Hi!
When running a given version of the swift frontend (say 5.7.1) on top of another version of the standard library (say 5.7) by using -resource-dir I have seen a different behaviour between Linux and macOS. The first time this is done on macOS the compiler will use the .swiftinterface files and compile the standard library, outputting the following remark:
<unknown>:0: remark: did not find a prebuilt standard library for target 'x86_64-apple-macos' compatible with this Swift compiler; building it may take a few minutes, but it should only happen once for this combination of compiler and target
However running something like
/opt/swift-5.7.1-RELEASE-ubuntu22.04/usr/bin/swift-frontend -c /dev/null -resource-dir /opt/swift-5.7-RELEASE-ubuntu22.04/usr/lib/swift
will instead result in
<unknown>:0: error: module compiled with Swift 5.7 cannot be imported by the Swift 5.7.1 compiler: /opt/swift-5.7-RELEASE-ubuntu22.04/usr/lib/swift/linux/Swift.swiftmodule/x86_64-unknown-linux-gnu.swiftmodule
even though the corresponding .swiftinterface is there (and the frontend would be able to compile it as can be tested with swift-frontend -compile-module-from-interface /opt/swift-5.7-RELEASE-ubuntu22.04/usr/lib/swift/linux/Swift.swiftmodule/x86_64-unknown-linux-gnu.swiftinterface -module-name Swift -parse-stdlib).
Is there any reason why automatic lazy compilation of the standard library does not happen on Linux?
We are building codeql support for Swift, and we built the solution on top of the swift frontend, and we rely on this auto-compilation to be able to ship one version of codeql able to analyse builds with older versions of swift, without having to ship any additional resources. Is there some magic command line flag I should provide to make it work? Did I hit a bug on the Linux version of the frontend?