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

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 @@ pub fn inhabited_predicate(
                 //     `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);

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -15,7 +15,7 @@
 };
 use rustc_middle::{bug, span_bug};
 use rustc_session::lint;
-use rustc_span::{DUMMY_SP, ErrorGuaranteed, Span};
+use rustc_span::{DUMMY_SP, ErrorGuaranteed, Span, sym};
 
 use crate::constructor::Constructor::*;
 use crate::constructor::{
@@ -232,10 +232,10 @@ fn reveal_and_alloc<'a, 'tcx>(
                             let is_visible =
                                 adt.is_enum() || field.vis.is_accessible_from(cx.module, cx.tcx);
                             let is_uninhabited = cx.is_uninhabited(*ty);
-                            let is_unstable = cx
-                                .tcx
-                                .lookup_stability(field.did)
-                                .is_some_and(|stab| stab.is_unstable());
+                            let is_unstable =
+                                cx.tcx.lookup_stability(field.did).is_some_and(|stab| {
+                                    stab.is_unstable() && stab.feature != sym::rustc_private
+                                });
                             let skip = is_uninhabited && (!is_visible || is_unstable);
                             (ty, PrivateUninhabitedField(skip))
                         });

tests/ui/uninhabited/uninhabited-pin-field.rs

@@ -0,0 +1,10 @@
+use std::pin::Pin;
+
+enum Void {}
+
+fn demo(x: Pin<Void>) {
+    match x {}
+    //~^ ERROR non-exhaustive patterns
+}
+
+fn main() {}

tests/ui/uninhabited/uninhabited-pin-field.stderr

@@ -0,0 +1,19 @@
+error[E0004]: non-exhaustive patterns: type `Pin<Void>` is non-empty
+  --> $DIR/uninhabited-pin-field.rs:6:11
+   |
+LL |     match x {}
+   |           ^
+   |
+note: `Pin<Void>` defined here
+  --> $SRC_DIR/core/src/pin.rs:LL:COL
+   = note: the matched value is of type `Pin<Void>`
+help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern as shown
+   |
+LL ~     match x {
+LL +         _ => todo!(),
+LL ~     }
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0004`.

tests/ui/uninhabited/uninhabited-unstable-field.current.stderr

@@ -14,7 +14,7 @@ help: ensure that all possible cases are being handled by adding a match arm wit
    |
 LL ~     match x {
 LL +         _ => todo!(),
-LL +     }
+LL ~     }
    |
 
 error: aborting due to 1 previous error