Extend check for UnsafeCell in consts to cover unions

A validity companion to changes from #90373.

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -14,6 +14,7 @@ use rustc_middle::mir::interpret::InterpError;
 use rustc_middle::ty;
 use rustc_middle::ty::layout::{LayoutOf, TyAndLayout};
 use rustc_span::symbol::{sym, Symbol};
+use rustc_span::DUMMY_SP;
 use rustc_target::abi::{Abi, Scalar as ScalarAbi, Size, VariantIdx, Variants, WrappingRange};
 
 use std::hash::Hash;
@@ -736,9 +737,15 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValueVisitor<'mir, 'tcx, M>
     #[inline(always)]
     fn visit_union(
         &mut self,
-        _op: &OpTy<'tcx, M::PointerTag>,
+        op: &OpTy<'tcx, M::PointerTag>,
         _fields: NonZeroUsize,
     ) -> InterpResult<'tcx> {
+        // Special check preventing `UnsafeCell` inside unions in the inner part of constants.
+        if matches!(self.ctfe_mode, Some(CtfeValidationMode::Const { inner: true, .. })) {
+            if !op.layout.ty.is_freeze(self.ecx.tcx.at(DUMMY_SP), self.ecx.param_env) {
+                throw_validation_failure!(self.path, { "`UnsafeCell` in a `const`" });
+            }
+        }
         Ok(())
     }