[Merge Modules] Omit merge-modules job when only one input is given.

And said input matches the expected output path.

Author: artemcm

Sources/SwiftDriver/Jobs/Planning.swift

@@ -392,6 +392,16 @@ extension Driver {
           !(moduleInputs.isEmpty && moduleInputsFromJobOutputs.isEmpty),
           compilerMode.usesPrimaryFileInputs
     else { return nil }
+
+    // If there is only 1 module input to this job and its path matches that which we intend
+    // to produce with this merge-modules job, skip it
+    let mergeOutput = moduleOutputInfo.output!.outputPath
+    if moduleInputs.count + moduleInputsFromJobOutputs.count == 1,
+       let soleInput = moduleInputs.first == nil ? moduleInputsFromJobOutputs.first : moduleInputs.first,
+       soleInput.fileHandle == mergeOutput {
+      return nil
+    }
+
     return try mergeModuleJob(inputs: moduleInputs, inputsFromOutputs: moduleInputsFromJobOutputs)
   }
 

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -1973,6 +1973,23 @@ final class SwiftDriverTests: XCTestCase {
       XCTAssertEqual(plannedJobs[1].inputs[0].file, .temporary(RelativePath("input.swiftmodule")))
       XCTAssertEqual(plannedJobs[1].outputs[0].file, .absolute(AbsolutePath("/tmp/test.swiftmodule")))
     }
+
+    do {
+      // -g is specified
+      // (partial and final product module are saved to temporary directory so merge-modules is omitted)
+      var driver = try Driver(args: ["swiftc", "-g", "input.swift"])
+      let plannedJobs = try driver.planBuild()
+      print(plannedJobs)
+      XCTAssertEqual(plannedJobs.count, 3)
+      // No .mergeModule job
+      #if os(macOS)
+      XCTAssertEqual(plannedJobs.map { $0.kind }, [.compile, .link, .generateDSYM])
+      #endif
+      #if os(Linux)
+      XCTAssertEqual(plannedJobs.map { $0.kind }, [.compile, .autolinkExtract, .link])
+      #endif
+      XCTAssertTrue(plannedJobs[0].outputs.contains(TypedVirtualPath(file: VirtualPath.temporary(RelativePath("input.swiftmodule")).intern(), type: .swiftModule)))
+    }
   }
 
   func testEmitModuleSeparately() throws {
@@ -2046,11 +2063,11 @@ final class SwiftDriverTests: XCTestCase {
     // dsymutil won't be found on other platforms
     #if os(macOS)
     do {
-      var driver = try Driver(args: ["swiftc", "-target", "x86_64-apple-macosx10.15", "-g", "foo.swift"])
+      var driver = try Driver(args: ["swiftc", "-target", "x86_64-apple-macosx10.15", "-g", "foo.swift", "bar.swift"])
       let plannedJobs = try driver.planBuild()
-      XCTAssertEqual(plannedJobs.count, 4)
+      XCTAssertEqual(plannedJobs.count, 5)
       // No module wrapping with Mach-O.
-      XCTAssertEqual(plannedJobs.map { $0.kind }, [.compile, .mergeModule, .link, .generateDSYM])
+      XCTAssertEqual(plannedJobs.map { $0.kind }, [.compile, .compile, .mergeModule, .link, .generateDSYM])
     }
     #endif
   }