IntelliSense for Swift in VSCode Fails When Project is Stored on BTRFS Volume in Windows 10

[ShikiSuen]

Description

I’ve observed that BTRFS on Windows 10 offers significantly higher read/write performance compared to NTFS, especially when compiling Swift Package projects. In my tests, it performs exceptionally well, even reducing unit test execution time by nearly half compared to NTFS. However, I’ve encountered an issue where VSCode’s IntelliSense for Swift stops working whenever the project is stored on a BTRFS volume. This issue does not occur when the same project is placed on an NTFS volume.

Steps to Reproduce

1. Set up a Windows 10 system with both NTFS and BTRFS volumes (e.g., using a BTRFS driver like WinBtrfs).
2. Install VSCode and the Swift extension (e.g., swift-server/vscode-swift).
3. Create or open a Swift Package project.
4. Place the project on a BTRFS volume and open it in VSCode.
5. Attempt to use IntelliSense (e.g., autocompletion, type hints) while editing Swift files.
6. Move the same project to an NTFS volume and repeat the process.

Expected Behavior

IntelliSense should work consistently for Swift files regardless of whether the project is stored on a BTRFS or NTFS volume.

Actual Behavior

When the project is on a BTRFS volume, IntelliSense fails to provide autocompletion, type information, or error highlighting. It works as expected when the project is on an NTFS volume.

Environment

• OS: Windows 10 22H2
• VSCode Version: The latest public release as of March 26, 2025 (1.98.2)
• Swift Extension Version: The latest public release as of March 26, 2025 (2.0.2)
• Swift Version: 6.0.3 public stable release.
• BTRFS Driver: 1.9

Additional Information

The issue seems specific to BTRFS volumes; NTFS works fine.

Compilation and unit testing performance on BTRFS is noticeably better, suggesting the filesystem itself is functional and efficient.

Restarting VSCode, reinstalling the Swift extension, or resetting the IntelliSense database does not resolve the issue.

Who to Contact?

I’m unsure whether this is an issue with the Swift extension, VSCode’s filesystem handling, or an interaction with the BTRFS driver on Windows. Could the maintainers please advise if this should also be reported to the VSCode team (e.g., microsoft/vscode) or the BTRFS driver maintainers?

Request

I’d appreciate any guidance on troubleshooting this further or confirmation if this is a known limitation. If possible, a fix or workaround would be fantastic, as BTRFS’s performance benefits are significant for Swift development on Windows.
Thank you!

[matthewbastien]

Thank you for the bug report @ShikiSuen!

Can you reproduce the issue by trying to use some language features, run the VS Code command Swift: Capture VS Code Swift Diagnostic Bundle, and attach it to this issue? It might contain more information as to why the language features aren't working.

Please note that these bundles can contain sensitive information. So, please review it before uploading!

[ShikiSuen]

@matthewbastien Thanks for your response.

I reformatted a new BTRFS volume trying to repeat this issue but failed. It seems that I have to wait until the background indexing process gets accomplished.

It worths mentioning that the background indexing process on BTRFS volume is extremely slow.

vscode-diagnostics-2025-03-27T11-53-09+08-00.zip

[matthewbastien]

Thanks for the logs! Like you said it does appear to be functioning now.

Does disabling background indexing by setting "swift.sourcekit-lsp.backgroundIndexing" to "off" fix the performance issues you're seeing? You'll have to make sure that you run a build before SourceKit-LSP will provide language features in that mode.

[ShikiSuen]

@matthewbastien I do wonder whether running swift build in VSCode built-in terminal works, because I don't know whether the GUI button is for building a Swift package project. Till now the only GUI approach I used for triggering a build is to run a unit test.

(I'll check the background indexing toggle later and will respond in another reply.)

[matthewbastien]

I believe just running a swift build on the command line should be sufficient. You can also trigger a build in VS Code by using the tasks automatically generated by the Swift extension. The shortcut in VS Code to start a build task is CMD + Shift + B on macOS and Ctrl + Shift + B on Linux and Windows. That'll pop up an input box where you can select which build you want to run. There should be one to build everything and additional ones to build only specific targets from your Package.swift.

[ShikiSuen]

Does disabling background indexing by setting "swift.sourcekit-lsp.backgroundIndexing" to "off" fix the performance issues you're seeing? You'll have to make sure that you run a build before SourceKit-LSP will provide language features in that mode.

Thanks. This action did suppress the issue.

[matthewbastien]

Glad to hear you're unblocked for now.

I'm moving this issue to SourceKit-LSP as it appears to be an issue with background indexing performance.

[ahoppen]

Synced to Apple’s issue tracker as rdar://148251414

[ShikiSuen]

@matthewbastien Thanks for moving this issue to the correct repo.

For this Windows-specific issue, I suggest testing the indexing performance for both BTRFS and APFS volumes.

• BTRFS on Windows can be supported by WinBTRFS.
• APFS on Windows can be supported by Paragon APFS. They also have APFS driver for Linux.