Use Hashes for swiftmodule incremental compilation

Sources/SwiftDriver/IncrementalCompilation/FirstWaveComputer.swift

@@ -202,9 +202,41 @@ extension IncrementalCompilationState.FirstWaveComputer {
       return false
     }
 
-    // Ensure that no output is older than any of the inputs
-    let oldestOutputModTime: TimePoint = try emitModuleJob.outputs.map { try fileSystem.lastModificationTime(for: $0.file) }.min() ?? .distantPast
-    return try emitModuleJob.inputs.swiftSourceFiles.allSatisfy({ try fileSystem.lastModificationTime(for: $0.typedFile.file) < oldestOutputModTime })
+    // If all the modules are older than the outputs, we can skip
+    let oldestOutputModTime = try emitModuleJob.outputs.map { try fileSystem.lastModificationTime(for: $0.file) }.min() ?? .distantPast
+    let areModulesOlderThanOutput = try emitModuleJob.inputs.swiftSourceFiles.allSatisfy({ try fileSystem.lastModificationTime(for: $0.typedFile.file) < oldestOutputModTime })
+    guard !areModulesOlderThanOutput else {
+        return true
+    }
+    // If we are not using hashes, we cannot skip
+    guard useHashes else {
+        return false
+    }
+    let inputs = emitModuleJob.inputs
+    for input in inputs {
+        guard let currentDate = buildRecordInfo.compilationInputModificationDates[input] else {
+            reporter?.report("Missing file metadata for: \(input)")
+            return false 
+        }
+
+        guard let currentHash = currentDate.hash else {
+            reporter?.report("Missing file hash data for: \(input)")
+            return false 
+        }
+
+        let inputInfos = buildRecord.inputInfos
+        guard let inputInfo = inputInfos[input.file] else {
+            reporter?.report("Missing incremental info for: \(input)")
+            return false
+        }
+
+        if currentHash != inputInfo.hash {
+            reporter?.report("Changed hash for: \(input)")
+            return false
+        }
+    }
+    return true
+
   }
 
   /// Figure out which compilation inputs are *not* mandatory at the start

Tests/SwiftDriverTests/IncrementalCompilationTests.swift

@@ -360,6 +360,35 @@ extension IncrementalCompilationTests {
     XCTAssertFalse(mandatoryJobInputs.contains("other.swift"))
   }
 
+  // Source file timestamps updated but contents are the same, with file-hashing emit-module job should be skipped
+  func testNullBuildNoEmitModuleWithHashing() throws {
+    let extraArguments = ["-experimental-emit-module-separately", "-emit-module", "-enable-incremental-file-hashing"]
+    try buildInitialState(extraArguments: extraArguments)
+    touch("main")
+    touch("other")
+    touch(try AbsolutePath(validating: explicitSwiftDependenciesPath.appending(component: "E.swiftinterface").pathString))
+    let driver = try doABuildWithoutExpectations(arguments: commonArgs + extraArguments + (try XCTUnwrap(Driver.sdkArgumentsForTesting())))
+    let mandatoryJobs = try XCTUnwrap(driver.incrementalCompilationState?.mandatoryJobsInOrder)
+    let mandatoryJobInputs = mandatoryJobs.flatMap { $0.inputs }.map { $0.file.basename }
+    XCTAssertFalse(mandatoryJobs.contains { $0.kind == .emitModule }, "emit-module should be skipped when using hashes and content unchanged")
+    XCTAssertFalse(mandatoryJobInputs.contains("main.swift"))
+    XCTAssertFalse(mandatoryJobInputs.contains("other.swift"))
+  }
+
+  // Source file updated, emit-module job should not be skipped regardless of file-hashing
+  func testEmitModuleWithHashingWhenContentChanges() throws {
+    let extraArguments = ["-experimental-emit-module-separately", "-emit-module", "-enable-incremental-file-hashing"]
+    try buildInitialState(extraArguments: extraArguments)
+    replace(contentsOf: "main", with: "let foo = 2")
+    let driver = try doABuildWithoutExpectations(arguments: commonArgs + extraArguments + (try XCTUnwrap(Driver.sdkArgumentsForTesting())))
+    let mandatoryJobs = try XCTUnwrap(driver.incrementalCompilationState?.mandatoryJobsInOrder)
+    let mandatoryJobInputs = mandatoryJobs.flatMap { $0.inputs }.map { $0.file.basename }
+    XCTAssertTrue(mandatoryJobs.contains { $0.kind == .emitModule }, "emit-module should run when using hashes and content has changed")
+    XCTAssertTrue(mandatoryJobInputs.contains("main.swift"))
+  }
+
+
+
   // External deps timestamp updated but contents are the same, and file-hashing is explicitly disabled 
   func testExplicitIncrementalBuildExternalDepsWithoutHashing() throws {
     replace(contentsOf: "other", with: "import E;let bar = foo")