Rollup merge of #100229 - RalfJung:extra-const-ub-checks, r=lcnr

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

Cc https://github.com/rust-lang/rust/issues/99923
r? `@oli-obk`

compiler/rustc_mir_transform/src/const_prop.rs

@@ -183,6 +183,18 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx>
 
     type MemoryKind = !;
 
+    #[inline(always)]
+    fn enforce_alignment(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+        // We do not check for alignment to avoid having to carry an `Align`
+        // in `ConstValue::ByRef`.
+        false
+    }
+
+    #[inline(always)]
+    fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+        false // for now, we don't enforce validity
+    }
+
     fn load_mir(
         _ecx: &InterpCx<'mir, 'tcx, Self>,
         _instance: ty::InstanceDef<'tcx>,