[Build-Script]: Extend Swift cmake options

[etcwilde]

The existing swift-cmake-options flag overwrites all flags computed by build-script. Sometimes it is useful to be able to append additional CMake flags without overwriting the existing flags. This patch adds --extra-swift-cmake-options that adds the specified flags to the Swift CMake configuration instead of overwriting them.

This also adds a similar --extra-llvm-cmake-options, which adds the new flags to the end, allowing one to replace and overwrite CMake flags that build-script computed.
Due to the parameter passing mechanisms in build-script-impl, while this behavior would be useful for Swift, it is not immediately apparent how one would best implement this at this time.

[etcwilde]

@AnthonyLatsis, this should help simplify #81284.
You should be able to avoid the SWIFT_CXX_FLAGS hack and use --extra-swift-cmake-options.

[etcwilde]

@swift-ci please test

[etcwilde]

preset=bootstrap_stage0
@swift-ci please test with preset Linux Platform

[etcwilde]

Bootstrap preset failed with unrelated failure:

[2025-05-07T20:28:09.097Z] [./swift/utils/build-script] ERROR: no value found for build_subdir in "%(build_subdir)s"

[etcwilde]

@swift-ci please test

[AnthonyLatsis]

Awesome, thank you!

The existing swift-cmake-options flag overwrites all flags computed by build-script

Where does this happen? They appear to be added to the front here:

swift/utils/build-script-impl

Lines 1604 to 1609 in 78234e2

	# Add any specific cmake options specified by build-script
	product_cmake_options_name=$(to_varname "${product%%_*}")_CMAKE_OPTIONS
	product_cmake_options=(${!product_cmake_options_name}) # convert to array
	cmake_options+=("${product_cmake_options[@]}")

[etcwilde]

Some of the options are computed in swift.py inside of the Python bits of build-script. Those are passed to build-script-impl via the --swift-cmake-options flag. When someone passes --swift-cmake-options explicitly, build-script will pass --swift-cmake-options to build-script-impl twice. Once with the flags that it computed, and once again with the flags that you set, rather than merging them.

build-script-impl only picks up the last instance of a flag, so that's the one we see.

e.g

./utils/build-script ... --enable-experimental-concurrency=1 --swift-cmake-options='-DI_AM_A_TEAPOT=YES'

results in a build-script-impl invocation with the following flags:

'--workspace', '/Users/ewilde/Work/Swift-Project', '--build-dir', ... '--swift-cmake-options=-DCMAKE_EXPORT_COMPILE_COMMANDS=TRUE ... -DSWIFT_ENABLE_EXPERIMENTAL_CONCURRENCY:BOOL=TRUE ...', ... '--swift-cmake-options=-DI_AM_A_TEAPOT=YES' ...

While I don't mind the idea of fixing that so that swift.py merges all of the flags and build-script (python) doesn't pass --swift-cmake-options again, I don't have time to fix the fallout from the resulting behavior change at the moment.

[AnthonyLatsis]

I see, thanks for clarifying.

[edymtt]

Thanks a lot for this improvement! Looking forward to adopt it in a fix I need for the compiler-rt logic.