IRGen: Skip lowering unavailable opaque type descriptors.

Part of rdar://107425181

Author: tshortli

lib/AST/Availability.cpp

@@ -184,6 +184,9 @@ AvailabilityInference::parentDeclForInferredAvailability(const Decl *D) {
   if (auto *PBD = dyn_cast<PatternBindingDecl>(D))
     return PBD->getAnchoringVarDecl(0);
 
+  if (auto *OTD = dyn_cast<OpaqueTypeDecl>(D))
+    return OTD->getNamingDecl();
+
   // Clang decls may be inaccurately parented rdar://53956555
   if (D->hasClangNode())
     return nullptr;

lib/AST/Decl.cpp

@@ -1098,6 +1098,8 @@ AvailabilityContext Decl::getAvailabilityForLinkage() const {
     return *containingContext;
   }
 
+  // FIXME: Adopt AvailabilityInference::parentDeclForInferredAvailability()
+  // here instead of duplicating the logic.
   if (auto *accessor = dyn_cast<AccessorDecl>(this))
     return accessor->getStorage()->getAvailabilityForLinkage();
 

lib/IRGen/GenStruct.cpp

@@ -1572,6 +1572,9 @@ void IRGenModule::emitStructDecl(StructDecl *st) {
 }
 
 void IRGenModule::maybeEmitOpaqueTypeDecl(OpaqueTypeDecl *opaque) {
+  if (Lowering::shouldSkipLowering(opaque))
+    return;
+
   if (IRGen.Opts.EnableAnonymousContextMangledNames) {
     // If we're emitting anonymous context mangled names for debuggability,
     // then emit all opaque type descriptors and make them runtime-discoverable

test/IRGen/unavailable_decl_optimization_opaque_type.swift

@@ -0,0 +1,27 @@
+// RUN: %target-swift-frontend -parse-as-library -module-name Test -validate-tbd-against-ir=missing %s -emit-ir | %FileCheck %s --check-prefixes=CHECK,CHECK-NO-STRIP
+
+// RUN: %target-swift-frontend -parse-as-library -module-name Test -validate-tbd-against-ir=missing -unavailable-decl-optimization=complete %s -emit-ir | %FileCheck %s --check-prefixes=CHECK,CHECK-STRIP
+
+// CHECK-NO-STRIP: s4Test1SV31unavailableFuncWithOpaqueReturnQryFQOMQ
+// CHECK-STRIP-NOT: s4Test1SV31unavailableFuncWithOpaqueReturnQryFQOMQ
+
+// CHECK-NO-STRIP: s4Test1SV30unavailableVarWithOpaqueReturnQrvpQOMQ
+// CHECK-STRIP-NOT: s4Test1SV30unavailableVarWithOpaqueReturnQrvpQOMQ
+
+public protocol P {}
+
+public struct S: P {
+  // CHECK-NO-STRIP: s4Test1SV31unavailableFuncWithOpaqueReturnQryF
+  // CHECK-STRIP-NOT: s4Test1SV31unavailableFuncWithOpaqueReturnQryF
+  @available(*, unavailable)
+  public func unavailableFuncWithOpaqueReturn() -> some P {
+    return self
+  }
+
+  // CHECK-NO-STRIP: s4Test1SV30unavailableVarWithOpaqueReturnQrvg
+  // CHECK-STRIP-NOT: s4Test1SV30unavailableVarWithOpaqueReturnQrvg
+  @available(*, unavailable)
+  public var unavailableVarWithOpaqueReturn: some P {
+    return self
+  }
+}