Moving project folder breaks semantic functionality until index-build folder is deleted

[rmecham]

Swift version

Swift version 6.1.2 (swift-6.1.2-RELEASE)

Platform

Ubuntu 24.04.2 LTS (running as devcontainer on macOS 15.5)

Editor

Visual Studio Code v1.101.2 with swiftlang.swift-vscode v2.6.1

Description

I've started a brand new Vapor / Fluent project in VSCode with a devcontainer. I've been unable to get sourcekit-lsp to work at all (as in, no autocomplete suggestions, error highlighting, etc.). I'm seeing the following errors in the Output tab in VSCode:

CreateUserMigration.swift:1:8: error: no such module 'Fluent'
import Fluent
       ^
sourcekit: [1:buildASTUnit: 0.0000] Compilation setup failed!!!
sourcekit: [1:failed: 0.0006] diagnostics failed: Loading the standard library failed
sourcekit: [1:buildASTUnit: 1.1802] Compilation setup failed!!!
sourcekit: [1:failed: 1.1804] diagnostics failed: Loading the standard library failed

I'm using the swift:6.1.2 Docker image as the base image for the devcontainer.

Steps to Reproduce

1. Start a new Vapor project using the Vapor toolbox CLI to start a new project from the standard Vapor template, including Fluent (and PostgreSQL support in Fluent).
2. Open that project in VSCode and use the built-in “Add Dev Container Configuration Files” command to add a Swift container to the project.
3. Re-open the project in the dev container, wait for the Swift plugin to fully initialize (including waiting for the background indexing to finish), and observe that no sourcekit-lsp features are working.
4. Open the Output tab, switch the pop-up menu to “SourceKit Language Server (6.1.2)” and observe the error messages.

Logging

No response

[rmecham]

After restarting things a couple of more times, I got this additional error message from the LSP:

<unknown>:0: error: PCH was compiled with module cache path '[...]', but the path is currently '[...]'

The paths listed were from an earlier project layout. That lead me to deleting the .build folder and sure enough things started working after that. I'm not sure why this particular error message didn't show up earlier. I suppose we can close this.

[ahoppen]

Ah, yes. I’ve seen that error before. I don’t think we’ve got an issue for this, so let’s keep this issue open. Moving your project folder shouldn’t cause semantic functionality to disappear. At the very least, we should give a more actionable error message.