Build: pass through Embedded flag to link jobs

Without passing this flag, Swift Driver links such products as if they're not built for embedded platforms at all, passing incorrect flags to the linker. This is not reproducible when using plain `swiftc`, as that combines both compile and link jobs by default, but SwiftPM prefers to run those separately, which requires this special handling.

### Modifications:

Directly checking in `ProductBuildDescription/linkArguments` whether all of the product's targets are built in the embedded mode. If so, we're passing the embedded mode flag to Swift Driver.

Unfortunately, `BuildSettings.AssignmentTable` is formed too early in the build process right in `PackageModel`, while we still need to run checks specific build settings quite late in the build stage. Because of that, there's no clean way to check if `Embedded` flag is passed other than directly rendering `BuildSettings.Scope` to a string and running a substring check on that. In the future we should move `BuildSettings` out of `PackageModel`, ensuring that SwiftPM clients don't rely on this behavior anymore.

### Result:

Products that have targets using Embedded Swift can be built with SwiftPM, assuming that Swift Driver handles other linker flags correctly.

Author: MaxDesiatov

Sources/Build/BuildDescription/ProductBuildDescription.swift

@@ -198,7 +198,7 @@ public final class ProductBuildDescription: SPMBuildCore.ProductBuildDescription
             // No arguments for static libraries.
             return []
         case .test:
-            // Test products are bundle when using objectiveC, executable when using test entry point.
+            // Test products are bundle when using Objective-C, executable when using test entry point.
             switch self.buildParameters.testingParameters.testProductStyle {
             case .loadableBundle:
                 args += ["-Xlinker", "-bundle"]
@@ -271,8 +271,24 @@ public final class ProductBuildDescription: SPMBuildCore.ProductBuildDescription
         }
         args += ["@\(self.linkFileListPath.pathString)"]
 
-        // Embed the swift stdlib library path inside tests and executables on Darwin.
         if containsSwiftTargets {
+            // A product is embedded if all of its static targets are embedded.
+            var isEmbeddedProduct = true
+            
+            for target in self.product.targets {
+                let flags = self.buildParameters.createScope(for: target).evaluate(.OTHER_SWIFT_FLAGS)
+
+                // FIXME: update this comparison to use `contains` on `Collection` when macOS 13 is available.
+                if !flags.joined(separator: " ").contains("-enable-experimental-feature Embedded") {
+                    isEmbeddedProduct = false
+                    break
+                }
+            }
+            if isEmbeddedProduct {
+                args += ["-enable-experimental-feature", "Embedded"]
+            }
+
+            // Embed the swift stdlib library path inside tests and executables on Darwin.
             let useStdlibRpath: Bool
             switch self.product.type {
             case .library(let type):
@@ -297,11 +313,9 @@ public final class ProductBuildDescription: SPMBuildCore.ProductBuildDescription
                     args += ["-Xlinker", "-rpath", "-Xlinker", backDeployedStdlib.pathString]
                 }
             }
-        }
-
-        // Don't link runtime compatibility patch libraries if there are no
-        // Swift sources in the target.
-        if !containsSwiftTargets {
+        } else {
+            // Don't link runtime compatibility patch libraries if there are no
+            // Swift sources in the target.
             args += ["-runtime-compatibility-version", "none"]
         }
 

Tests/BuildTests/ProductBuildDescriptionTests.swift

@@ -0,0 +1,75 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift open source project
+//
+// Copyright (c) 2014-2021 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+@testable
+import Build
+
+import class Basics.ObservabilitySystem
+import class TSCBasic.InMemoryFileSystem
+
+import class PackageModel.Manifest
+import struct PackageModel.TargetDescription
+
+@testable
+import struct PackageGraph.ResolvedProduct
+
+import func SPMTestSupport.loadPackageGraph
+import func SPMTestSupport.mockBuildParameters
+import func SPMTestSupport.XCTAssertNoDiagnostics
+import XCTest
+
+final class ProductBuildDescriptionTests: XCTestCase {
+    func testEmbeddedProducts() throws {
+        let fs = InMemoryFileSystem(
+            emptyFiles:
+            "/Pkg/Sources/exe/main.swift"
+        )
+
+        let observability = ObservabilitySystem.makeForTesting()
+        let graph = try loadPackageGraph(
+            fileSystem: fs,
+            manifests: [
+                Manifest.createRootManifest(
+                    displayName: "Pkg",
+                    path: "/Pkg",
+                    targets: [
+                        TargetDescription(
+                            name: "exe",
+                            settings: [.init(tool: .swift, kind: .enableExperimentalFeature("Embedded"))]
+                        ),
+                    ]
+                ),
+            ],
+            observabilityScope: observability.topScope
+        )
+        XCTAssertNoDiagnostics(observability.diagnostics)
+
+        let id = ResolvedProduct.ID(targetName: "exe", packageIdentity: .plain("pkg"), buildTriple: .destination)
+        let package = try XCTUnwrap(graph.rootPackages.first)
+        let product = try XCTUnwrap(graph.allProducts[id])
+
+        let buildDescription = try ProductBuildDescription(
+            package: package,
+            product: product,
+            toolsVersion: .v5_9,
+            buildParameters: mockBuildParameters(environment: .init(platform: .macOS)),
+            fileSystem: fs,
+            observabilityScope: observability.topScope
+        )
+
+        XCTAssertTrue(
+            try buildDescription.linkArguments()
+                .joined(separator: " ")
+                .contains("-enable-experimental-feature Embedded")
+        )
+    }
+}