Rollup merge of #121893 - RalfJung:const-interior-mut-tests, r=oli-obk

Add tests (and a bit of cleanup) for interior mut handling in promotion and const-checking

Basically these are the parts of https://github.com/rust-lang/rust/pull/121786 that can be salvaged.

r? ``@oli-obk``

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -412,10 +412,10 @@ pub fn const_validate_mplace<'mir, 'tcx>(
             _ if cid.promoted.is_some() => CtfeValidationMode::Promoted,
             Some(mutbl) => CtfeValidationMode::Static { mutbl }, // a `static`
             None => {
-                // In normal `const` (not promoted), the outermost allocation is always only copied,
-                // so having `UnsafeCell` in there is okay despite them being in immutable memory.
-                let allow_immutable_unsafe_cell = cid.promoted.is_none() && !inner;
-                CtfeValidationMode::Const { allow_immutable_unsafe_cell }
+                // This is a normal `const` (not promoted).
+                // The outermost allocation is always only copied, so having `UnsafeCell` in there
+                // is okay despite them being in immutable memory.
+                CtfeValidationMode::Const { allow_immutable_unsafe_cell: !inner }
             }
         };
         ecx.const_validate_operand(&mplace.into(), path, &mut ref_tracking, mode)?;

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -30,7 +30,7 @@ type QualifResults<'mir, 'tcx, Q> =
     rustc_mir_dataflow::ResultsCursor<'mir, 'tcx, FlowSensitiveAnalysis<'mir, 'mir, 'tcx, Q>>;
 
 #[derive(Default)]
-pub struct Qualifs<'mir, 'tcx> {
+pub(crate) struct Qualifs<'mir, 'tcx> {
     has_mut_interior: Option<QualifResults<'mir, 'tcx, HasMutInterior>>,
     needs_drop: Option<QualifResults<'mir, 'tcx, NeedsDrop>>,
     needs_non_const_drop: Option<QualifResults<'mir, 'tcx, NeedsNonConstDrop>>,

compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs

@@ -284,6 +284,7 @@ where
                 if Q::in_adt_inherently(cx, def, args) {
                     return true;
                 }
+                // Don't do any value-based reasoning for unions.
                 if def.is_union() && Q::in_any_value_of_ty(cx, rvalue.ty(cx.body, cx.tcx)) {
                     return true;
                 }