[SE-0362] Add enableUpcomingFeature and enableExperimentalFeature Swift settings

Sources/PackageDescription/BuildSettings.swift

@@ -296,6 +296,55 @@ public struct SwiftSetting: Encodable {
     public static func unsafeFlags(_ flags: [String], _ condition: BuildSettingCondition? = nil) -> SwiftSetting {
         return SwiftSetting(name: "unsafeFlags", value: flags, condition: condition)
     }
+
+    /// Enable an upcoming feature with the given name.
+    ///
+    /// An upcoming feature is one that has been accepted into Swift as of a
+    /// certain language version, but is not available by default in prior
+    /// language modes because it has some impact on source compatibility.
+    ///
+    /// Multiple upcoming features can be added to a given target, and can
+    /// be used in a target without affecting its dependencies. An unknown
+    /// upcoming feature will be ignored by the implementation.
+    ///
+    /// - Since: First available in PackageDescription 5.8.
+    ///
+    /// - Parameters:
+    ///   - name: The name of the upcoming feature, e.g., ConciseMagicFile.
+    ///   - condition: A condition that restricts the application of the build
+    /// setting.
+    @available(_PackageDescription, introduced: 5.8)
+    public static func enableUpcomingFeature(
+        _ name: String,
+        _ condition: BuildSettingCondition? = nil
+    ) -> SwiftSetting {
+        return SwiftSetting(
+            name: "upcomingFeatures", value: [name], condition: condition)
+    }
+
+    /// Enable an experimental feature with the given name.
+    ///
+    /// An experimental feature is one that is in development, but
+    /// has not been accepted into Swift as a language feature.
+    ///
+    /// Multiple experimental features can be added to a given target, and can
+    /// be used in a target without affecting its dependencies. An unknown
+    /// experimental feature will be ignored by the implementation.
+    ///
+    /// - Since: First available in PackageDescription 5.8.
+    ///
+    /// - Parameters:
+    ///   - name: The name of the experimental feature, e.g., VariadicGenerics.
+    ///   - condition: A condition that restricts the application of the build
+    /// setting.
+    @available(_PackageDescription, introduced: 5.8)
+    public static func enableExperimentalFeature(
+        _ name: String,
+        _ condition: BuildSettingCondition? = nil
+    ) -> SwiftSetting {
+        return SwiftSetting(
+            name: "experimentalFeatures", value: [name], condition: condition)
+    }
 }
 
 /// A linker build setting.

Sources/PackageLoading/ManifestJSONParser.swift

@@ -425,6 +425,10 @@ enum ManifestJSONParser {
             kind = .linkedFramework(value)
         case "unsafeFlags":
             kind = .unsafeFlags(values)
+        case "upcomingFeatures":
+            kind = .upcomingFeatures(values)
+        case "experimentalFeatures":
+            kind = .experimentalFeatures(values)
         default:
             throw InternalError("invalid build setting \(name)")
         }

Sources/PackageLoading/PackageBuilder.swift

@@ -977,6 +977,30 @@ public final class PackageBuilder {
                 case .linker:
                     decl = .OTHER_LDFLAGS
                 }
+
+            case .upcomingFeatures(let _values):
+                switch setting.tool {
+                case .c, .cxx, .linker:
+                    throw InternalError("only Swift supports future features")
+
+                case .swift:
+                    decl = .OTHER_SWIFT_FLAGS
+                }
+
+                values = _values.precedeElements(with: "-enable-future-feature")
+
+            case .experimentalFeatures(let _values):
+                switch setting.tool {
+                case .c, .cxx, .linker:
+                    throw InternalError(
+                        "only Swift supports experimental features")
+
+                case .swift:
+                    decl = .OTHER_SWIFT_FLAGS
+                }
+
+                values = _values.precedeElements(
+                    with: "-enable-experimental-feature")
             }
 
             // Create an assignment for this setting.
@@ -1455,3 +1479,23 @@ extension PackageBuilder {
             }
     }
 }
+
+fileprivate extension Sequence {
+    /// Construct a new array where each of the elements in the \c self
+    /// sequence is preceded by the \c prefixElement.
+    ///
+    /// For example:
+    /// ```
+    /// ["Alice", "Bob", "Charlie"].precedeElements(with: "Hi")
+    /// ```
+    ///
+    /// produces `["Hi", "Alice", "Hi", "Bob", "Hi", "Charlie"]`.
+    func precedeElements(with prefixElement: Element) -> [Element] {
+        var results: [Element] = []
+        for element in self {
+            results.append(prefixElement)
+            results.append(element)
+        }
+        return results
+    }
+}

Sources/PackageModel/Manifest/TargetBuildSettingDescription.swift

@@ -29,6 +29,8 @@ public enum TargetBuildSettingDescription {
         case linkedFramework(String)
 
         case unsafeFlags([String])
+        case upcomingFeatures([String])
+        case experimentalFeatures([String])
     }
 
     /// An individual build setting.

Sources/PackageModel/ManifestSourceGeneration.swift

@@ -487,7 +487,7 @@ fileprivate extension SourceCodeFragment {
                 params.append(SourceCodeFragment(from: condition))
             }
             self.init(enum: setting.kind.name, subnodes: params)
-        case .unsafeFlags(let values):
+        case .unsafeFlags(let values), .upcomingFeatures(let values), .experimentalFeatures(let values):
             params.append(SourceCodeFragment(strings: values))
             if let condition = setting.condition {
                 params.append(SourceCodeFragment(from: condition))
@@ -639,6 +639,10 @@ extension TargetBuildSettingDescription.Kind {
             return "linkedFramework"
         case .unsafeFlags:
             return "unsafeFlags"
+        case .upcomingFeatures:
+            return "upcomingFeatures"
+        case .experimentalFeatures:
+            return "experimentalFeatures"
         }
     }
 }

Tests/BuildTests/BuildPlanTests.swift

@@ -3219,6 +3219,8 @@ final class BuildPlanTests: XCTestCase {
                         .init(tool: .swift, kind: .define("RLINUX"), condition: .init(platformNames: ["linux"], config: "release")),
                         .init(tool: .swift, kind: .define("DMACOS"), condition: .init(platformNames: ["macos"], config: "debug")),
                         .init(tool: .swift, kind: .unsafeFlags(["-Isfoo", "-L", "sbar"])),
+                        .init(tool: .swift, kind: .upcomingFeatures(["BestFeature"])),
+                        .init(tool: .swift, kind: .upcomingFeatures(["WorstFeature"]), condition: .init(platformNames: ["macos"], config: "debug"))
                     ]
                 ),
                 try TargetDescription(
@@ -3283,7 +3285,7 @@ final class BuildPlanTests: XCTestCase {
             XCTAssertMatch(cbar, [.anySequence, "-DCCC=2", "-I\(A.appending(components: "Sources", "cbar", "Sources", "headers"))", "-I\(A.appending(components: "Sources", "cbar", "Sources", "cppheaders"))", "-Icfoo", "-L", "cbar", "-Icxxfoo", "-L", "cxxbar", .end])
 
             let bar = try result.target(for: "bar").swiftTarget().compileArguments()
-            XCTAssertMatch(bar, [.anySequence, "-DLINUX", "-Isfoo", "-L", "sbar", .end])
+            XCTAssertMatch(bar, [.anySequence, "-DLINUX", "-Isfoo", "-L", "sbar", "-enable-future-feature", "BestFeature", .end])
 
             let exe = try result.target(for: "exe").swiftTarget().compileArguments()
             XCTAssertMatch(exe, [.anySequence, "-DFOO", .end])
@@ -3299,7 +3301,7 @@ final class BuildPlanTests: XCTestCase {
             XCTAssertMatch(cbar, [.anySequence, "-DCCC=2", "-I\(A.appending(components: "Sources", "cbar", "Sources", "headers"))", "-I\(A.appending(components: "Sources", "cbar", "Sources", "cppheaders"))", "-Icfoo", "-L", "cbar", "-Icxxfoo", "-L", "cxxbar", .end])
 
             let bar = try result.target(for: "bar").swiftTarget().compileArguments()
-            XCTAssertMatch(bar, [.anySequence, "-DDMACOS", "-Isfoo", "-L", "sbar", .end])
+            XCTAssertMatch(bar, [.anySequence, "-DDMACOS", "-Isfoo", "-L", "sbar", "-enable-future-feature", "BestFeature", "-enable-future-feature", "WorstFeature", .end])
 
             let exe = try result.target(for: "exe").swiftTarget().compileArguments()
             XCTAssertMatch(exe, [.anySequence, "-DFOO", .end])