Generalize the opt-in

Author: stmontgomery

Sources/Testing/ABI/ABI.swift

@@ -94,6 +94,33 @@ extension ABI {
 #endif
 }
 
+/// The value of the environment variable flag which enables experimental event
+/// stream fields, if any.
+private let _shouldIncludeExperimentalFlags = Environment.flag(named: "SWT_EXPERIMENTAL_EVENT_STREAM_FIELDS_ENABLED")
+
+extension ABI.Version {
+  /// Whether or not experimental fields should be included when using this
+  /// ABI version.
+  ///
+  /// The value of this property is `true` if any of the following conditions
+  /// are satisfied:
+  ///
+  /// - The version number is less than 6.3. This is to preserve compatibility
+  ///   with existing clients before the inclusion of experimental fields became
+  ///   opt-in starting in 6.3.
+  /// - The version number is greater than or equal to 6.3 and the environment
+  ///   variable flag `SWT_EXPERIMENTAL_EVENT_STREAM_FIELDS_ENABLED` is set to a
+  ///   true value.
+  /// - The version number is greater than or equal to that of ``ABI/ExperimentalVersion``.
+  ///
+  /// Otherwise, the value of this property is `false`.
+  static var includesExperimentalFields: Bool {
+    versionNumber < ABI.v6_3.versionNumber
+      || (versionNumber >= ABI.v6_3.versionNumber && _shouldIncludeExperimentalFlags == true)
+      || versionNumber >= ABI.ExperimentalVersion.versionNumber
+  }
+}
+
 // MARK: - Concrete ABI versions
 
 extension ABI {

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

@@ -99,8 +99,8 @@ extension ABI {
       self.messages = messages.map(EncodedMessage.init)
       testID = event.testID.map(EncodedTest.ID.init)
 
-      // Experimental
-      if V.versionNumber >= ABI.ExperimentalVersion.versionNumber {
+      // Experimental fields
+      if V.includesExperimentalFields {
         if eventContext.test?.isParameterized == true {
           _testCase = eventContext.testCase.map(EncodedTestCase.init)
         }

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

@@ -72,8 +72,8 @@ extension ABI {
         isFailure = issue.isFailure
       }
 
-      // Experimental
-      if V.versionNumber >= ABI.ExperimentalVersion.versionNumber {
+      // Experimental fields
+      if V.includesExperimentalFields {
         if let backtrace = issue.sourceContext.backtrace {
           _backtrace = EncodedBacktrace(encoding: backtrace, in: eventContext)
         }

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

@@ -90,25 +90,12 @@ extension ABI {
       sourceLocation = test.sourceLocation
       id = ID(encoding: test.id)
 
-      // 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 fields
+      if V.includesExperimentalFields {
+        if isParameterized == 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:))