Add '@_alwaysEmitConformanceMetadata' protocol attribute

This attribute will, in the near future, be used to inform IRGen that a nominal type that conforms to such protocol must have its type metadata always emitted into the binary, regardless of whether it is used/public.

Author: artemcm

docs/ReferenceGuides/UnderscoredAttributes.md

@@ -852,3 +852,8 @@ the distributed actor isolation checks. This is used for things like `whenLocal`
 where the actor passed to the closure is known-to-be-local, and similarly a 
 `self` of obtained from an _isolated_ function inside a distributed actor is 
 also guaranteed to be local by construction.
+
+
+## `@_alwaysEmitConformanceMetadata`
+
+Forces conformances of the attributed protocol to always have their Type Metadata get emitted into the binary and prevents it from being optimized away or stripped by the linker.

include/swift/AST/Attr.def

@@ -740,6 +740,10 @@ SIMPLE_DECL_ATTR(_moveOnly, MoveOnly,
   UserInaccessible |
   ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
   131)
+  
+SIMPLE_DECL_ATTR(_alwaysEmitConformanceMetadata, AlwaysEmitConformanceMetadata,
+  OnProtocol | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  132)
 
 // If you're adding a new underscored attribute here, please document it in
 // docs/ReferenceGuides/UnderscoredAttributes.md.

include/swift/AST/DiagnosticsSema.def

@@ -1415,6 +1415,9 @@ ERROR(transparent_in_protocols_not_supported,none,
 ERROR(transparent_in_classes_not_supported,none,
       "'@_transparent' attribute is not supported on declarations within classes", ())
 
+ERROR(always_emit_conformance_attr_protocols_only,none,
+      "'@_alwaysEmitConformanceMetadata' attribute is only valid on protocol declarations", ())
+
 ERROR(iboutlet_nonobjc_class,none,
       "@IBOutlet property cannot %select{have|be an array of}0 "
       "non-'@objc' class type %1", (bool, Type))

lib/AST/Attr.cpp

@@ -1269,7 +1269,7 @@ bool DeclAttribute::printImpl(ASTPrinter &Printer, const PrintOptions &Options,
     Printer << ")";
     break;
   }
-
+  // ACTODO
   case DAK_Count:
     llvm_unreachable("exceed declaration attribute kinds");
 

lib/Sema/TypeCheckAttr.cpp

@@ -311,6 +311,8 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
   void visitNonisolatedAttr(NonisolatedAttr *attr);
 
   void visitNoImplicitCopyAttr(NoImplicitCopyAttr *attr);
+  
+  void visitAlwaysEmitConformanceMetadataAttr(AlwaysEmitConformanceMetadataAttr *attr);
 
   void visitUnavailableFromAsyncAttr(UnavailableFromAsyncAttr *attr);
 
@@ -381,6 +383,14 @@ void AttributeChecker::visitNoImplicitCopyAttr(NoImplicitCopyAttr *attr) {
   }
 }
 
+void AttributeChecker::visitAlwaysEmitConformanceMetadataAttr(AlwaysEmitConformanceMetadataAttr *attr) {
+  DeclContext *dc = D->getDeclContext();
+  // Only protocol declarations can be annotated `_alwaysEmitConformanceMetadata`
+  if (!isa<ProtocolDecl>(dc))
+    diagnoseAndRemoveAttr(attr, diag::always_emit_conformance_attr_protocols_only);
+  return;
+}
+
 void AttributeChecker::visitTransparentAttr(TransparentAttr *attr) {
   DeclContext *dc = D->getDeclContext();
   // Protocol declarations cannot be transparent.

lib/Sema/TypeCheckDeclOverride.cpp

@@ -1612,6 +1612,7 @@ namespace  {
 
     UNINTERESTING_ATTR(UnsafeInheritExecutor)
     UNINTERESTING_ATTR(CompilerInitialized)
+    UNINTERESTING_ATTR(AlwaysEmitConformanceMetadata)
 #undef UNINTERESTING_ATTR
 
     void visitAvailableAttr(AvailableAttr *attr) {

lib/Serialization/ModuleFormat.h

@@ -58,7 +58,7 @@ const uint16_t SWIFTMODULE_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 SWIFTMODULE_VERSION_MINOR = 698; // opaque decl with unavailability conditions
+const uint16_t SWIFTMODULE_VERSION_MINOR = 699; // @_alwaysEmitConformanceMetadata
 
 /// A standard hash seed used for all string hashes in a serialized module.
 ///