Reapply "merge partial SIL modules" again

lib/Driver/ToolChains.cpp

@@ -687,6 +687,14 @@ ToolChain::constructInvocation(const MergeModuleJobAction &job,
   // serialized ASTs.
   Arguments.push_back("-parse-as-library");
 
+  // Merge serialized SIL from partial modules.
+  Arguments.push_back("-sil-merge-partial-modules");
+
+  // Disable SIL optimization passes; we've already optimized the code in each
+  // partial mode.
+  Arguments.push_back("-disable-diagnostic-passes");
+  Arguments.push_back("-disable-sil-perf-optzns");
+
   addCommonFrontendArgs(*this, context.OI, context.Output, context.Args,
                         Arguments);
   context.Args.AddLastArg(Arguments, options::OPT_import_objc_header);

lib/SIL/SILVerifier.cpp

@@ -4516,13 +4516,20 @@ void SILModule::verify() const {
   // Uniquing set to catch symbol name collisions.
   llvm::StringSet<> symbolNames;
 
+  // When merging partial modules, we only link functions from the current
+  // module, without enabling "LinkAll" mode or running the SILLinker pass;
+  // in this case, we need to relax some of the checks.
+  bool SingleFunction = false;
+  if (getOptions().MergePartialModules)
+    SingleFunction = true;
+
   // Check all functions.
   for (const SILFunction &f : *this) {
     if (!symbolNames.insert(f.getName()).second) {
       llvm::errs() << "Symbol redefined: " << f.getName() << "!\n";
       assert(false && "triggering standard assertion failure routine");
     }
-    f.verify(/*SingleFunction=*/false);
+    f.verify(SingleFunction);
   }
 
   // Check all globals.

lib/Sema/CodeSynthesis.cpp

@@ -1699,7 +1699,7 @@ void swift::maybeAddAccessorsToVariable(VarDecl *var, TypeChecker &TC) {
         getter->setAccess(var->getFormalAccess());
 
         // Make a setter if the behavior property has one.
-        if (auto valueSetter = valueProp->getSetter()) {
+        if (valueProp->getSetter()) {
           ParamDecl *newValueParam = nullptr;
           setter = createSetterPrototype(var, newValueParam, TC);
           if (mightBeMutating && valueProp->isSetterMutating())

test/Frontend/sil-merge-partial-modules-stdlib.swift

@@ -0,0 +1,9 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-swift-frontend -emit-module -primary-file %s -module-name test -o %t/partial.swiftmodule -O
+
+// RUN: %target-swift-frontend -emit-module %t/partial.swiftmodule -module-name test -sil-merge-partial-modules -disable-diagnostic-passes -disable-sil-perf-optzns -o %t/test.swiftmodule
+
+public func makeMirror(object x: Any) -> Mirror {
+  return Mirror(reflecting: x)
+}

validation-test/Evolution/test_function_change_transparent_body.swift

@@ -1,9 +1,6 @@
-// RUN: %target-resilience-test-wmo
+// RUN: %target-resilience-test
 // REQUIRES: executable_test
 
-// FIXME: shouldn't need -whole-module-optimization here; we need to fix the
-// frontend to merge serialized SIL functions from different translation units
-
 import StdlibUnittest
 import function_change_transparent_body