Add the min_exhaustive_patterns feature gate

compiler/rustc_feature/src/unstable.rs

@@ -516,6 +516,9 @@ 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.
+    (incomplete, min_exhaustive_patterns, "CURRENT_RUSTC_VERSION", Some(119612)),
     /// A minimal, sound subset of specialization intended to be used by the
     /// standard library until the soundness issues with specialization
     /// are fixed.

compiler/rustc_pattern_analysis/src/lib.rs

@@ -95,6 +95,7 @@ pub trait TypeCx: Sized + fmt::Debug {
     type PatData: Clone;
 
     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

@@ -182,7 +182,9 @@ 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
@@ -960,6 +962,9 @@ 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 ctor_arity(&self, ctor: &crate::constructor::Constructor<Self>, ty: &Self::Ty) -> usize {
         self.ctor_arity(ctor, *ty)

compiler/rustc_pattern_analysis/src/usefulness.rs

@@ -548,11 +548,12 @@
 //! [`ValidityConstraint::specialize`].
 //!
 //! Having said all that, in practice we don't fully follow what's been presented in this section.
-//! Under `exhaustive_patterns`, we allow omitting empty arms even in `!known_valid` places, for
-//! backwards-compatibility until we have a better alternative. Without `exhaustive_patterns`, we
-//! mostly treat empty types as inhabited, except specifically a non-nested `!` or empty enum. In
-//! this specific case we also allow the empty match regardless of place validity, for
-//! backwards-compatibility. Hopefully we can eventually deprecate this.
+//! Let's call "toplevel exception" the case where the match scrutinee itself has type `!` or
+//! `EmptyEnum`. First, on stable rust, we require `_` patterns for empty types in all cases apart
+//! from the toplevel exception. The `exhaustive_patterns` and `min_exaustive_patterns` allow
+//! omitting patterns in the cases described above. There's a final detail: in the toplevel
+//! exception or with the `exhaustive_patterns` feature, we ignore place validity when checking
+//! whether a pattern is required for exhaustiveness. I (Nadrieril) hope to deprecate this behavior.
 //!
 //!
 //!
@@ -1440,10 +1441,17 @@ fn compute_exhaustiveness_and_usefulness<'a, 'p, Cx: TypeCx>(
     // We treat match scrutinees of type `!` or `EmptyEnum` differently.
     let is_toplevel_exception =
         is_top_level && matches!(ctors_for_ty, ConstructorSet::NoConstructors);
-    // Whether empty patterns can be omitted for exhaustiveness.
-    let can_omit_empty_arms = is_toplevel_exception || mcx.tycx.is_exhaustive_patterns_feature_on();
-    // Whether empty patterns are counted as useful or not.
-    let empty_arms_are_unreachable = place_validity.is_known_valid() && can_omit_empty_arms;
+    // Whether empty patterns are counted as useful or not. We only warn an empty arm unreachable if
+    // it is guaranteed unreachable by the opsem (i.e. if the place is `known_valid`).
+    let empty_arms_are_unreachable = place_validity.is_known_valid()
+        && (is_toplevel_exception
+            || mcx.tycx.is_exhaustive_patterns_feature_on()
+            || mcx.tycx.is_min_exhaustive_patterns_feature_on());
+    // Whether empty patterns can be omitted for exhaustiveness. We ignore place validity in the
+    // toplevel exception and `exhaustive_patterns` cases for backwards compatibility.
+    let can_omit_empty_arms = empty_arms_are_unreachable
+        || is_toplevel_exception
+        || mcx.tycx.is_exhaustive_patterns_feature_on();
 
     // Analyze the constructors present in this column.
     let ctors = matrix.heads().map(|p| p.ctor());

compiler/rustc_span/src/symbol.rs

@@ -1023,6 +1023,7 @@ symbols! {
         min_const_fn,
         min_const_generics,
         min_const_unsafe_fn,
+        min_exhaustive_patterns,
         min_specialization,
         min_type_alias_impl_trait,
         minnumf32,