Swift 6.1/6.2 can not cross-compile executables with the latest static linux Musl SDK bundle snapshots

[finagolfin]

I was just testing out cross-compilation of Swift packages that use macros, and it works fine with the latest 6.0 release for the first time:

> cd swift-syntax/Examples
> ~/swift-6.0.2-RELEASE-fedora39/usr/bin/swift sdk install ~/swift-6.0.2-RELEASE_static-linux-0.0.1.artifactbundle.tar.gz
Swift SDK bundle at `swift-6.0.2-RELEASE_static-linux-0.0.1.artifactbundle.tar.gz` is assumed to be an archive, unpacking...
Swift SDK bundle at `swift-6.0.2-RELEASE_static-linux-0.0.1.artifactbundle.tar.gz` successfully installed as swift-6.0.2-RELEASE_static-linux-0.0.1.artifactbundle.
> ~/swift-6.0.2-RELEASE-fedora39/usr/bin/swift build --swift-sdk aarch64-swift-linux-musl
Building for debugging...
Build complete! (1.04s)

However, if I try the same with the latest trunk 6.1 Musl or Android SDK bundles, it fails:

> ~/swift-6.0.2-RELEASE-fedora39/usr/bin/swift sdk remove swift-6.0.2-RELEASE_static-linux-0.0.1
Swift SDK bundle at path `/home/finagolfin/.swiftpm/swift-sdks/swift-6.0.2-RELEASE_static-linux-0.0.1.artifactbundle` was successfully removed from the file system.
> ~/swift-6.0.2-RELEASE-fedora39/usr/bin/swift package clean
> ~/swift-DEVELOPMENT-SNAPSHOT-2024-09-25-a-ubi9/usr/bin/swift sdk install ~/swift-DEVELOPMENT-SNAPSHOT-2024-09-25-a_static-linux-0.0.1.artifactbundle.tar.gz
Swift SDK bundle at `/home/finagolfin/swift-DEVELOPMENT-SNAPSHOT-2024-09-25-a_static-linux-0.0.1.artifactbundle.tar.gz` is assumed to be an archive, unpacking...
Swift SDK bundle at `/home/finagolfin/swift-DEVELOPMENT-SNAPSHOT-2024-09-25-a_static-linux-0.0.1.artifactbundle.tar.gz` successfully installed as swift-DEVELOPMENT-SNAPSHOT-2024-09-25-a_static-linux-0.0.1.artifactbundle.
> ~/swift-DEVELOPMENT-SNAPSHOT-2024-09-25-a-ubi9/usr/bin/swift build --swift-sdk aarch64-swift-linux-musl                  
Building for debugging...
error: link command failed with exit code 1 (use -v to see invocation)
clang: error: no such file or directory: '/home/finagolfin/.swiftpm/swift-sdks/swift-DEVELOPMENT-SNAPSHOT-2024-09-25-a_static-linux-0.0.1.artifactbundle/swift-DEVELOPMENT-SNAPSHOT-2024-09-25-a_static-linux-0.0.1/swift-linux-musl/musl-1.2.5.sdk/aarch64/usr/lib/swift/linux-static/aarch64/swiftrt.o'

If I pass in -Xswiftc -disallow-use-new-driver, it works again, so clearly a regression in this swift-driver, most likely related to #1667, which I argued against at the time.

@al45tair, can you confirm this regression and advise?

[finagolfin]

I looked into this a bit more: the problem is still reproducible with the latest dev snapshots of the static Musl SDK for 6.1 and trunk, from Jan. 31 and Jan. 5 respectively. Turns out the 6.1 and trunk Android SDK bundles have a different error caused by another bug, which I will address separately.

This Musl SDK problem is actually different and larger, breaking building all executables with the static Musl SDK. As noted above, #1667 is the culprit, as it assumes that there is always a swiftrt.o in the shared Swift resource directory, which is not the case for the static Musl SDK. It's an easy fix, but I have a larger fix in mind to get rid of that swiftrt.o linking in this driver altogether, so will submit that instead.