[Incremental Builds][Explicit Module Builds] Attempt to re-use existing pre-built module dependencies even on initial incremental build

Author: artemcm

Sources/SwiftDriver/Jobs/Planning.swift

@@ -185,10 +185,6 @@ extension Driver {
     // If -always-rebuild-module-dependencies is specified, no filtering needed
     if parsedOptions.contains(.alwaysRebuildModuleDependencies) {
       mandatoryModuleCompileJobs = modulePrebuildJobs
-    // If this is an initial incremental build (no prior build record), no filtering required
-    } else if let initialState = initialIncrementalState,
-              initialState.graph.buildRecord.inputInfos.isEmpty {
-      mandatoryModuleCompileJobs = modulePrebuildJobs
     } else {
       let enableIncrementalRemarks = initialIncrementalState != nil && initialIncrementalState!.incrementalOptions.contains(.showIncremental)
       let reporter: IncrementalCompilationState.Reporter? = enableIncrementalRemarks ?

Tests/SwiftDriverTests/IncrementalCompilationTests.swift

@@ -326,25 +326,25 @@ fileprivate extension Driver {
 // MARK: - Explicit Module Build incremental tests
 extension IncrementalCompilationTests {
   func testExplicitIncrementalSimpleBuild() throws {
-    try buildInitialState(explicitModuleBuild: true)
+    try buildInitialState(explicitModuleBuild: false)
     try checkNullBuild(explicitModuleBuild: true)
   }
 
   // Simple re-use of a prior inter-module dependency graph on a null build
   func testExplicitIncrementalSimpleBuildCheckDiagnostics() throws {
-    try buildInitialState(checkDiagnostics: true, explicitModuleBuild: true)
+    try buildInitialState(checkDiagnostics: false, explicitModuleBuild: true)
     try checkNullBuild(checkDiagnostics: true, explicitModuleBuild: true)
   }
 
   // Source files have changed but the inter-module dependency graph still up-to-date
   func testExplicitIncrementalBuildCheckGraphReuseOnChange() throws {
-    try buildInitialState(checkDiagnostics: true, explicitModuleBuild: true)
+    try buildInitialState(checkDiagnostics: false, explicitModuleBuild: true)
     try checkReactionToTouchingAll(checkDiagnostics: true, explicitModuleBuild: true)
   }
 
   // Adding an import invalidates prior inter-module dependency graph.
   func testExplicitIncrementalBuildNewImport() throws {
-    try buildInitialState(checkDiagnostics: true, explicitModuleBuild: true)
+    try buildInitialState(checkDiagnostics: false, explicitModuleBuild: true)
     // Introduce a new import. This will cause a re-scan and a re-build of 'other.swift'
     replace(contentsOf: "other", with: "import E;let bar = foo")
     try doABuild(