Add a guide for upcoming language feature migration

Author: bnbarham

Guide.docc/FeatureMigration.md

@@ -0,0 +1,98 @@
+# Migrating to upcoming language features
+
+Migrate your project to upcoming language features.
+
+Upcoming language features can be enabled in the Swift compiler via a `-enable-upcoming-feature
+<FeatureName>` flag. Some of these features also support a migration mode. This mode does not
+actually enable the desired feature. Instead, it produces compiler warnings with the necessary
+fix-its to make the existing code both source- and binary-compatible with the feature. The exact
+semantics of such a migration is dependent on the feature, see their [corresponding
+documentation](https://docs.swift.org/compiler/documentation/diagnostics/upcoming-language-features)
+for more details.
+
+## SwiftPM
+
+> Note: This feature is in active development. Test with a [nightly
+> snapshot](https://www.swift.org/install) for best results.
+
+`swift package migrate` builds and applies migration fix-its to allow for semi-automated migration.
+Make sure to start with a clean working tree (no current changes staged or otherwise) and a working
+build - applying the fix-its requires there to be no build errors and will modify files in the
+package *in place*.
+
+To eg. migrate all targets in your package to `NonisolatedNonsendingByDefault`:
+```sh
+swift package migrate --to-feature NonisolatedNonsendingByDefault
+```
+
+Or a target at a time with `--targets`:
+```sh
+swift package migrate --targets TargetA --to-feature NonisolatedNonsendingByDefault
+```
+
+This will start a build, apply any migration fix-its, and then update the manifest:
+```
+> Starting the build.
+... regular build output with migration diagnostics ...
+> Applying fix-its.
+> Updating manifest.
+```
+
+Check out the changes with your usual version control tooling, e.g., `git diff`:
+```diff
+diff --git a/Package.swift b/Package.swift
+index a1e587c..11097be 100644
+--- a/Package.swift
++++ b/Package.swift
+@@ -14,10 +14,16 @@ let package = Package(
+     targets: [
+         .target(
+             name: "TargetA",
++            swiftSettings: [
++                .enableUpcomingFeature("NonisolatedNonsendingByDefault"),
++            ]
+         ),
+     ]
+
+diff --git a/Sources/packtest/packtest.swift b/Sources/packtest/packtest.swift
+index 85253f5..8498bb5 100644
+--- a/Sources/TargetA/TargetA.swift
++++ b/Sources/TargetA/TargetA.swift
+@@ -1,5 +1,5 @@
+ struct S: Sendable {
+-  func alwaysSwitch() async {}
++  @concurrent func alwaysSwitch() async {}
+ }
+```
+
+In some cases, the automated application of upcoming features to a target in the package manifest
+can fail for more complicated packages, e.g., if settings have been factored out into a variable
+that's then applied to multiple targets:
+```
+error: Could not update manifest for 'TargetA' (unable to find array literal for 'swiftSettings' argument). Please enable 'NonisolatedNonsendingByDefault' features manually.
+```
+
+If this happens, manually add a `.enableUpcomingFeature("SomeFeature")` Swift setting to complete
+the migration:
+```swift
+// swift-tools-version: 6.2
+
+let targetSettings: [SwiftSetting] = [
+    // ...
+    .enableUpcomingFeature("NonisolatedNonsendingByDefault")
+]
+
+let targetSettings:
+let package = Package(
+    name: "MyPackage",
+    products: [
+        // ...
+    ],
+    targets: [
+        .target(
+            name: "TargetA",
+            swiftSettings: targetSettings
+        ),
+    ]
+)
+```