[clangd] Set correct argv[0] for clangd to fix c++ header search

Clang looks for the C++ standard headers relative to the clang binary
that we provide in the compilation database. So pass through a real path
so that we get the correct headers.

Author: benlangmuir

Sources/SourceKit/SourceKitServer.swift

@@ -556,8 +556,8 @@ public func languageService(
   switch language {
 
   case .c, .cpp, .objective_c, .objective_cpp:
-    guard let clangd = toolchain.clangd else { return nil }
-    return try makeJSONRPCClangServer(client: client, clangd: clangd, buildSettings: (client as? SourceKitServer)?.workspace?.buildSettings)
+    guard toolchain.clangd != nil else { return nil }
+    return try makeJSONRPCClangServer(client: client, toolchain: toolchain, buildSettings: (client as? SourceKitServer)?.workspace?.buildSettings)
 
   case .swift:
     guard let sourcekitd = toolchain.sourcekitd else { return nil }

Sources/SourceKit/clangd/ClangLanguageServer.swift

@@ -26,11 +26,14 @@ final class ClangLanguageServerShim: LanguageServer {
 
   let buildSystem: BuildSystem
 
+  let clang: AbsolutePath?
+
   /// Creates a language server for the given client using the sourcekitd dylib at the specified path.
-  public init(client: Connection, clangd: Connection, buildSystem: BuildSystem) throws {
+  public init(client: Connection, clangd: Connection, buildSystem: BuildSystem, clang: AbsolutePath?) throws {
 
     self.clangd = clangd
     self.buildSystem = buildSystem
+    self.clang = clang
     super.init(client: client)
   }
 
@@ -106,7 +109,7 @@ extension ClangLanguageServerShim {
     if let settings = settings {
       clangd.send(DidChangeConfiguration(settings: .clangd(
         ClangWorkspaceSettings(
-          compilationDatabaseChanges: [url.path: ClangCompileCommand(settings)]))))
+          compilationDatabaseChanges: [url.path: ClangCompileCommand(settings, clang: clang)]))))
     }
 
     clangd.send(note.params)
@@ -121,7 +124,11 @@ extension ClangLanguageServerShim {
   }
 }
 
-func makeJSONRPCClangServer(client: MessageHandler, clangd: AbsolutePath, buildSettings: BuildSystem?) throws -> Connection {
+func makeJSONRPCClangServer(client: MessageHandler, toolchain: Toolchain, buildSettings: BuildSystem?) throws -> Connection {
+
+  guard let clangd = toolchain.clangd else {
+    preconditionFailure("missing clang from toolchain \(toolchain.identifier)")
+  }
 
   let clientToServer: Pipe = Pipe()
   let serverToClient: Pipe = Pipe()
@@ -137,8 +144,8 @@ func makeJSONRPCClangServer(client: MessageHandler, clangd: AbsolutePath, buildS
   let shim = try ClangLanguageServerShim(
     client: connectionToClient,
     clangd: connection,
-    buildSystem: buildSettings ?? BuildSystemList()
-  )
+    buildSystem: buildSettings ?? BuildSystemList(),
+    clang: toolchain.clang)
 
   connectionToShim.start(handler: shim)
   connectionToClient.start(handler: client)
@@ -172,10 +179,10 @@ func makeJSONRPCClangServer(client: MessageHandler, clangd: AbsolutePath, buildS
 }
 
 extension ClangCompileCommand {
-  init(_ settings: FileBuildSettings) {
+  init(_ settings: FileBuildSettings, clang: AbsolutePath?) {
     // Clang expects the first argument to be the program name, like argv.
     self.init(
-      compilationCommand: ["clang"] + settings.compilerArguments,
+      compilationCommand: [clang?.pathString ?? "clang"] + settings.compilerArguments,
       workingDirectory: settings.workingDirectory ?? "")
   }
 }

Tests/INPUTS/ClangStdHeaderCanary/project.json

@@ -0,0 +1,4 @@
+{
+  "clang_flags": ["-fno-modules", "-Wunused-variable"],
+  "sources": ["std_header_canary.cpp"]
+}

Tests/INPUTS/ClangStdHeaderCanary/std_header_canary.cpp

@@ -0,0 +1,9 @@
+// Note: tests generally should avoid including system headers
+// to keep them fast and portable. This test is specifically
+// ensuring clangd can find libc++ and builtin headers.
+
+#include <cstdint>
+
+void test() {
+  uint64_t /*unused_b*/b;
+}

Tests/SourceKitTests/LocalClangTests.swift

@@ -141,4 +141,33 @@ final class LocalClangTests: XCTestCase {
     let resp = try! sk.sendSync(FoldingRangeRequest(textDocument: TextDocumentIdentifier(url)))
     XCTAssertNil(resp)
   }
+
+
+  func testClangStdHeaderCanary() throws {
+    guard let ws = try staticSourceKitTibsWorkspace(name: "ClangStdHeaderCanary") else { return }
+    if ToolchainRegistry.shared.default?.clangd == nil { return }
+
+    let loc = ws.testLoc("unused_b")
+
+    let expectation = XCTestExpectation(description: "diagnostics")
+
+    ws.sk.handleNextNotification { (note: Notification<PublishDiagnostics>) in
+      XCTAssertEqual(note.params.diagnostics, [
+        Diagnostic(
+          range: Position(loc) ..< Position(line: loc.line - 1, utf16index: loc.column),
+          severity: .warning,
+          source: nil,
+          message: "Unused variable 'b'")
+      ])
+
+      expectation.fulfill()
+    }
+
+    try ws.openDocument(loc.url, language: .cpp)
+
+    let result = XCTWaiter.wait(for: [expectation], timeout: 15)
+    if result != .completed {
+      fatalError("error \(result) waiting for diagnostics notification")
+    }
+  }
 }