Get rid of const eval_* and try_eval_* helpers

compiler/rustc_const_eval/src/const_eval/mod.rs

@@ -43,7 +43,7 @@ pub(crate) fn try_destructure_mir_constant_for_user_output<'tcx>(
 
     // We go to `usize` as we cannot allocate anything bigger anyway.
     let (field_count, variant, down) = match ty.kind() {
-        ty::Array(_, len) => (len.eval_target_usize(tcx.tcx, param_env) as usize, None, op),
+        ty::Array(_, len) => (len.try_to_target_usize(tcx.tcx)? as usize, None, op),
         ty::Adt(def, _) if def.variants().is_empty() => {
             return None;
         }

compiler/rustc_const_eval/src/interpret/cast.rs

@@ -391,7 +391,9 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 let ptr = self.read_pointer(src)?;
                 let val = Immediate::new_slice(
                     ptr,
-                    length.eval_target_usize(*self.tcx, self.param_env),
+                    length
+                        .try_to_target_usize(*self.tcx)
+                        .expect("expected monomorphic const in const eval"),
                     self,
                 );
                 self.write_immediate(val, dest)