Create an xunit results file even if no tests are run.

Fixtures/Miscellaneous/EmptyTestsPkg/.gitignore

@@ -0,0 +1,4 @@
+.DS_Store
+/.build
+/Packages
+/*.xcodeproj

Fixtures/Miscellaneous/EmptyTestsPkg/Package.swift

@@ -0,0 +1,14 @@
+// swift-tools-version:4.2
+import PackageDescription
+
+let package = Package(
+    name: "EmptyTestsPkg",
+    targets: [
+        .target(
+            name: "EmptyTestsPkg",
+            dependencies: []),
+        .testTarget(
+            name: "EmptyTestsPkgTests",
+            dependencies: ["EmptyTestsPkg"]),
+    ]
+)

Fixtures/Miscellaneous/EmptyTestsPkg/Sources/EmptyTestsPkg/EmptyTestsPkg.swift

@@ -0,0 +1,5 @@
+struct EmptyTests {
+
+    var text = "Hello, World!"
+    var bool = false
+}

Fixtures/Miscellaneous/EmptyTestsPkg/Tests/EmptyTestsPkgTests/EmptyTestsTests.swift

@@ -0,0 +1,2 @@
+import XCTest
+@testable import EmptyTestsPkg

Sources/Commands/SwiftTestTool.swift

@@ -241,6 +241,7 @@ public struct SwiftTestTool: SwiftCommand {
             // If there were no matches, emit a warning and exit.
             if tests.isEmpty {
                 swiftTool.observabilityScope.emit(.noMatchingTests)
+                try generateXUnitOutputIfRequested(for: [], swiftTool: swiftTool)
                 return
             }
 
@@ -264,14 +265,7 @@ public struct SwiftTestTool: SwiftCommand {
 
             let testResults = try runner.run(tests)
 
-            // Generate xUnit file if requested
-            if let xUnitOutput = options.xUnitOutput {
-                let generator = XUnitGenerator(
-                    fileSystem: swiftTool.fileSystem,
-                    results: testResults
-                )
-                try generator.generate(at: xUnitOutput)
-            }
+            try generateXUnitOutputIfRequested(for: testResults, swiftTool: swiftTool)
 
             // process code Coverage if request
             if self.options.enableCodeCoverage, runner.ranSuccessfully {
@@ -325,6 +319,19 @@ public struct SwiftTestTool: SwiftCommand {
         }
     }
 
+    /// Generate xUnit file if requested.
+    private func generateXUnitOutputIfRequested(for testResults: [ParallelTestRunner.TestResult], swiftTool: SwiftTool) throws {
+        guard let xUnitOutput = options.xUnitOutput else {
+            return
+        }
+
+        let generator = XUnitGenerator(
+            fileSystem: swiftTool.fileSystem,
+            results: testResults
+        )
+        try generator.generate(at: xUnitOutput)
+    }
+
     // MARK: - swift-testing
 
     private func swiftTestingRun(_ swiftTool: SwiftTool) throws {

Tests/CommandsTests/TestToolTests.swift

@@ -128,6 +128,22 @@ final class TestToolTests: CommandsTestCase {
         }
     }
 
+    func testSwiftTestXMLOutputWhenEmpty() throws {
+        try fixture(name: "Miscellaneous/EmptyTestsPkg") { fixturePath in
+            let xUnitOutput = fixturePath.appending("result.xml")
+            // Run tests in parallel with verbose output.
+            let stdout = try SwiftPM.Test.execute(["--parallel", "--verbose", "--xunit-output", xUnitOutput.pathString], packagePath: fixturePath).stdout
+            // in "swift test" test output goes to stdout
+            XCTAssertNoMatch(stdout, .contains("passed"))
+            XCTAssertNoMatch(stdout, .contains("failed"))
+
+            // Check the xUnit output.
+            XCTAssertFileExists(xUnitOutput)
+            let contents: String = try localFileSystem.readFileContents(xUnitOutput)
+            XCTAssertMatch(contents, .contains("tests=\"0\" failures=\"0\""))
+        }
+    }
+
     func testSwiftTestFilter() throws {
         try fixture(name: "Miscellaneous/SkipTests") { fixturePath in
             let (stdout, _) = try SwiftPM.Test.execute(["--filter", ".*1"], packagePath: fixturePath)