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``

2 files changed, 2 insertions, 1 deletions

diff --git a/compiler/rustc_const_eval/src/transform/check_consts/check.rs b/compiler/rustc_const_eval/src/transform/check_consts/check.rs
index 53308cd7f90..a93e8138aa4 100644
--- a/compiler/rustc_const_eval/src/transform/check_consts/check.rs
+++ b/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>>,
diff --git a/compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs b/compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs
index 7eb3c181d69..1847847d9d2 100644
--- a/compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs
+++ b/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;
                 }