Define an experimental event stream version and use it to conditionalize relevant content

Sources/Testing/ABI/ABI.swift

@@ -45,7 +45,8 @@ extension ABI {
   /// The current supported ABI version (ignoring any experimental versions.)
   typealias CurrentVersion = v0
 
-  /// The highest supported ABI version (including any experimental versions.)
+  /// The highest defined and supported ABI version (including any experimental
+  /// versions.)
   typealias HighestVersion = v6_3
 
 #if !hasFeature(Embedded)
@@ -125,6 +126,14 @@ extension ABI {
       VersionNumber(6, 3)
     }
   }
+
+  /// A namespace and type representing the ABI version whose symbols are
+  /// considered experimental.
+  enum ExperimentalVersion: Sendable, Version {
+    static var versionNumber: VersionNumber {
+      VersionNumber(99, 0)
+    }
+  }
 }
 
 /// A namespace for ABI version 0 symbols.

Sources/Testing/ABI/Encoded/ABI.EncodedEvent.swift

@@ -98,8 +98,12 @@ extension ABI {
       instant = EncodedInstant(encoding: event.instant)
       self.messages = messages.map(EncodedMessage.init)
       testID = event.testID.map(EncodedTest.ID.init)
-      if eventContext.test?.isParameterized == true {
-        _testCase = eventContext.testCase.map(EncodedTestCase.init)
+
+      // Experimental
+      if V.versionNumber >= ABI.ExperimentalVersion.versionNumber {
+        if eventContext.test?.isParameterized == true {
+          _testCase = eventContext.testCase.map(EncodedTestCase.init)
+        }
       }
     }
   }

Sources/Testing/ABI/Encoded/ABI.EncodedIssue.swift

@@ -73,11 +73,13 @@ extension ABI {
       }
 
       // Experimental
-      if let backtrace = issue.sourceContext.backtrace {
-        _backtrace = EncodedBacktrace(encoding: backtrace, in: eventContext)
-      }
-      if let error = issue.error {
-        _error = EncodedError(encoding: error, in: eventContext)
+      if V.versionNumber >= ABI.ExperimentalVersion.versionNumber {
+        if let backtrace = issue.sourceContext.backtrace {
+          _backtrace = EncodedBacktrace(encoding: backtrace, in: eventContext)
+        }
+        if let error = issue.error {
+          _error = EncodedError(encoding: error, in: eventContext)
+        }
       }
     }
   }

Sources/Testing/ABI/Encoded/ABI.EncodedTest.swift

@@ -76,29 +76,39 @@ extension ABI {
     /// The tags associated with the test.
     ///
     /// - Warning: Tags are not yet part of the JSON schema.
-    ///
-    /// @Metadata {
-    ///   @Available("Swift Testing ABI", introduced: 6.3)
-    /// }
     var _tags: [String]?
 
     init(encoding test: borrowing Test) {
       if test.isSuite {
         kind = .suite
       } else {
         kind = .function
-        let testIsParameterized = test.isParameterized
-        isParameterized = testIsParameterized
-        if testIsParameterized {
-          _testCases = test.uncheckedTestCases?.map(EncodedTestCase.init(encoding:))
-        }
+        isParameterized = test.isParameterized
       }
       name = test.name
       displayName = test.displayName
       sourceLocation = test.sourceLocation
       id = ID(encoding: test.id)
 
-      if V.versionNumber >= ABI.v6_3.versionNumber {
+      // Experimental test case encoding: This field was included in v0 with an
+      // underscore-prefixed name despite not having been formally proposed, and
+      // a prominent client (the VS Code Swift plugin) depends on it. To avoid
+      // breaking existing versions of that plugin, continue unconditionally
+      // including this field in versions earlier than 6.3 (including v0). In
+      // 6.3 and later, don't include it by default but allow opting-in to it
+      // via an environment variable. (This is to maintain compatibility until
+      // the field has been formally proposed and accepted, and discourage new
+      // clients from becoming dependent on it in the mean time.) Finally,
+      // in the special experimental version always include this field.
+      if isParameterized == true,
+         (V.versionNumber < ABI.v6_3.versionNumber
+          || V.versionNumber >= ABI.ExperimentalVersion.versionNumber
+          || Environment.flag(named: "SWT_ENABLE_EXPERIMENTAL_EVENT_STREAM_TEST_CASE_ENCODING") == true) {
+        _testCases = test.uncheckedTestCases?.map(EncodedTestCase.init(encoding:))
+      }
+
+      // Experimental
+      if V.versionNumber >= ABI.ExperimentalVersion.versionNumber {
         let tags = test.tags
         if !tags.isEmpty {
           _tags = tags.map(String.init(describing:))

Sources/Testing/ABI/EntryPoints/EntryPoint.swift

@@ -57,10 +57,10 @@ func entryPoint(passing args: __CommandLineArguments_v0?, eventHandler: Event.Ha
       // Check for experimental console output flag
       if Environment.flag(named: "SWT_ENABLE_EXPERIMENTAL_CONSOLE_OUTPUT") == true {
         // Use experimental AdvancedConsoleOutputRecorder
-        var advancedOptions = Event.AdvancedConsoleOutputRecorder<ABI.HighestVersion>.Options()
+        var advancedOptions = Event.AdvancedConsoleOutputRecorder<ABI.ExperimentalVersion>.Options()
         advancedOptions.base = .for(.stderr)
 
-        let eventRecorder = Event.AdvancedConsoleOutputRecorder<ABI.HighestVersion>(options: advancedOptions) { string in
+        let eventRecorder = Event.AdvancedConsoleOutputRecorder<ABI.ExperimentalVersion>(options: advancedOptions) { string in
           try? FileHandle.stderr.write(string)
         }
 

Sources/Testing/ExitTests/ExitTest.swift

@@ -541,10 +541,9 @@ extension ABI {
   /// The ABI version to use for encoding and decoding events sent over the back
   /// channel.
   ///
-  /// The back channel always uses the latest ABI version (even if experimental)
-  /// since both the producer and consumer use this exact version of the testing
-  /// library.
-  fileprivate typealias BackChannelVersion = v6_3
+  /// The back channel always uses the experimental ABI version since both the
+  /// producer and consumer use this exact version of the testing library.
+  fileprivate typealias BackChannelVersion = ExperimentalVersion
 }
 
 @_spi(ForToolsIntegrationOnly)

Tests/TestingTests/SwiftPMTests.swift

@@ -376,7 +376,7 @@ struct SwiftPMTests {
     }
     #expect(testRecords.count == 1)
     for testRecord in testRecords {
-      if version.versionNumber >= ABI.v6_3.versionNumber {
+      if version.versionNumber >= ABI.ExperimentalVersion.versionNumber {
         #expect(testRecord._tags != nil)
       } else {
         #expect(testRecord._tags == nil)