(layout.size)?;
+                let val = self.read_scalar(args[0])?.not_undef()?;
+                let val_bits = self.force_bits(val, layout.size)?;
+                let raw_shift = self.read_scalar(args[1])?.not_undef()?;
+                let raw_shift_bits = self.force_bits(raw_shift, layout.size)?;
                 let width_bits = layout.size.bits() as u128;
                 let shift_bits = raw_shift_bits % width_bits;
                 let inv_shift_bits = (width_bits - shift_bits) % width_bits;
diff --git a/src/librustc_mir/interpret/operand.rs b/src/librustc_mir/interpret/operand.rs
index 7a545e8ad6f79..b5aab992e3adb 100644
--- a/src/librustc_mir/interpret/operand.rs
+++ b/src/librustc_mir/interpret/operand.rs
@@ -629,11 +629,10 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         // post-process
         Ok(match *discr_kind {
             layout::DiscriminantKind::Tag => {
-                let bits_discr = match raw_discr.to_bits(discr_val.layout.size) {
-                    Ok(raw_discr) => raw_discr,
-                    Err(_) =>
-                        throw_unsup!(InvalidDiscriminant(raw_discr.erase_tag())),
-                };
+                let bits_discr = raw_discr
+                    .not_undef()
+                    .and_then(|raw_discr| self.force_bits(raw_discr, discr_val.layout.size))
+                    .map_err(|_| err_unsup!(InvalidDiscriminant(raw_discr.erase_tag())))?;
                 let real_discr = if discr_val.layout.ty.is_signed() {
                     // going from layout tag type to typeck discriminant type
                     // requires first sign extending with the layout discriminant
diff --git a/src/librustc_mir/interpret/traits.rs b/src/librustc_mir/interpret/traits.rs
index a2fc75739ffa0..34a10de7de7fc 100644
--- a/src/librustc_mir/interpret/traits.rs
+++ b/src/librustc_mir/interpret/traits.rs
@@ -144,11 +144,13 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         let size = alloc.read_ptr_sized(
             self,
             vtable.offset(pointer_size, self)?
-        )?.to_bits(pointer_size)? as u64;
+        )?.not_undef()?;
+        let size = self.force_bits(size, pointer_size)? as u64;
         let align = alloc.read_ptr_sized(
             self,
             vtable.offset(pointer_size * 2, self)?,
-        )?.to_bits(pointer_size)? as u64;
+        )?.not_undef()?;
+        let align = self.force_bits(align, pointer_size)? as u64;
         Ok((Size::from_bytes(size), Align::from_bytes(align).unwrap()))
     }
 }