[GSB] Mark conformances used by the GenericSignatureBuilder as “used”.

Prior to the removal of the “lookup conformance function”, whenever
the type checker created a GenericSignatureBuilder, its lookup conformance
function would mark any referenced conformance as “used”. Do so now via
a LazyResolver callback, which fixes a regression in multi-file type
checking scenarios.

Author: DougGregor

include/swift/AST/LazyResolver.h

@@ -17,6 +17,7 @@
 #ifndef SWIFT_AST_LAZYRESOLVER_H
 #define SWIFT_AST_LAZYRESOLVER_H
 
+#include "swift/AST/ProtocolConformanceRef.h"
 #include "swift/AST/TypeLoc.h"
 #include "llvm/ADT/PointerEmbeddedInt.h"
 
@@ -102,6 +103,10 @@ class LazyResolver {
   /// is usable for the given type.
   virtual bool isProtocolExtensionUsable(DeclContext *dc, Type type,
                                          ExtensionDecl *protocolExtension) = 0;
+
+  /// Mark the given conformance as "used" from the given declaration context.
+  virtual void markConformanceUsed(ProtocolConformanceRef conformance,
+                                   DeclContext *dc) = 0;
 };
 
 /// An implementation of LazyResolver that delegates to another.
@@ -171,6 +176,11 @@ class DelegatingLazyResolver : public LazyResolver {
                                  ExtensionDecl *protocolExtension) override {
     return Principal.isProtocolExtensionUsable(dc, type, protocolExtension);
   }
+
+  void markConformanceUsed(ProtocolConformanceRef conformance,
+                           DeclContext *dc) override {
+    return Principal.markConformanceUsed(conformance, dc);
+  }
 };
 
 class LazyMemberLoader;

lib/AST/GenericSignatureBuilder.cpp

@@ -2919,8 +2919,12 @@ GenericSignatureBuilder::lookupConformance(CanType dependentType,
   // FIXME: When lookupConformance() starts respecting modules, we'll need
   // to do some filtering here.
   ModuleDecl *searchModule = conformedProtocol->getDecl()->getParentModule();
-  return searchModule->lookupConformance(conformingReplacementType,
-                                         conformedProtocol->getDecl());
+  auto result = searchModule->lookupConformance(conformingReplacementType,
+                                                conformedProtocol->getDecl());
+  if (result && getLazyResolver())
+    getLazyResolver()->markConformanceUsed(*result, searchModule);
+
+  return result;
 }
 
 LazyResolver *GenericSignatureBuilder::getLazyResolver() const { 

lib/Sema/TypeChecker.h

@@ -1961,7 +1961,7 @@ class TypeChecker final : public LazyResolver {
   /// Mark the given protocol conformance as "used" from the given declaration
   /// context.
   void markConformanceUsed(ProtocolConformanceRef conformance,
-                           DeclContext *dc);
+                           DeclContext *dc) override final;
 
   /// Functor class suitable for use as a \c LookupConformanceFn to look up a
   /// conformance through a particular declaration context using the given