Set an environment variable in swift test to indicate which testing library is in use.

Fixtures/Miscellaneous/CheckTestLibraryEnvironmentVariable/Package.swift

@@ -0,0 +1,10 @@
+// swift-tools-version: 5.10
+
+import PackageDescription
+
+let package = Package(
+    name: "CheckTestLibraryEnvironmentVariable",
+    targets: [
+        .testTarget(name: "CheckTestLibraryEnvironmentVariableTests"),
+    ]
+)

Fixtures/Miscellaneous/CheckTestLibraryEnvironmentVariable/Tests/CheckTestLibraryEnvironmentVariableTests/CheckTestLibraryEnvironmentVariableTests.swift

@@ -0,0 +1,8 @@
+import XCTest
+
+final class CheckTestLibraryEnvironmentVariableTests: XCTestCase {
+    func testEnviromentVariable() throws {
+        let envvar = ProcessInfo.processInfo.environment["SWIFT_PM_TEST_LIBRARY"]
+        XCTAssertEqual(envvar, "XCTest")
+    }
+}

Sources/Commands/SwiftTestCommand.swift

@@ -421,7 +421,8 @@ public struct SwiftTestCommand: AsyncSwiftCommand {
         let testEnv = try TestingSupport.constructTestEnvironment(
             toolchain: toolchain,
             destinationBuildParameters: productsBuildParameters,
-            sanitizers: globalOptions.build.sanitizers
+            sanitizers: globalOptions.build.sanitizers,
+            library: library
         )
 
         let runner = TestRunner(
@@ -702,7 +703,8 @@ extension SwiftTestCommand {
             let testEnv = try TestingSupport.constructTestEnvironment(
                 toolchain: toolchain,
                 destinationBuildParameters: productsBuildParameters,
-                sanitizers: globalOptions.build.sanitizers
+                sanitizers: globalOptions.build.sanitizers,
+                library: .swiftTesting
             )
 
             let additionalArguments = ["--list-tests"] + CommandLine.arguments.dropFirst()
@@ -1014,7 +1016,8 @@ final class ParallelTestRunner {
         let testEnv = try TestingSupport.constructTestEnvironment(
             toolchain: self.toolchain,
             destinationBuildParameters: self.productsBuildParameters,
-            sanitizers: self.buildOptions.sanitizers
+            sanitizers: self.buildOptions.sanitizers,
+            library: .xctest // swift-testing does not use ParallelTestRunner
         )
 
         // Enqueue all the tests.

Sources/Commands/Utilities/PluginDelegate.swift

@@ -236,7 +236,8 @@ final class PluginDelegate: PluginInvocationDelegate {
         let testEnvironment = try TestingSupport.constructTestEnvironment(
             toolchain: toolchain,
             destinationBuildParameters: toolsBuildParameters,
-            sanitizers: swiftCommandState.options.build.sanitizers
+            sanitizers: swiftCommandState.options.build.sanitizers,
+            library: .xctest // FIXME: support both libraries
         )
 
         // Iterate over the tests and run those that match the filter.

Sources/Commands/Utilities/TestingSupport.swift

@@ -121,7 +121,8 @@ enum TestingSupport {
                     experimentalTestOutput: experimentalTestOutput,
                     library: .xctest
                 ).productsBuildParameters,
-                sanitizers: sanitizers
+                sanitizers: sanitizers,
+                library: .xctest
             )
 
             try TSCBasic.Process.checkNonZeroExit(arguments: args, environment: env)
@@ -136,7 +137,8 @@ enum TestingSupport {
                 shouldSkipBuilding: shouldSkipBuilding,
                 library: .xctest
             ).productsBuildParameters,
-            sanitizers: sanitizers
+            sanitizers: sanitizers,
+            library: .xctest
         )
         args = [path.description, "--dump-tests-json"]
         let data = try Process.checkNonZeroExit(arguments: args, environment: env)
@@ -149,7 +151,8 @@ enum TestingSupport {
     static func constructTestEnvironment(
         toolchain: UserToolchain,
         destinationBuildParameters buildParameters: BuildParameters,
-        sanitizers: [Sanitizer]
+        sanitizers: [Sanitizer],
+        library: BuildParameters.Testing.Library
     ) throws -> EnvironmentVariables {
         var env = EnvironmentVariables.process()
 
@@ -161,6 +164,10 @@ enum TestingSupport {
             env["NO_COLOR"] = "1"
         }
 
+        // Set an environment variable to indicate which library's test product
+        // is being executed.
+        env["SWIFT_PM_TEST_LIBRARY"] = String(describing: library)
+
         // Add the code coverage related variables.
         if buildParameters.testingParameters.enableCodeCoverage {
             // Defines the path at which the profraw files will be written on test execution.

Tests/CommandsTests/TestCommandTests.swift

@@ -303,4 +303,10 @@ final class TestCommandTests: CommandsTestCase {
         }
     }
 #endif
+
+    func testLibraryEnvironmentVariable() throws {
+      try fixture(name: "Miscellaneous/CheckTestLibraryEnvironmentVariable") { fixturePath in
+        XCTAssertNoThrow(try SwiftPM.Test.execute(packagePath: fixturePath))
+      }
+    }
 }