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 {
-            simplify::remove_dead_blocks(body);
-        }
+        // Do not remove dead blocks, let `SimplifyCfg` do it.
     }
 }