Implement feature gate logic

Author: Nadrieril

Diff for: 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;

Diff for: 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)

Diff for: compiler/rustc_pattern_analysis/src/usefulness.rs

@@ -1439,10 +1439,19 @@ 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 can be omitted for exhaustiveness. Empty arms on a `!known_valid`
+    // place are not guaranteed unreachable by the operational semantics, but we allow omitting them
+    // in some cases nonetheless.
+    let can_omit_empty_arms = is_toplevel_exception
+        || mcx.tycx.is_exhaustive_patterns_feature_on()
+        || (mcx.tycx.is_min_exhaustive_patterns_feature_on() && place_validity.is_known_valid());
+    // Whether empty patterns are counted as useful or not. Unlike for omitting arms, we only warn
+    // an 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());
 
     // Analyze the constructors present in this column.
     let ctors = matrix.heads().map(|p| p.ctor());