Simplify empty pattern logic

This is logically equivalent to what was done previously; I derived it
pretty mechanically.

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

@@ -287,7 +287,8 @@ impl<'p, 'tcx> MatchVisitor<'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

@@ -1003,17 +1003,6 @@ impl<Cx: TypeCx> ConstructorSet<Cx> {
             }
         }
 
-        // We have now grouped all the constructors into 3 buckets: present, missing, missing_empty.
-        // 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.is_scrutinee && matches!(self, Self::NoConstructors))
-        {
-            // Treat all missing constructors as nonempty.
-            // This clears `missing_empty`.
-            missing.append(&mut missing_empty);
-        }
-
         SplitConstructorSet { present, missing, missing_empty }
     }
 }

compiler/rustc_pattern_analysis/src/usefulness.rs

@@ -1375,39 +1375,41 @@ 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();
     // 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();
+
+    // For backwards-compatibility 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 are counted as useful or not.
+    let empty_arms_are_unreachable = place_validity.is_known_valid()
+        && (is_toplevel_exception || mcx.tycx.is_exhaustive_patterns_feature_on());
+    // Whether empty patterns can be omitted for exhaustiveness.
+    let can_omit_empty_arms = 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());
-    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);
     let all_missing = split_set.present.is_empty();
-
     // Build the set of constructors we will specialize with. It must cover the whole type.
     let mut split_ctors = split_set.present;
-    if !split_set.missing.is_empty() {
-        // We need to iterate over a full set of constructors, so we add `Missing` to represent the
-        // missing ones. This is explained under "Constructor Splitting" at the top of this file.
-        split_ctors.push(Constructor::Missing);
-    } else if !split_set.missing_empty.is_empty() && !place_validity.is_known_valid() {
-        // The missing empty constructors are reachable if the place can contain invalid data.
+    if !split_set.missing.is_empty()
+        || (!split_set.missing_empty.is_empty() && !empty_arms_are_unreachable)
+    {
         split_ctors.push(Constructor::Missing);
     }
 
     // Decide what constructors to report.
+    let is_integers = matches!(ctors_for_ty, ConstructorSet::Integers { .. });
     let always_report_all = is_top_level && !is_integers;
     // Whether we should report "Enum::A and Enum::C are missing" or "_ is missing".
     let report_individual_missing_ctors = always_report_all || !all_missing;
     // Which constructors are considered missing. We ensure that `!missing_ctors.is_empty() =>
     // split_ctors.contains(Missing)`. The converse usually holds except in the
     // `MaybeInvalidButAllowOmittingArms` backwards-compatibility case.
     let mut missing_ctors = split_set.missing;
-    if !place_validity.allows_omitting_empty_arms() {
+    if !can_omit_empty_arms {
         missing_ctors.extend(split_set.missing_empty);
     }