[Explicit Module Builds] Always rebuild the target module in explicit -compile-module-from-interface builds.

We have long switched to delegate the checking of whether a module is up-to-date to the build system (SwiftDriver) and stopped serializing file dependencies for interface-built binary modules.

Resolves rdar://162881032

include/swift/Basic/LangOptions.h

@@ -273,9 +273,6 @@ namespace swift {
     /// Emit a remark when indexing a system module.
     bool EnableIndexingSystemModuleRemarks = false;
 
-    /// Emit a remark on early exit in explicit interface build
-    bool EnableSkipExplicitInterfaceModuleBuildRemarks = false;
-
     ///
     /// Support for alternate usage modes
     ///

lib/Frontend/CompilerInvocation.cpp

@@ -1478,8 +1478,6 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableMacroLoadingRemarks = Args.hasArg(OPT_remark_macro_loading);
   Opts.EnableIndexingSystemModuleRemarks = Args.hasArg(OPT_remark_indexing_system_module);
 
-  Opts.EnableSkipExplicitInterfaceModuleBuildRemarks = Args.hasArg(OPT_remark_skip_explicit_interface_build);
-
   if (Args.hasArg(OPT_experimental_skip_non_exportable_decls)) {
     // Only allow -experimental-skip-non-exportable-decls if either library
     // evolution is enabled (in which case the module's ABI is independent of

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -1609,27 +1609,6 @@ bool ModuleInterfaceLoader::buildExplicitSwiftModuleFromSwiftInterface(
     StringRef outputPath, bool ShouldSerializeDeps,
     ArrayRef<std::string> CompiledCandidates,
     DependencyTracker *tracker) {
-
-  if (!Instance.getInvocation().getIRGenOptions().AlwaysCompile) {
-    // First, check if the expected output already exists and possibly
-    // up-to-date w.r.t. all of the dependencies it was built with. If so, early
-    // exit.
-    UpToDateModuleCheker checker(
-        Instance.getASTContext());
-    ModuleRebuildInfo rebuildInfo;
-    SmallVector<FileDependency, 3> allDeps;
-    std::unique_ptr<llvm::MemoryBuffer> moduleBuffer;
-    if (checker.swiftModuleIsUpToDate(outputPath, rebuildInfo, allDeps,
-                                      moduleBuffer)) {
-      if (Instance.getASTContext()
-              .LangOpts.EnableSkipExplicitInterfaceModuleBuildRemarks) {
-        Instance.getDiags().diagnose(
-            SourceLoc(), diag::explicit_interface_build_skipped, outputPath);
-      }
-      return false;
-    }
-  }
-
   // Read out the compiler version.
   llvm::BumpPtrAllocator alloc;
   llvm::StringSaver ArgSaver(alloc);

test/Frontend/output_determinism_check.swift

@@ -14,11 +14,9 @@
 // RUN: %target-swift-frontend -scan-dependencies -module-name test -o %t/test.json %s -enable-deterministic-check 2>&1 | %FileCheck %s --check-prefix=DEPSCAN_OUTPUT
 /// TODO: Implicit module build use a different compiler instance so it doesn't support checking yet.
 // RUN: %target-swift-frontend -typecheck -emit-module-interface-path %t/test.swiftinterface %s -O -enable-deterministic-check 2>&1 | %FileCheck %s --check-prefix=INTERFACE_OUTPUT
+
 /// Hit cache and not emit the second time.
-// RUN: rm %t/test.swiftmodule
-// RUN: not %target-swift-frontend -compile-module-from-interface %t/test.swiftinterface -explicit-interface-module-build -o %t/test.swiftmodule -enable-deterministic-check 2>&1 | %FileCheck --check-prefix=MODULE_MISMATCH %s
-/// Force swiftmodule generation.
-// RUN: %target-swift-frontend -compile-module-from-interface %t/test.swiftinterface -explicit-interface-module-build -o %t/test.swiftmodule -enable-deterministic-check -always-compile-output-files 2>&1 | %FileCheck --check-prefix=MODULE_OUTPUT %s
+// RUN: %target-swift-frontend -compile-module-from-interface %t/test.swiftinterface -explicit-interface-module-build -o %t/test.swiftmodule -enable-deterministic-check 2>&1 | %FileCheck --check-prefix=MODULE_OUTPUT %s
 
 // RUN: %target-swift-frontend -emit-pcm -module-name UserClangModule -o %t/test.pcm %S/Inputs/dependencies/module.modulemap -enable-deterministic-check 2>&1 | %FileCheck %s --check-prefix=PCM_OUTPUT
 
@@ -34,7 +32,6 @@
 // OBJECT_MISMATCH: error: output file '{{.*}}{{/|\\}}test.o' is missing from second compilation for deterministic check
 // DEPSCAN_OUTPUT: remark: produced matching output file '{{.*}}{{/|\\}}test.json'
 // INTERFACE_OUTPUT: remark: produced matching output file '{{.*}}{{/|\\}}test.swiftinterface'
-// MODULE_MISMATCH: error: output file '{{.*}}{{/|\\}}test.swiftmodule' is missing from second compilation for deterministic check
 // PCM_OUTPUT: remark: produced matching output file '{{.*}}{{/|\\}}test.pcm'
 
 public var x = 1

test/ModuleInterface/repeated-explicit-interface-build-nop.swiftinterface

@@ -1,14 +0,0 @@
-// swift-interface-format-version: 1.0
-// swift-module-flags: -parse-stdlib
-
-// RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend -compile-module-from-interface -module-name ExplicitModule -explicit-interface-module-build -o %/t/ExplicitModule.swiftmodule %s
-
-// RUN: %target-swift-frontend -compile-module-from-interface -module-name ExplicitModule -explicit-interface-module-build -o %/t/ExplicitModule.swiftmodule %s -Rskip-explicit-interface-build 2>&1 | %FileCheck %s
-
-import Swift
-extension Int {
-  public static var fortytwo: Int = 42
-}
-
-// CHECK: <unknown>:0: remark: Skipped rebuilding module at {{.*}}ExplicitModule.swiftmodule - up-to-date

test/ScanDependencies/explicit-interface-build.swift

@@ -0,0 +1,25 @@
+// RUN: %empty-directory(%t)
+// RUN: %empty-directory(%t/module-cache)
+// RUN: %empty-directory(%t/Foo.swiftmodule)
+// RUN: split-file %s %t
+
+// Build module from interface
+// RUN: %target-swift-frontend -compile-module-from-interface -o %t/Foo.swiftmodule/Foo.swiftmodule -module-name Foo %t/Foo.swiftmodule/Foo.swiftinterface -explicit-interface-module-build
+
+// Add to the textual interface
+// RUN: echo "struct GoodBye {}" >> %t/Foo.swiftmodule/Foo.swiftinterface
+
+// Build module from interface
+// RUN: %target-swift-frontend -compile-module-from-interface -o %t/Foo.swiftmodule/Foo.swiftmodule -module-name Foo %t/Foo.swiftmodule/Foo.swiftinterface -explicit-interface-module-build
+
+// Get the list of strings and ensure the binary module reflects the new addition
+// RUN: %llvm-strings %t/Foo.swiftmodule/Foo.swiftmodule > %t/module_strings.txt
+
+// RUN: cat %t/module_strings.txt | %FileCheck %s
+// CHECK-DAG: Hello
+// CHECK-DAG: GoodBye
+
+//--- Foo.swiftmodule/Foo.swiftinterface
+// swift-interface-format-version: 1.0
+// swift-module-flags: -module-name Foo
+struct Hello {}