[Swift Build] Wire up --jobs

Sources/SwiftBuildSupport/SwiftBuildSystem.swift

@@ -1072,16 +1072,27 @@ public final class SwiftBuildSystem: SPMBuildCore.BuildSystem {
         return params
     }
 
-    public func makeBuildRequest(session: SWBBuildServiceSession, configuredTargets: [SWBTargetGUID], derivedDataPath: Basics.AbsolutePath, symbolGraphOptions: BuildOutput.SymbolGraphOptions?) async throws -> SWBBuildRequest {
+    public func makeBuildRequest(
+        session: SWBBuildServiceSession,
+        configuredTargets: [SWBTargetGUID],
+        derivedDataPath: Basics.AbsolutePath,
+        symbolGraphOptions: BuildOutput.SymbolGraphOptions?,
+        setToolchainSetting: Bool = true,
+        ) async throws -> SWBBuildRequest {
         var request = SWBBuildRequest()
-        request.parameters = try await makeBuildParameters(session: session, symbolGraphOptions: symbolGraphOptions)
+        request.parameters = try await makeBuildParameters(
+            session: session,
+            symbolGraphOptions: symbolGraphOptions,
+            setToolchainSetting: setToolchainSetting,
+        )
         request.configuredTargets = configuredTargets.map { SWBConfiguredTarget(guid: $0.rawValue, parameters: request.parameters) }
         request.useParallelTargets = true
         request.useImplicitDependencies = false
         request.useDryRun = false
         request.hideShellScriptEnvironment = true
         request.showNonLoggedProgress = true
         request.recordBuildBacktraces = buildParameters.outputParameters.enableTaskBacktraces
+        request.schedulerLaneWidthOverride = buildParameters.workers
 
         // Override the arena. We need to apply the arena info to both the request-global build
         // parameters as well as the target-specific build parameters, since they may have been

Sources/_InternalTestSupport/MockBuildTestHelper.swift

@@ -98,6 +98,7 @@ public func mockBuildParameters(
     enableXCFrameworksOnLinux: Bool = false,
     prepareForIndexing: BuildParameters.PrepareForIndexingMode = .off,
     sanitizers: [Sanitizer] = [],
+    numberOfWorkers: UInt32 = 3,
 ) -> BuildParameters {
     try! BuildParameters(
         destination: destination,
@@ -108,7 +109,7 @@ public func mockBuildParameters(
         flags: flags,
         buildSystemKind: buildSystemKind,
         pkgConfigDirectories: [],
-        workers: 3,
+        workers: numberOfWorkers,
         sanitizers: EnabledSanitizers(Set(sanitizers)),
         indexStoreMode: indexStoreMode,
         prepareForIndexing: prepareForIndexing,

Tests/SwiftBuildSupportTests/SwiftBuildSystemTests.swift

@@ -269,4 +269,36 @@ struct SwiftBuildSystemTests {
             )
        }
     }
+
+    @Test(
+        arguments: [
+            0,
+            1,
+            2,
+            10,
+        ],
+    )
+    func numberOfWorkersBuildParameterSetsTheExpectedSwiftBuildRequest(
+        expectedNumberOfWorkers: UInt32,
+    ) async throws {
+        try await withTemporaryDirectory { tempDir in
+            try await withInstantiatedSwiftBuildSystem(
+                fromFixture: "PIFBuilder/Simple",
+                buildParameters: mockBuildParameters(
+                    destination: .host,
+                    numberOfWorkers: expectedNumberOfWorkers,
+                ),
+            ) { swiftBuild, session, observabilityScope, buildParameters in
+                let buildRequest = try await swiftBuild.makeBuildRequest(
+                    session: session,
+                    configuredTargets: [],
+                    derivedDataPath: tempDir,
+                    symbolGraphOptions: nil,
+                    setToolchainSetting: false
+                )
+
+                #expect(buildRequest.schedulerLaneWidthOverride == expectedNumberOfWorkers)
+            }
+        }
+    }
 }