Dont consider fields that are forced unstable due to -Zforce-unstable-if-unmarked to be uninhabited

Author: compiler-errors

compiler/rustc_middle/src/ty/inhabitedness/mod.rs

@@ -43,6 +43,7 @@
 //! This code should only compile in modules where the uninhabitedness of `Foo`
 //! is visible.
 
+use rustc_span::sym;
 use rustc_type_ir::TyKind::*;
 use tracing::instrument;
 
@@ -90,7 +91,13 @@ impl<'tcx> VariantDef {
                 //     `let pred = pred.or(InhabitedPredicate::IsUnstable(field.did));`
                 // but this is unnecessary for now, since it would only affect nightly-only
                 // code or code within the standard library itself.
-                if tcx.lookup_stability(field.did).is_some_and(|stab| stab.is_unstable()) {
+                // HACK: We filter out `rustc_private` fields since with the flag
+                // `-Zforce-unstable-if-unmarked` we consider all unmarked fields to be
+                // unstable when building the compiler.
+                if tcx
+                    .lookup_stability(field.did)
+                    .is_some_and(|stab| stab.is_unstable() && stab.feature != sym::rustc_private)
+                {
                     return InhabitedPredicate::True;
                 }
                 let pred = tcx.type_of(field.did).instantiate_identity().inhabited_predicate(tcx);