Enable index store based on Clang feature detection

Sources/Build/BuildDescription/ClangTargetBuildDescription.swift

@@ -225,16 +225,11 @@ public final class ClangTargetBuildDescription {
         args += activeCompilationConditions
         args += ["-fblocks"]
 
-        let buildTriple = self.buildParameters.triple
         // Enable index store, if appropriate.
-        //
-        // This feature is not widely available in OSS clang. So, we only enable
-        // index store for Apple's clang or if explicitly asked to.
-        if ProcessEnv.vars.keys.contains("SWIFTPM_ENABLE_CLANG_INDEX_STORE") {
-            args += self.buildParameters.indexStoreArguments(for: target)
-        } else if buildTriple.isDarwin(),
-                  (try? self.buildParameters.toolchain._isClangCompilerVendorApple()) == true
-        {
+        if let supported = try? ClangSupport.supportsFeature(
+            name: "index-unit-output-path",
+            toolchain: self.buildParameters.toolchain
+        ), supported {
             args += self.buildParameters.indexStoreArguments(for: target)
         }
 

Sources/Build/CMakeLists.txt

@@ -24,6 +24,7 @@ add_library(Build
   BuildPlan/BuildPlan+Product.swift
   BuildPlan/BuildPlan+Swift.swift
   BuildPlan/BuildPlan+Test.swift
+  ClangSupport.swift
   SwiftCompilerOutputParser.swift
   TestObservation.swift)
 target_link_libraries(Build PUBLIC

Sources/Build/ClangSupport.swift

@@ -0,0 +1,41 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift open source project
+//
+// Copyright (c) 2024 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
+//
+//===----------------------------------------------------------------------===//
+
+import Basics
+import Foundation
+import PackageModel
+
+public enum ClangSupport {
+    private struct Feature: Decodable {
+        let name: String
+        let value: [String]?
+    }
+
+    private struct Features: Decodable {
+        let features: [Feature]
+    }
+
+    private static var cachedFeatures = ThreadSafeBox<Features>()
+
+    public static func supportsFeature(name: String, toolchain: PackageModel.Toolchain) throws -> Bool {
+        let features = try cachedFeatures.memoize {
+            let clangPath = try toolchain.getClangCompiler()
+            let featuresPath = clangPath.parentDirectory.parentDirectory.appending(components: ["share", "clang", "features.json"])
+            return try JSONDecoder.makeWithDefaults().decode(
+                path: featuresPath,
+                fileSystem: localFileSystem,
+                as: Features.self
+            )
+        }
+        return features.features.first(where: { $0.name == name }) != nil
+    }
+}

Tests/BuildTests/BuildPlanTests.swift

@@ -3613,7 +3613,11 @@ final class BuildPlanTests: XCTestCase {
 
         func check(for mode: BuildParameters.IndexStoreMode, config: BuildConfiguration) throws {
             let result = try BuildPlanResult(plan: BuildPlan(
-                buildParameters: mockBuildParameters(config: config, indexStoreMode: mode),
+                buildParameters: mockBuildParameters(
+                    config: config,
+                    toolchain: try UserToolchain.default,
+                    indexStoreMode: mode
+                ),
                 graph: graph,
                 fileSystem: fs,
                 observabilityScope: observability.topScope
@@ -3622,17 +3626,10 @@ final class BuildPlanTests: XCTestCase {
             let lib = try result.target(for: "lib").clangTarget()
             let path = StringPattern.equal(result.plan.destinationBuildParameters.indexStore.pathString)
 
-            #if os(macOS)
             XCTAssertMatch(
                 try lib.basicArguments(isCXX: false),
                 [.anySequence, "-index-store-path", path, .anySequence]
             )
-            #else
-            XCTAssertNoMatch(
-                try lib.basicArguments(isCXX: false),
-                [.anySequence, "-index-store-path", path, .anySequence]
-            )
-            #endif
 
             let exe = try result.target(for: "exe").swiftTarget().compileArguments()
             XCTAssertMatch(exe, [.anySequence, "-index-store-path", path, .anySequence])

Tests/BuildTests/ClangTargetBuildDescriptionTests.swift

@@ -0,0 +1,63 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift open source project
+//
+// Copyright (c) 2024 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
+//
+//===----------------------------------------------------------------------===//
+
+import Basics
+@testable import Build
+import PackageGraph
+import PackageModel
+import SPMTestSupport
+import XCTest
+
+final class ClangTargetBuildDescriptionTests: XCTestCase {
+    func testClangIndexStorePath() throws {
+        let targetDescription = try makeTargetBuildDescription()
+        XCTAssertTrue(try targetDescription.basicArguments().contains("-index-store-path"))
+    }
+
+    private func makeClangTarget() throws -> ClangTarget {
+        try ClangTarget(
+            name: "dummy",
+            cLanguageStandard: nil,
+            cxxLanguageStandard: nil,
+            includeDir: .root,
+            moduleMapType: .none,
+            type: .library,
+            path: .root,
+            sources: .init(paths: [.root.appending(component: "foo.c")], root: .root),
+            usesUnsafeFlags: false
+        )
+    }
+
+    private func makeResolvedTarget() throws -> ResolvedTarget {
+        ResolvedTarget(
+            packageIdentity: .plain("dummy"),
+            underlying: try makeClangTarget(),
+            dependencies: [],
+            supportedPlatforms: [],
+            platformVersionProvider: .init(implementation: .minimumDeploymentTargetDefault)
+        )
+    }
+
+    private func makeTargetBuildDescription() throws -> ClangTargetBuildDescription {
+        let observability = ObservabilitySystem.makeForTesting(verbose: false)
+        return try ClangTargetBuildDescription(
+            target: try makeResolvedTarget(),
+            toolsVersion: .current,
+            buildParameters: mockBuildParameters(
+                toolchain: try UserToolchain.default,
+                indexStoreMode: .on
+            ),
+            fileSystem: localFileSystem,
+            observabilityScope: observability.topScope
+        )
+    }
+}