Error while building library with Windows: "lld-link: error: subsystem must be defined"

[cmcgee1024]

SwiftPM libraries are failing to build with the swiftbuild build system with this error when building on Windows x86.

Steps to reproduce:

• Clone the latest SwiftPM on the main branch
• Create a simple library package using the init function:

emptydir=$(mktemp -d)
(cd "$emptydir"; swift package init --type=library)

• Build the project with the swiftbuild build system

swift run swift-build --package-path="$emptydir" --build-system=swiftbuild

Expected Result:
The package should build without error on Windows x86

Actual Result:
The package fails to build with the linker error: "lld-link: error: subsystem must be defined"

[cmcgee1024]

This could be caused by the need for #104 , which causes the linker to be invoked to do "partial linking", which will attempt to link object files together, and then use the result of those as an object file to the linker in the next stage.

[dschaefer2]

Unless the library is dynamic, we shouldn't be linking here at all. We need to rethink how we're building targets on Windows.

[dschaefer2]

In the end it comes down to the lack of support for partial linking on Windows. We need to decide what the build output should be as a replacement.