Burn some invariants we keep up into code

src/librustc_mir/const_eval.rs

@@ -76,7 +76,7 @@ pub fn op_to_const<'tcx>(
             _ => false,
         };
     let normalized_op = if normalize {
-        ecx.try_read_immediate(op)?
+        Ok(*ecx.read_immediate(op).expect("normalization works on validated constants"))
     } else {
         match *op {
             Operand::Indirect(mplace) => Err(mplace),

src/librustc_mir/interpret/operand.rs

@@ -269,7 +269,7 @@ pub(super) fn from_known_layout<'tcx>(
 impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M> {
     /// Try reading an immediate in memory; this is interesting particularly for ScalarPair.
     /// Returns `None` if the layout does not permit loading this as a value.
-    pub(super) fn try_read_immediate_from_mplace(
+    fn try_read_immediate_from_mplace(
         &self,
         mplace: MPlaceTy<'tcx, M::PointerTag>,
     ) -> EvalResult<'tcx, Option<Immediate<M::PointerTag>>> {
@@ -323,7 +323,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
     /// Note that for a given layout, this operation will either always fail or always
     /// succeed!  Whether it succeeds depends on whether the layout can be represented
     /// in a `Immediate`, not on which data is stored there currently.
-    pub(crate) fn try_read_immediate(
+    pub(super) fn try_read_immediate(
         &self,
         src: OpTy<'tcx, M::PointerTag>,
     ) -> EvalResult<'tcx, Result<Immediate<M::PointerTag>, MemPlace<M::PointerTag>>> {