-Xcc flags should be forwarded to swift compilation via build request overrides

Sources/SwiftBuildSupport/SwiftBuildSystem.swift

@@ -908,7 +908,16 @@ public final class SwiftBuildSystem: SPMBuildCore.BuildSystem {
             }
         }
 
-        let swiftCompilerFlags = buildParameters.toolchain.extraFlags.swiftCompilerFlags + buildParameters.flags.swiftCompilerFlags
+        var swiftCompilerFlags = buildParameters.toolchain.extraFlags.swiftCompilerFlags + buildParameters.flags.swiftCompilerFlags
+        swiftCompilerFlags += buildParameters.toolchain.extraFlags.cCompilerFlags.asSwiftcCCompilerFlags()
+        // User arguments (from -Xcc) should follow generated arguments to allow user overrides
+        swiftCompilerFlags += buildParameters.flags.cCompilerFlags.asSwiftcCCompilerFlags()
+
+        // TODO: Pass -Xcxx flags to swiftc (#6491)
+        // Uncomment when downstream support arrives.
+        // swiftCompilerFlags += buildParameters.toolchain.extraFlags.cxxCompilerFlags.rawFlags.asSwiftcCXXCompilerFlags()
+        // // User arguments (from -Xcxx) should follow generated arguments to allow user overrides
+        // swiftCompilerFlags += buildParameters.flags.cxxCompilerFlags.rawFlags.asSwiftcCXXCompilerFlags()
         let compilerAndLinkerFlags = [
             "OTHER_CFLAGS": buildParameters.toolchain.extraFlags.cCompilerFlags + buildParameters.flags.cCompilerFlags,
             "OTHER_CPLUSPLUSFLAGS": buildParameters.toolchain.extraFlags.cxxCompilerFlags + buildParameters.flags.cxxCompilerFlags,
@@ -1360,4 +1369,10 @@ fileprivate extension [BuildFlag] {
             }
         }.map { $0.value }
     }
+
+    /// Converts a set of C compiler flags into an equivalent set to be
+    /// indirected through the Swift compiler instead.
+    func asSwiftcCCompilerFlags() -> [BuildFlag] {
+        self.flatMap { [BuildFlag(value: "-Xcc", source: $0.source), $0] }
+    }
 }

Tests/SwiftBuildSupportTests/SwiftBuildSystemTests.swift

@@ -399,6 +399,32 @@ struct SwiftBuildSystemTests {
         }
     }
 
+    @Test
+    func cFlagsAppliedToSwiftInBuildRequest() async throws {
+        try await withTemporaryDirectory { tempDir in
+            try await withInstantiatedSwiftBuildSystem(
+                fromFixture: "PIFBuilder/Simple",
+                buildParameters: mockBuildParameters(
+                    destination: .host,
+                    flags: .init(cCompilerFlags: [BuildFlag(value: "-DFoo", source: .commandLineOptions)]),
+                    buildSystemKind: .swiftbuild
+                ),
+            ) { swiftBuild, service, session, observabilityScope, buildParameters in
+                let buildRequest = try await swiftBuild.makeBuildRequest(
+                    service: service,
+                    session: session,
+                    configuredTargets: [],
+                    derivedDataPath: tempDir,
+                    symbolGraphOptions: nil,
+                    setToolchainSetting: false
+                )
+
+                #expect(buildRequest.parameters.overrides.synthesized?.table["OTHER_CFLAGS"]?.contains("-DFoo") == true)
+                #expect(buildRequest.parameters.overrides.synthesized?.table["OTHER_SWIFT_FLAGS"]?.contains("-Xcc -DFoo") == true)
+            }
+        }
+    }
+
     @Suite
     struct DebuggingSettingsTests {
 

Tests/SwiftPMBuildServerTests/BuildServerTests.swift

@@ -196,5 +196,21 @@ struct SwiftPMBuildServerTests {
             }
         }
     }
+
+    @Test
+    func extraSwiftcArgs() async throws {
+        try await withSwiftPMBSP(fixtureName: "Miscellaneous/Simple", extraBSPArgs: ["-Xswiftc", "-DFoo", "-Xcc", "-DBar"]) { connection, _, _ in
+            let targetResponse = try await connection.send(WorkspaceBuildTargetsRequest())
+            let fooID = try #require(targetResponse.targets.first(where: { $0.displayName == "Foo" })).id
+            let sourcesResponse = try await connection.send(BuildTargetSourcesRequest(targets: [fooID]))
+            let item = try #require(sourcesResponse.items.only?.sources.only)
+
+            _ = try await connection.send(BuildTargetPrepareRequest(targets: [fooID]))
+
+            let settingsResponse = try #require(try await connection.send(TextDocumentSourceKitOptionsRequest(textDocument: TextDocumentIdentifier(item.uri), target: fooID, language: .swift)))
+            #expect(settingsResponse.compilerArguments.contains("-DFoo"))
+            #expect(settingsResponse.compilerArguments.contains(["-Xcc", "-DBar"]))
+        }
+    }
 }
 #endif