Explicit Module Builds interop with vfsoverlays

[chiragramani]

We have an existing Xcode project where we make use of vfsoverlays. These overlays have been working well in our previous setups. However, after enabling the Explicit Module Build feature in Xcode 16, our builds start to fail.

It seems like the VFS overlays, although configured correctly and pointing to valid files, are not being honored during the build process. We are unsure if this is a configuration issue or a limitation with the new explicit module build system.

Any insights or suggestions?

[chiragramani]

cc: @artemcm

[chiragramani]

Hi @artemcm, @compnerd, @cachemeifyoucan, could you assist us with this?

[artemcm]

@chiragramani would you be able to provide a reduced sample project that demonstrates the issue?

[chiragramani]

@chiragramani would you be able to provide a reduced sample project that demonstrates the issue?

Sure, I’ll try to share it by tomorrow.

[chiragramani]

ReproBase.zip

Hey @artemcm,
I’ve prepared a reproducible project to demonstrate the issue. Please follow these steps:

1. Unarchive the file and set the extracted folder as your current directory.
2. Run the setup script:

   ./setup.sh

   This script updates the template variables with the absolute paths required for the project to work.
3. Open the project CorePrebuild/CorePrebuild.xcodeproj in Xcode and build the target. This is the unfocused target.
4. Next, open FocussedProj/FocussedProj.xcodeproj in Xcode and build the target. This is the focused project, which uses an overlay pointing to CorePrebuild.swiftmodule (externally built in step 3).
5. In the default setup, which does not use the Explicitly Built Modules feature (as controlled in the .xcconfig file), everything builds successfully.
6. However, when the Explicitly Built Modules feature is enabled in the .xcconfig file, the build fails with the following error:

   Build input file cannot be found: 
   .../FocussedProj/FocussedProj/includes/FocussedProj/CorePrebuild.swiftmodule/arm64-apple-ios-simulator.swiftmodule. 
   Did you forget to declare this file as an output of a script phase or custom build rule which produces it?
   

Let me know if further clarification or tweaks to the setup are needed!