add -Zextra-const-ub-checks to enable more UB checking in const-eval

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -236,6 +236,16 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
 
     const PANIC_ON_ALLOC_FAIL: bool = false; // will be raised as a proper error
 
+    #[inline(always)]
+    fn enforce_alignment(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+        ecx.tcx.sess.opts.unstable_opts.extra_const_ub_checks
+    }
+
+    #[inline(always)]
+    fn enforce_validity(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+        ecx.tcx.sess.opts.unstable_opts.extra_const_ub_checks
+    }
+
     fn load_mir(
         ecx: &InterpCx<'mir, 'tcx, Self>,
         instance: ty::InstanceDef<'tcx>,

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -1005,6 +1005,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     /// It will error if the bits at the destination do not match the ones described by the layout.
     #[inline(always)]
     pub fn validate_operand(&self, op: &OpTy<'tcx, M::Provenance>) -> InterpResult<'tcx> {
+        // Note that we *could* actually be in CTFE here with `-Zextra-const-ub-checks`, but it's
+        // still correct to not use `ctfe_mode`: that mode is for validation of the final constant
+        // value, it rules out things like `UnsafeCell` in awkward places. It also can make checking
+        // recurse through references which, for now, we don't want here, either.
         self.validate_operand_internal(op, vec![], None, None)
     }
 }