Use mergeable symbols in embedded

Sources/Build/BuildDescription/SwiftModuleBuildDescription.swift

@@ -490,9 +490,19 @@ public final class SwiftModuleBuildDescription {
             args += ["-v"]
         }
 
-        // Enable batch mode whenever WMO is off.
-        if !self.useWholeModuleOptimization {
-            args += ["-enable-batch-mode"]
+        if self.useWholeModuleOptimization {
+            args.append("-whole-module-optimization")
+            args.append("-num-threads")
+            args.append(String(ProcessInfo.processInfo.activeProcessorCount))
+        } else {
+            args.append("-incremental")
+            args.append("-enable-batch-mode")
+        }
+
+        // Workaround for https://github.com/swiftlang/swift-package-manager/issues/8648
+        if self.useMergeableSymbols {
+            args.append("-Xfrontend")
+            args.append("-mergeable-symbols")
         }
 
         args += ["-serialize-diagnostics"]
@@ -779,14 +789,6 @@ public final class SwiftModuleBuildDescription {
             result.append(outputFileMapPath.pathString)
         }
 
-        if self.useWholeModuleOptimization {
-            result.append("-whole-module-optimization")
-            result.append("-num-threads")
-            result.append(String(ProcessInfo.processInfo.activeProcessorCount))
-        } else {
-            result.append("-incremental")
-        }
-
         result.append("-c")
         result.append(contentsOf: self.sources.map(\.pathString))
 
@@ -1044,6 +1046,12 @@ public final class SwiftModuleBuildDescription {
             return true
         }
     }
+
+    // Workaround for https://github.com/swiftlang/swift-package-manager/issues/8648
+    /// Whether to build Swift code with -Xfrontend -mergeable-symbols.
+    package var useMergeableSymbols: Bool {
+        return self.target.underlying.isEmbeddedSwiftTarget
+    }
 }
 
 extension SwiftModuleBuildDescription {

Tests/BuildTests/BuildPlanTests.swift

@@ -2240,17 +2240,61 @@ class BuildPlanTestCase: BuildSystemProviderTestCase {
         )
         XCTAssertNoDiagnostics(observability.diagnostics)
 
-        // WMO should always be on with Embedded
-        let plan = try await mockBuildPlan(
+        // -Xfrontend -mergeable symbols should be passed with Embedded
+        let result = try await BuildPlanResult(plan: mockBuildPlan(
             graph: graph,
             fileSystem: fs,
             observabilityScope: observability.topScope
+        ))
+        result.checkTargetsCount(1)
+
+        // Compile Swift Target
+        let aCompileArguments = try result.moduleBuildDescription(for: "A").swift().compileArguments()
+        let aCompileArgumentsPattern: [StringPattern] = ["-whole-module-optimization"]
+        let aCompileArgumentsNegativePattern: [StringPattern] = ["-wmo"]
+        XCTAssertMatch(aCompileArguments, aCompileArgumentsPattern)
+        XCTAssertNoMatch(aCompileArguments, aCompileArgumentsNegativePattern)
+    }
+
+    // Workaround for: https://github.com/swiftlang/swift-package-manager/issues/8648
+    func test_mergeableSymbols_enabledInEmbedded() async throws {
+        let Pkg: AbsolutePath = "/Pkg"
+        let fs: FileSystem = InMemoryFileSystem(
+            emptyFiles:
+                Pkg.appending(components: "Sources", "A", "A.swift").pathString
         )
 
-        let a = try BuildPlanResult(plan: plan)
-            .moduleBuildDescription(for: "A").swift().emitCommandLine()
-        XCTAssertMatch(a, ["-whole-module-optimization"])
-        XCTAssertNoMatch(a, ["-wmo"])
+        let observability = ObservabilitySystem.makeForTesting()
+        let graph = try loadModulesGraph(
+            fileSystem: fs,
+            manifests: [
+                Manifest.createRootManifest(
+                    displayName: "Pkg",
+                    path: .init(validating: Pkg.pathString),
+                    targets: [
+                        TargetDescription(
+                            name: "A",
+                            settings: [.init(tool: .swift, kind: .enableExperimentalFeature("Embedded"))]
+                        ),
+                    ]
+                ),
+            ],
+            observabilityScope: observability.topScope
+        )
+        XCTAssertNoDiagnostics(observability.diagnostics)
+
+        // -Xfrontend -mergeable symbols should be passed with Embedded
+        let result = try await BuildPlanResult(plan: mockBuildPlan(
+            graph: graph,
+            fileSystem: fs,
+            observabilityScope: observability.topScope
+        ))
+        result.checkTargetsCount(1)
+
+        // Compile Swift Target
+        let aCompileArguments = try result.moduleBuildDescription(for: "A").swift().compileArguments()
+        let aCompileArgumentsPattern: [StringPattern] = ["-Xfrontend", "-mergeable-symbols"]
+        XCTAssertMatch(aCompileArguments, aCompileArgumentsPattern)
     }
 
     func testREPLArguments() async throws {