min_exhaustive_patterns

compiler/rustc_feature/src/unstable.rs

@@ -512,6 +512,9 @@ declare_features! (
     (unstable, macro_metavar_expr, "1.61.0", Some(83527)),
     /// Allows `#[marker]` on certain traits allowing overlapping implementations.
     (unstable, marker_trait_attr, "1.30.0", Some(29864)),
+    /// Allows exhaustive pattern matching on types that contain uninhabited types in cases that are
+    /// unambiguously sound.
+    (unstable, min_exhaustive_patterns, "CURRENT_RUSTC_VERSION", None),
     /// A minimal, sound subset of specialization intended to be used by the
     /// standard library until the soundness issues with specialization
     /// are fixed.

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -298,7 +298,8 @@ impl<'thir, 'p, 'tcx> MatchVisitor<'thir, 'p, 'tcx> {
     fn is_known_valid_scrutinee(&self, scrutinee: &Expr<'tcx>) -> bool {
         use ExprKind::*;
         match &scrutinee.kind {
-            // Both pointers and references can validly point to a place with invalid data.
+            // Pointers can validly point to a place with invalid data. It is undecided whether
+            // references can too, so we conservatively assume they can.
             Deref { .. } => false,
             // Inherit validity of the parent place, unless the parent is an union.
             Field { lhs, .. } => {

compiler/rustc_pattern_analysis/src/constructor.rs

@@ -1009,6 +1009,7 @@ impl<Cx: TypeCx> ConstructorSet<Cx> {
         // In the absence of the `exhaustive_patterns` feature however, we don't count nested empty
         // types as empty. Only non-nested `!` or `enum Foo {}` are considered empty.
         if !pcx.mcx.tycx.is_exhaustive_patterns_feature_on()
+            && !pcx.mcx.tycx.is_min_exhaustive_patterns_feature_on()
             && !(pcx.is_scrutinee && matches!(self, Self::NoConstructors))
         {
             // Treat all missing constructors as nonempty.

compiler/rustc_pattern_analysis/src/lib.rs

@@ -64,6 +64,7 @@ pub trait TypeCx: Sized + Clone + fmt::Debug {
 
     fn is_opaque_ty(ty: Self::Ty) -> bool;
     fn is_exhaustive_patterns_feature_on(&self) -> bool;
+    fn is_min_exhaustive_patterns_feature_on(&self) -> bool;
 
     /// The number of fields for this constructor.
     fn ctor_arity(&self, ctor: &Constructor<Self>, ty: Self::Ty) -> usize;

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -119,7 +119,9 @@ impl<'p, 'tcx> RustcMatchCheckCtxt<'p, 'tcx> {
             // `field.ty()` doesn't normalize after substituting.
             let ty = cx.tcx.normalize_erasing_regions(cx.param_env, ty);
             let is_visible = adt.is_enum() || field.vis.is_accessible_from(cx.module, cx.tcx);
-            let is_uninhabited = cx.tcx.features().exhaustive_patterns && cx.is_uninhabited(ty);
+            let is_uninhabited = (cx.tcx.features().exhaustive_patterns
+                || cx.tcx.features().min_exhaustive_patterns)
+                && cx.is_uninhabited(ty);
 
             if is_uninhabited && (!is_visible || is_non_exhaustive) {
                 None
@@ -873,6 +875,9 @@ impl<'p, 'tcx> TypeCx for RustcMatchCheckCtxt<'p, 'tcx> {
     fn is_exhaustive_patterns_feature_on(&self) -> bool {
         self.tcx.features().exhaustive_patterns
     }
+    fn is_min_exhaustive_patterns_feature_on(&self) -> bool {
+        self.tcx.features().min_exhaustive_patterns
+    }
     fn is_opaque_ty(ty: Self::Ty) -> bool {
         matches!(ty.kind(), ty::Alias(ty::Opaque, ..))
     }

compiler/rustc_pattern_analysis/src/usefulness.rs

@@ -1199,17 +1199,23 @@ fn compute_exhaustiveness_and_usefulness<'a, 'p, Cx: TypeCx>(
 
     debug!("ty: {ty:?}");
     let pcx = &PlaceCtxt { mcx, ty, is_scrutinee: is_top_level };
+    let ctors_for_ty = pcx.ctors_for_ty();
+    let is_integers = matches!(ctors_for_ty, ConstructorSet::Integers { .. }); // For diagnostics.
 
     // Whether the place/column we are inspecting is known to contain valid data.
-    let place_validity = matrix.place_validity[0];
-    // For backwards compability we allow omitting some empty arms that we ideally shouldn't.
-    let place_validity = place_validity.allow_omitting_side_effecting_arms();
+    let mut place_validity = matrix.place_validity[0];
+    if !mcx.tycx.is_min_exhaustive_patterns_feature_on()
+        || (is_top_level && matches!(ctors_for_ty, ConstructorSet::NoConstructors))
+    {
+        // For backwards compability we allow omitting some empty arms that we ideally shouldn't.
+        place_validity = place_validity.allow_omitting_side_effecting_arms();
+    }
 
     // Analyze the constructors present in this column.
     let ctors = matrix.heads().map(|p| p.ctor());
-    let ctors_for_ty = pcx.ctors_for_ty();
-    let is_integers = matches!(ctors_for_ty, ConstructorSet::Integers { .. }); // For diagnostics.
     let split_set = ctors_for_ty.split(pcx, ctors);
+
+    // Decide what constructors to report.
     let all_missing = split_set.present.is_empty();
 
     // Build the set of constructors we will specialize with. It must cover the whole type.

compiler/rustc_span/src/symbol.rs

@@ -1015,6 +1015,7 @@ symbols! {
         min_const_fn,
         min_const_generics,
         min_const_unsafe_fn,
+        min_exhaustive_patterns,
         min_specialization,
         min_type_alias_impl_trait,
         minnumf32,