Document the official distribution locations and the recommendation to use a built-in copy

Documentation/Distributions.md

@@ -0,0 +1,100 @@
+# Obtaining Swift Testing
+
+<!--
+This source file is part of the Swift.org open source project
+
+Copyright (c) 2025 Apple Inc. and the Swift project authors
+Licensed under Apache License v2.0 with Runtime Library Exception
+
+See https://swift.org/LICENSE.txt for license information
+See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+-->
+
+There are multiple ways to obtain Swift Testing, and they have different
+tradeoffs to consider. This document discusses the various ways Swift Testing is
+distributed and offers recommended workflows.
+
+## Distribution locations
+
+Swift Testing is distributed in the following places:
+
+* In [Swift.org toolchains][install], versions 6.0 and later (for
+  [supported platforms][]), as a dynamic library.
+* In Apple’s [Xcode IDE][], versions 16.0 and later, as a framework.
+* In Apple’s [Command Line Tools for Xcode package][], versions 16.0 and later,
+  as a framework.
+
+The locations above are considered **built-in** because they're included with a
+larger collection of software (such as a toolchain, IDE, or system package) and
+consist of _pre-compiled_ copies of the `Testing` module, its associated runtime
+libraries, and its macro plugin.
+
+> [!IMPORTANT]
+> Prefer using a built-in copy of Swift Testing unless you're making changes to
+> Swift Testing itself.
+
+Swift Testing is also available as a Swift **package library product** from the
+[swiftlang/swift-testing][swift-testing] repository. This copy is _not_
+considered built-in because it must be downloaded and compiled separately by
+each client. The package version is generally considered to have a lower level
+of support than the built-in copies above due to the [known caveats][caveats]
+described in the following section.
+
+## Caveats when using Swift Testing as a package
+
+Although Swift Testing is available as a Swift package and you _can_ declare a
+dependency on [swift-testing][] to use it, doing so is not generally recommended
+because it has several downsides:
+
+* **It requires building the Swift Testing runtime library.** This increases
+  your build time, and since builds for testing are typically for debug
+  configuration, it will not include performance optimizations.
+* **It requires building Swift Testing’s macro plugin.** This also increases
+  build time, especially because it often requires building [SwiftSyntax][] as
+  well. (SwiftSyntax now offers [prebuilt copies][prebuilt-swift-syntax], but
+  Swift Testing doesn't always declare a dependency on one of the prebuilt tags,
+  circumventing this time-saver.) Additionally, the locally-built macro plugin
+  and SwiftSyntax will be built for debug, without optimizations.
+* **It may not integrate as well with spporting tools/IDEs as a built-in copy.**
+  Tools which integrate with Swift Testing such as Swift Package Manager or
+  Apple's Xcode IDE often optimize for the copy included in the same
+  distribution. Some features may not work as well or be missing entirely when
+  using Swift Testing as a package.
+* **It may encounter build failures when another package uses Swift
+  Testing.** If you use Swift Testing as a package, but you depend on a library
+  from another package which uses a built-in copy of Swift Testing (as this
+  document recommends), this can cause build failures:
+  * The other package may fail to build non-deterministically due to not having
+    a target dependency on the `Testing` target from the locally-built
+    [swift-testing][] package.
+  * On platforms which don't support a two-level linker namespace, it can fail
+    to link due to duplicate defintions for the symbols in the `Testing` library.
+* **It may misbehave at runtime.** Even if your build doesn't encounter one of
+  the failures mentioned above, mixing built-in and package copies of Swift
+  Testing can lead to runtime problems, such as issues (e.g. `#expect` failures)
+  being silently ignored.
+
+## When to use Swift Testing as a package
+
+The primary reason Swift Testing is available to be used as a Swift package is
+to support its own development. The core contributors regularly develop Swift
+Testing by building it locally as a package, following workflows described in
+[Contributing][], and its CI builds that way as well.
+
+It's also sometimes helpful to use Swift Testing as a package in order to
+validate how changes made to the testing library will impact supporting tools,
+or to test changes to both the testing library and a related tool in conjunction
+with each other. When using one of these workflows locally, it's important to be
+mindful of the [caveats][] above, but during local development it's often
+possible to take extra care and control things sufficiently to avoid those
+problems.
+
+[install]: https://www.swift.org/install
+[supported platforms]: https://github.com/swiftlang/swift-testing/blob/main/README.md#cross-platform-support
+[Xcode IDE]: https://developer.apple.com/xcode/
+[Command Line Tools for Xcode package]: https://developer.apple.com/documentation/xcode/installing-the-command-line-tools/
+[swift-testing]: https://github.com/swiftlang/swift-testing
+[SwiftSyntax]: https://github.com/swiftlang/swift-syntax
+[Contributing]: https://github.com/swiftlang/swift-testing/blob/main/CONTRIBUTING.md
+[caveats]: #caveats-when-using-swift-testing-as-a-package
+[prebuilt-swift-syntax]: https://www.swift.org/blog/swift-6.2-released/#macro-build-performance

Documentation/README.md

@@ -23,6 +23,10 @@ as supplemental content located in the
 [`Sources/Testing/Testing.docc/`](https://github.com/swiftlang/swift-testing/tree/main/Sources/Testing/Testing.docc)
 directory.
 
+[Obtaining Swift Testing](https://github.com/swiftlang/swift-testing/blob/main/Documentation/Distributions.md)
+discusses the various ways Swift Testing is distributed and offers recommended
+usage workflows.
+
 ## Vision document and API proposals
 
 The [Vision document](https://github.com/swiftlang/swift-evolution/blob/main/visions/swift-testing.md)