Change the default JSON schema version to v0.

This PR changes the default JSON schema version for event stream output to
version 0 (the version documented in #479.) Previously, if a caller did not
specify a version number for the event stream, it would default to encoding
"snapshots" of various values from the testing library.

Xcode 16 Beta 1 uses this legacy format, so to continue supporting newer betas
of Xcode 16 transitionally, this PR temporarily enables a "-1" JSON schema
version that continues to emit snapshot encodings. This JSON schema version will
be removed (along with the snapshots code in general) in a future update once
Xcode has migrated.

Resolves #527.

Author: grynspan

Sources/Testing/EntryPoints/ABIv0/ABIv0.Record+Streaming.swift

@@ -57,8 +57,8 @@ extension ABIv0.Record {
 /// External adopters are not necessarily written in Swift and are expected to
 /// decode the JSON produced for this type in implementation-specific ways.
 ///
-/// - Warning: This type will be removed when the ABI version 0 JSON schema is
-///   finalized.
+/// - Warning: This type supports early Xcode 16 betas and will be removed in a
+///   future update.
 struct EventAndContextSnapshot {
   /// A snapshot of the event.
   var event: Event.Snapshot
@@ -86,8 +86,8 @@ extension EventAndContextSnapshot: Codable {}
 /// performs additional postprocessing before writing JSON data to ensure it
 /// does not contain any newline characters.
 ///
-/// - Warning: This function will be removed when the ABI version 0 JSON schema
-///   is finalized.
+/// - Warning: This function supports early Xcode 16 betas and will be removed
+///   in a future update.
 func eventHandlerForStreamingEventSnapshots(
   to eventHandler: @escaping @Sendable (_ eventAndContextJSON: UnsafeRawBufferPointer) -> Void
 ) -> Event.Handler {

Sources/Testing/EntryPoints/EntryPoint.swift

@@ -220,12 +220,12 @@ public struct __CommandLineArguments_v0: Sendable {
   /// The version of the event stream schema to use when writing events to
   /// ``experimentalEventStreamOutput``.
   ///
-  /// If the value of this property is `nil`, events are encoded verbatim (using
-  /// ``Event/Snapshot``.) Otherwise, the corresponding stable schema is used
-  /// (e.g. ``ABIv0/Record`` for `0`.)
+  /// The corresponding stable schema is used to encode events to the event
+  /// stream (for example, ``ABIv0/Record`` is used if the value of this
+  /// property is `0`.)
   ///
-  /// - Warning: The behavior of this property will change when the ABI version
-  ///   0 JSON schema is finalized.
+  /// If the value of this property is `nil`, the testing library assumes a
+  /// value of `0` instead.
   public var experimentalEventStreamVersion: Int?
 
   /// The value(s) of the `--filter` argument.
@@ -482,9 +482,11 @@ public func configurationForEntryPoint(from args: __CommandLineArguments_v0) thr
 /// - Throws: If `version` is not a supported ABI version.
 func eventHandlerForStreamingEvents(version: Int?, forwardingTo eventHandler: @escaping @Sendable (UnsafeRawBufferPointer) -> Void) throws -> Event.Handler {
   switch version {
-  case nil:
+  case -1:
+    // Legacy support for Xcode 16 betas. Support for this undocumented version
+    // will be removed in a future update. Do not use it.
     eventHandlerForStreamingEventSnapshots(to: eventHandler)
-  case 0:
+  case nil, 0:
     ABIv0.Record.eventHandler(forwardingTo: eventHandler)
   case let .some(unsupportedVersion):
     throw _EntryPointError.invalidArgument("--experimental-event-stream-version", value: "\(unsupportedVersion)")