[AllocBoxToStack] Make alloc_stacks lexical.

When the -enable-experimental-lexical-lifetimes flag is enabled, the
alloc_stack instructions which the pass replaces alloc_box instructions
with have the lexical attribute.

Author: nate-chandler

lib/SILOptimizer/Transforms/AllocBoxToStack.cpp

@@ -533,11 +533,15 @@ static bool rewriteAllocBoxAsAllocStack(AllocBoxInst *ABI) {
   SILBuilderWithScope Builder(ABI);
   assert(ABI->getBoxType()->getLayout()->getFields().size() == 1
          && "rewriting multi-field box not implemented");
+  bool isLexical = ABI->getFunction()
+                       ->getModule()
+                       .getASTContext()
+                       .LangOpts.EnableExperimentalLexicalLifetimes;
   auto *ASI = Builder.createAllocStack(
       ABI->getLoc(),
       getSILBoxFieldType(TypeExpansionContext(*ABI->getFunction()),
                          ABI->getBoxType(), ABI->getModule().Types, 0),
-      ABI->getVarInfo(), ABI->hasDynamicLifetime());
+      ABI->getVarInfo(), ABI->hasDynamicLifetime(), isLexical);
 
   // Transfer a mark_uninitialized if we have one.
   SILValue StackBox = ASI;

test/SILOptimizer/allocbox_to_stack_lifetime.sil

@@ -0,0 +1,30 @@
+// RUN: %target-sil-opt -enable-sil-verify-all %s -allocbox-to-stack -enable-experimental-lexical-lifetimes | %FileCheck %s
+
+import Builtin
+
+struct Int {
+  var _value: Builtin.Int64
+}
+
+struct Bool {
+  var _value: Builtin.Int1
+}
+
+protocol Error {}
+
+// CHECK-LABEL: sil [ossa] @simple_promotion
+sil [ossa] @simple_promotion : $@convention(thin) (Int) -> Int {
+bb0(%0 : $Int):
+  %1 = alloc_box ${ var Int }
+  %1a = project_box %1 : ${ var Int }, 0
+  store %0 to [trivial] %1a : $*Int
+
+  %3 = load [trivial] %1a : $*Int
+  destroy_value %1 : ${ var Int }
+  return %3 : $Int
+// CHECK: alloc_stack [lexical]
+// CHECK-NOT: alloc_box
+// CHECK-NOT: destroy_value
+// CHECK: return
+}
+