Auto merge of #133889 - compiler-errors:inh-unstable, r=Nadrieril

Consider fields to be inhabited if they are unstable

Fixes #133885 with a simple heuristic

r? Nadrieril

Not totally certain if this needs T-lang approval or a crater run.

1 files changed, 16 insertions, 0 deletions

diff --git a/compiler/rustc_middle/src/ty/inhabitedness/mod.rs b/compiler/rustc_middle/src/ty/inhabitedness/mod.rs
index 855e5043e84..32988965a35 100644
--- a/compiler/rustc_middle/src/ty/inhabitedness/mod.rs
+++ b/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;
 
@@ -84,6 +85,21 @@ impl<'tcx> VariantDef {
         InhabitedPredicate::all(
             tcx,
             self.fields.iter().map(|field| {
+                // Unstable fields are always considered to be inhabited. In the future,
+                // this could be extended to be conditional on the field being unstable
+                // only within the module that's querying the inhabitedness, like:
+                //     `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.
+                // 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);
                 if adt.is_enum() {
                     return pred;