Automatically enable swift-testing support in swift test when it's a dependency. (#7220)

This PR causes `swift test` to check the dependencies of the package
under test; if they include an explicit reference to the public
swift-testing repository on GitHub, then
`--enable-experimental-swift-testing` is assumed. A developer can still
opt out by explicitly writing `--disable-experimental-swift-testing`.

Resolves rdar://99155992.

Author: grynspan

Sources/Commands/SwiftTestTool.swift

@@ -78,11 +78,39 @@ struct SharedOptions: ParsableArguments {
           help: "Enable support for XCTest")
     var enableXCTestSupport: Bool = true
 
-    /// Whether to enable support for swift-testing.
+    /// Storage for whether to enable support for swift-testing.
     @Flag(name: .customLong("experimental-swift-testing"),
           inversion: .prefixedEnableDisable,
           help: "Enable experimental support for swift-testing")
-    var enableSwiftTestingLibrarySupport: Bool = false
+    var _enableSwiftTestingLibrarySupport: Bool?
+
+    /// Whether to enable support for swift-testing.
+    func enableSwiftTestingLibrarySupport(swiftTool: SwiftTool) throws -> Bool {
+        // Honor the user's explicit command-line selection, if any.
+        if let callerSuppliedValue = _enableSwiftTestingLibrarySupport {
+            return callerSuppliedValue
+        }
+
+        // If the active package has a dependency on swift-testing, automatically enable support for it so that extra steps are not needed.
+        let workspace = try swiftTool.getActiveWorkspace()
+        let root = try swiftTool.getWorkspaceRoot()
+        let rootManifests = try temp_await {
+            workspace.loadRootManifests(
+                packages: root.packages,
+                observabilityScope: swiftTool.observabilityScope,
+                completion: $0
+            )
+        }
+        let isEnabledByDependency = rootManifests.values.lazy
+            .flatMap(\.dependencies)
+            .map(\.identity)
+            .map(String.init(describing:))
+            .contains("swift-testing")
+        if isEnabledByDependency {
+            swiftTool.observabilityScope.emit(debug: "Enabling swift-testing support due to its presence as a package dependency.")
+        }
+        return isEnabledByDependency
+    }
 }
 
 struct TestToolOptions: ParsableArguments {
@@ -359,7 +387,7 @@ public struct SwiftTestTool: SwiftCommand {
             let command = try List.parse()
             try command.run(swiftTool)
         } else {
-            if options.sharedOptions.enableSwiftTestingLibrarySupport {
+            if try options.sharedOptions.enableSwiftTestingLibrarySupport(swiftTool: swiftTool) {
                 try swiftTestingRun(swiftTool)
             }
             if options.sharedOptions.enableXCTestSupport {
@@ -651,7 +679,7 @@ extension SwiftTestTool {
         // MARK: - Common implementation
 
         func run(_ swiftTool: SwiftTool) throws {
-            if sharedOptions.enableSwiftTestingLibrarySupport {
+            if try sharedOptions.enableSwiftTestingLibrarySupport(swiftTool: swiftTool) {
                 try swiftTestingRun(swiftTool)
             }
             if sharedOptions.enableXCTestSupport {
@@ -1212,7 +1240,7 @@ extension SwiftTool {
             experimentalTestOutput: options.enableExperimentalTestOutput,
             library: library
         )
-        if options.sharedOptions.enableSwiftTestingLibrarySupport {
+        if try options.sharedOptions.enableSwiftTestingLibrarySupport(swiftTool: self) {
             result.flags.swiftCompilerFlags += ["-DSWIFT_PM_SUPPORTS_SWIFT_TESTING"]
         }
         return result