Remove the @_downgrade_exhaustivity_check hack

This hack was only needed for Swift 3 mode in a narrow case.  Flush it out of the compiler so we can simplify the space engine.

Author: CodaFi

include/swift/AST/Attr.def

@@ -345,12 +345,7 @@ DECL_ATTR(_restatedObjCConformance, RestatedObjCConformance,
   OnProtocol |
   LongAttribute | RejectByParser |
   NotSerialized, 70)
-// HACK: Attribute needed to preserve source compatibility by downgrading errors
-// due to an SDK change in Dispatch
-SIMPLE_DECL_ATTR(_downgrade_exhaustivity_check, DowngradeExhaustivityCheck,
-  OnEnumElement |
-  LongAttribute | UserInaccessible,
-  71)
+// NOTE: 71 is unused
 SIMPLE_DECL_ATTR(_implicitly_unwrapped_optional, ImplicitlyUnwrappedOptional,
   OnFunc | OnAccessor | OnVar | OnParam | OnSubscript | OnConstructor |
   RejectByParser,

include/swift/Serialization/ModuleFormat.h

@@ -55,7 +55,7 @@ const uint16_t VERSION_MAJOR = 0;
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
 /// Don't worry about adhering to the 80-column limit for this line.
-const uint16_t VERSION_MINOR = 438; // Last change: add isbitwisetakable
+const uint16_t VERSION_MINOR = 439; // Last change: remove @_downgrade_exhaustivity_check attribute
 
 using DeclIDField = BCFixed<31>;
 

lib/AST/ASTDumper.cpp

@@ -862,8 +862,6 @@ namespace {
 
     void visitEnumElementDecl(EnumElementDecl *EED) {
       printCommon(EED, "enum_element_decl");
-      if (EED->getAttrs().hasAttribute<DowngradeExhaustivityCheckAttr>())
-        OS << "@_downgrade_exhaustivity_check";
       PrintWithColorRAII(OS, ParenthesisColor) << ')';
     }
 

lib/SILGen/SILGenPattern.cpp

@@ -1806,8 +1806,7 @@ CaseBlocks::CaseBlocks(
       enumDecl->isEffectivelyExhaustive(SGF.getModule().getSwiftModule(),
                                         SGF.F.getResilienceExpansion());
   if (canAssumeExhaustive) {
-    // Check that Sema didn't let any cases slip through. (This can happen
-    // with @_downgrade_exhaustivity_check.)
+    // Check that Sema didn't let any cases slip through.
     canAssumeExhaustive = llvm::all_of(enumDecl->getAllElements(),
                                        [&](const EnumElementDecl *elt) {
       return caseToIndex.count(elt);

lib/Sema/TypeCheckAttr.cpp

@@ -78,7 +78,6 @@ class AttributeEarlyChecker : public AttributeVisitor<AttributeEarlyChecker> {
   IGNORED_ATTR(ClangImporterSynthesizedType)
   IGNORED_ATTR(Convenience)
   IGNORED_ATTR(DiscardableResult)
-  IGNORED_ATTR(DowngradeExhaustivityCheck)
   IGNORED_ATTR(DynamicMemberLookup)
   IGNORED_ATTR(Effects)
   IGNORED_ATTR(Exported)
@@ -777,7 +776,6 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
     IGNORED_ATTR(ClangImporterSynthesizedType)
     IGNORED_ATTR(Consuming)
     IGNORED_ATTR(Convenience)
-    IGNORED_ATTR(DowngradeExhaustivityCheck)
     IGNORED_ATTR(Dynamic)
     IGNORED_ATTR(Effects)
     IGNORED_ATTR(Exported)

lib/Sema/TypeCheckDeclOverride.cpp

@@ -1159,7 +1159,6 @@ namespace  {
     UNINTERESTING_ATTR(RestatedObjCConformance)
     UNINTERESTING_ATTR(Implements)
     UNINTERESTING_ATTR(StaticInitializeObjCMetadata)
-    UNINTERESTING_ATTR(DowngradeExhaustivityCheck)
     UNINTERESTING_ATTR(ImplicitlyUnwrappedOptional)
     UNINTERESTING_ATTR(ClangImporterSynthesizedType)
     UNINTERESTING_ATTR(WeakLinked)