No such module 'PackagePlugin' while editing package plugins

[tomwyr]

Swift version

swiftlang-6.0.2.1.2 clang-1600.0.26.4

Platform

macOS 15.3.2

Editor

Visual Studio Code with Swift extension 2.2.0

Description

I noticed that module resolution stopped working recently in a Swift plugin project I've been working on. Swift analyzer displays No such module 'PackagePlugin' and stops any further code syntax analysis (code completion and documentation for other modules work but both became slower).

I'm noticing the same issue after setting up a new empty plugin project:

// Package.swift
// swift-tools-version: 6.0

import PackageDescription // No such module 'PackageDescription'

let package = Package(
  name: "PrintMessage",
  platforms: [.macOS(.v14)],
  products: [
    .plugin(
      name: "PrintMessage",
      targets: ["PrintMessage"]
    )
  ],
  targets: [
    .plugin(
      name: "PrintMessage",
      capability: .command(
        intent: .custom(
          verb: "print-message",
          description: "Prints message"
        )
      )
    )
  ]
)

// Plugins/PrintMessage/Plugin.swift

import PackagePlugin // No such module 'PackagePlugin'

@main
struct PrintMessagePlugin: CommandPlugin {
  func performCommand(context: PluginContext, arguments: [String]) throws {
    print("Message")
  }
}

The plugin command runs without any issues but not being able to see syntax analysis in the editor severely decreases the ability to develop projects that include plugins.

Steps to Reproduce

No response

Logging

sourcekit-lsp-diagnose-2025-04-16T16-30-38+02-00.zip

Not sure if I should've attached the entire directory that the diagnose command produced. Let me know, if there's more logs data that'd be helpful.

[bnbarham]

Given the version, I suspect this is https://github.com/swiftlang/sourcekit-lsp/issues/1704. Would you be able to try 6.1/a recent nightly @tomwyr? If you still hit this, would you be able to enable extended logging (https://github.com/swiftlang/sourcekit-lsp/blob/main/Documentation/Enable%20Extended%20Logging.md) and attach another diagnose?

[tomwyr]

I tried 6.1 and nightly (6.2-dev) using swiftly to install the toolchain but with no luck.

Here's the lsp diagnose output logged for 6.2-dev: sourcekit-lsp-diagnose-2025-04-26T11-08-29+02-00.zip

[ahoppen]

The issue appears to be this FIXME

https://github.com/swiftlang/swift-package-manager/blob/e0afabb43ea2081ab276e5cef80d43872e62e8b7/Sources/SourceKitLSPAPI/PluginTargetBuildDescription.swift#L76

As far as I can tell, we use the compiler arguments for the package manifest for files in package plugins. And the package manifest compiler arguments don’t contain an include path to usr/lib/swift/pm/PluginAPI, which contains PackagePlugin.swiftmodule.