Reorder passes.

compiler/rustc_mir_transform/src/lib.rs

@@ -568,10 +568,11 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         &[
             &check_alignment::CheckAlignment,
             &lower_slice_len::LowerSliceLenCalls, // has to be done before inlining, otherwise actual call will be almost always inlined. Also simple, so can just do first
-            &unreachable_prop::UnreachablePropagation,
-            &uninhabited_enum_branching::UninhabitedEnumBranching,
-            &o1(simplify::SimplifyCfg::AfterUninhabitedEnumBranching),
             &inline::Inline,
+            // Substitutions during inlining may introduce switch on enums with uninhabited branches.
+            &uninhabited_enum_branching::UninhabitedEnumBranching,
+            &unreachable_prop::UnreachablePropagation,
+            &o1(simplify::SimplifyCfg::AfterUninhabitedEnumBranching),
             &remove_storage_markers::RemoveStorageMarkers,
             &remove_zsts::RemoveZsts,
             &normalize_array_len::NormalizeArrayLen, // has to run after `slice::len` lowering

compiler/rustc_mir_transform/src/unreachable_prop.rs

@@ -2,7 +2,6 @@
 //! when all of their successors are unreachable. This is achieved through a
 //! post-order traversal of the blocks.
 
-use crate::simplify;
 use crate::MirPass;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_middle::mir::*;
@@ -52,8 +51,6 @@ impl MirPass<'_> for UnreachablePropagation {
             body.basic_blocks_mut()[bb].statements.clear();
         }
 
-        let replaced = !replacements.is_empty();
-
         for (bb, terminator_kind) in replacements {
             if !tcx.consider_optimizing(|| {
                 format!("UnreachablePropagation {:?} ", body.source.def_id())
@@ -64,9 +61,7 @@ impl MirPass<'_> for UnreachablePropagation {
             body.basic_blocks_mut()[bb].terminator_mut().kind = terminator_kind;
         }
 
-        if replaced {
+        // Do not remove dead blocks, let `SimplifyCfg` do it.
-            simplify::remove_dead_blocks(body);
-        }
     }
 }
 

tests/mir-opt/separate_const_switch.identity.SeparateConstSwitch.diff

@@ -52,7 +52,7 @@
           StorageLive(_10);
           StorageLive(_11);
           _9 = discriminant(_1);
-          switchInt(move _9) -> [0: bb7, 1: bb5, otherwise: bb6];
+          switchInt(move _9) -> [0: bb6, 1: bb5, otherwise: bb3];
       }
   
       bb1: {
@@ -92,10 +92,6 @@
       }
   
       bb6: {
-          unreachable;
-      }
-  
-      bb7: {
           _10 = ((_1 as Ok).0: i32);
           _3 = ControlFlow::<Result<Infallible, i32>, i32>::Continue(move _10);
           goto -> bb1;

tests/mir-opt/unreachable.main.UnreachablePropagation.panic-abort.diff

@@ -24,8 +24,7 @@
   
       bb1: {
           _2 = discriminant(_1);
--         switchInt(move _2) -> [1: bb2, otherwise: bb6];
+          switchInt(move _2) -> [1: bb2, otherwise: bb6];
-+         switchInt(move _2) -> [1: bb2, otherwise: bb3];
       }
   
       bb2: {
@@ -36,29 +35,31 @@
 -         StorageLive(_6);
 -         _6 = const true;
 -         switchInt(move _6) -> [0: bb4, otherwise: bb3];
--     }
++         unreachable;
-- 
+      }
--     bb3: {
+  
+      bb3: {
 -         _4 = const 21_i32;
 -         _5 = const ();
 -         goto -> bb5;
--     }
++         unreachable;
-- 
+      }
--     bb4: {
+  
+      bb4: {
 -         _4 = const 42_i32;
 -         _5 = const ();
 -         goto -> bb5;
--     }
++         unreachable;
-- 
+      }
--     bb5: {
+  
+      bb5: {
 -         StorageDead(_6);
 -         StorageDead(_5);
 -         StorageLive(_7);
           unreachable;
       }
   
--     bb6: {
+      bb6: {
-+     bb3: {
           _0 = const ();
           StorageDead(_1);
           return;

tests/mir-opt/unreachable.main.UnreachablePropagation.panic-unwind.diff

@@ -24,8 +24,7 @@
   
       bb1: {
           _2 = discriminant(_1);
--         switchInt(move _2) -> [1: bb2, otherwise: bb6];
+          switchInt(move _2) -> [1: bb2, otherwise: bb6];
-+         switchInt(move _2) -> [1: bb2, otherwise: bb3];
       }
   
       bb2: {
@@ -36,29 +35,31 @@
 -         StorageLive(_6);
 -         _6 = const true;
 -         switchInt(move _6) -> [0: bb4, otherwise: bb3];
--     }
++         unreachable;
-- 
+      }
--     bb3: {
+  
+      bb3: {
 -         _4 = const 21_i32;
 -         _5 = const ();
 -         goto -> bb5;
--     }
++         unreachable;
-- 
+      }
--     bb4: {
+  
+      bb4: {
 -         _4 = const 42_i32;
 -         _5 = const ();
 -         goto -> bb5;
--     }
++         unreachable;
-- 
+      }
--     bb5: {
+  
+      bb5: {
 -         StorageDead(_6);
 -         StorageDead(_5);
 -         StorageLive(_7);
           unreachable;
       }
   
--     bb6: {
+      bb6: {
-+     bb3: {
           _0 = const ();
           StorageDead(_1);
           return;

tests/mir-opt/unreachable.rs

@@ -1,5 +1,6 @@
-// skip-filecheck
+// unit-test: UnreachablePropagation
 // EMIT_MIR_FOR_EACH_PANIC_STRATEGY
+
 enum Empty {}
 
 fn empty() -> Option<Empty> {
@@ -8,6 +9,21 @@ fn empty() -> Option<Empty> {
 
 // EMIT_MIR unreachable.main.UnreachablePropagation.diff
 fn main() {
+    // CHECK-LABEL: fn main(
+    // CHECK: bb0: {
+    // CHECK: {{_.*}} = empty()
+    // CHECK: bb1: {
+    // CHECK: switchInt({{.*}}) -> [1: bb2, otherwise: bb6];
+    // CHECK: bb2: {
+    // CHECK: unreachable;
+    // CHECK: bb3: {
+    // CHECK: unreachable;
+    // CHECK: bb4: {
+    // CHECK: unreachable;
+    // CHECK: bb5: {
+    // CHECK: unreachable;
+    // CHECK: bb6: {
+    // CHECK: return;
     if let Some(_x) = empty() {
         let mut _y;
 

tests/mir-opt/unreachable_diverging.rs

@@ -1,5 +1,6 @@
-// skip-filecheck
+// unit-test: UnreachablePropagation
 // EMIT_MIR_FOR_EACH_PANIC_STRATEGY
+
 pub enum Empty {}
 
 fn empty() -> Option<Empty> {
@@ -12,6 +13,21 @@ fn loop_forever() {
 
 // EMIT_MIR unreachable_diverging.main.UnreachablePropagation.diff
 fn main() {
+    // CHECK-LABEL: fn main(
+    // CHECK: bb0: {
+    // CHECK: {{_.*}} = empty()
+    // CHECK: bb1: {
+    // CHECK: switchInt({{.*}}) -> [1: bb2, otherwise: bb6];
+    // CHECK: bb2: {
+    // CHECK: switchInt({{.*}}) -> [0: bb4, otherwise: bb3];
+    // CHECK: bb3: {
+    // CHECK: {{_.*}} = loop_forever()
+    // CHECK: bb4: {
+    // CHECK: unreachable;
+    // CHECK: bb5: {
+    // CHECK: unreachable;
+    // CHECK: bb6: {
+    // CHECK: return;
     let x = true;
     if let Some(bomb) = empty() {
         if x {