Compute correct compiler arguments for build plugins

[ahoppen]

The computation of compiler arguments for package plugins was plain out broken and didn’t even include the search path to lib/swift/pm/PluginAPI. Call into the same logic with which we compute the build settings during building from SourceKitLSPAPI.

Tested as part of swiftlang/sourcekit-lsp#2446.

Fixes swiftlang/sourcekit-lsp#2115

[ahoppen]

swiftlang/sourcekit-lsp#2446

@swift-ci Please test

[ahoppen]

swiftlang/sourcekit-lsp#2446

@swift-ci Please test

[ahoppen]

swiftlang/sourcekit-lsp#2446

@swift-ci Please test

[jakepetroules]

What about Swift Build? Is it already correct when using that backend?

[ahoppen]

I haven’t been able to test it. The following test in SourceKit-LSP fails with issues from the BSP server. Regardless, I think it’s worth fixing this in the native build system and having a test for it that will catch any similar issues as we qualify the swiftbuild build system for SourceKit-LSP. So, I don’t think that fixing this in swiftbuild should be a blocker for this PR anyway.

Details

  func testPackagePlugin() async throws {
    var options = try await SourceKitLSPOptions.testDefault()
    options.swiftPMOrDefault.buildSystem = .swiftbuild
    let project = try await SwiftPMTestProject(
      files: [
        "Test.swift": "",
        "Plugins/PrintMessage/Plugin.swift": """
        import PackagePlugin

        @main
        struct PrintMessagePlugin: CommandPlugin {
          func performCommand(context: PluginContext, arguments: [String]) throws {
            print("Message")
            let x: String = 1
          }
        }
        """,
      ],
      manifest: """
        import PackageDescription

        let package = Package(
          name: "PrintMessage",
          platforms: [.macOS(.v14)],
          products: [
            .plugin(
              name: "PrintMessage",
              targets: ["PrintMessage"]
            )
          ],
          targets: [
            .target(name: "MyLibrary"),
            .plugin(
              name: "PrintMessage",
              capability: .command(
                intent: .custom(
                  verb: "print-message",
                  description: "Prints message"
                )
              )
            )
          ]
        )
        """,
      options: options
    )
    let (uri, _) = try! project.openDocument("Plugin.swift")
    let diags = try await project.testClient.send(DocumentDiagnosticsRequest(textDocument: TextDocumentIdentifier(uri)))
    XCTAssertEqual(
      diags.fullReport?.items.map(\.message),
      ["Cannot convert value of type 'Int' to specified type 'String'"]
    )
  }

[org.swift.sourcekit-lsp.tests:default] info 2026-03-23 12:05:48.0630 +0100
Sending notification to client
window/logMessage
{
  "logName" : "SourceKit-LSP: Indexing",
  "message" : "Error generating build description: No such file or directory (2)",
  "type" : 4
}
…
[org.swift.sourcekit-lsp.tests:default] info 2026-03-23 12:05:48.0630 +0100
Sending notification to client
window/logMessage
{
  "logName" : "SourceKit-LSP: Indexing",
  "message" : "Computing targets list",
  "type" : 4
}
...
[org.swift.sourcekit-lsp.tests:default] error 2026-03-23 12:05:48.0650 +0100
Received error for request "sk-2" from BSP-Server
workspace/buildTargets
ResponseError(code: LanguageServerProtocol.ErrorCode(rawValue: -32001), message: "No build description", data: nil)

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test Windows

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test Windows