SWIFT_DRIVER_SWIFT_FRONTEND_EXEC Ignored by swift-build system

[rjmansfield]

When building with the Swift Build System (--build-system swiftbuild), the environment variable SWIFT_DRIVER_SWIFT_FRONTEND_EXEC is ignored. As a result, swift build always invokes the system toolchain’s swift-frontend instead of the custom frontend specified by the environment variable.

Steps to Reproduce

Create a simple Swift package:

mkdir -p /tmp/swiftbuild-env-test && cd /tmp/swiftbuild-env-test
swift package init --type executable --name EnvTest

Test 1: Native Build System (works)

cd /tmp/swiftbuild-env-test
rm -rf .build
# Use a custom swift-frontend and pass -Xswiftc -v to confirm
env SWIFT_DRIVER_SWIFT_FRONTEND_EXEC=/Users/ryan_mansfield/swift/build/Ninja-RelWithDebInfoAssert/swift-macosx-arm64/bin/swift-frontend \
  swift build -Xswiftc -v 2>&1 | grep "swift-frontend.*-frontend.*-c" | head -1

Result:

Shows the custom frontend being used:
/Users/ryan_mansfield/swift/build/Ninja-RelWithDebInfoAssert/swift-macosx-arm64/bin/swift-frontend -frontend -c -primary-file /private/tmp/swiftbuild-env-test/Sources/EnvTest/main.swift ...

Test 2: Swift Build System (doesn't work)

cd /tmp/swiftbuild-env-test
rm -rf .build

# Use the same custom frontend with the Swift Build system
env SWIFT_DRIVER_SWIFT_FRONTEND_EXEC=/Users/ryan_mansfield/swift/build/Ninja-RelWithDebInfoAssert/swift-macosx-arm64/bin/swift-frontend \
  swift build --build-system swiftbuild -Xswiftc -v 2>&1 | grep "swift-frontend.*-frontend.*-c" | head -1

Result:

The system toolchain frontend is used instead:

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-frontend -frontend -c -primary-file /private/tmp/swiftbuild-env-test/Sources/EnvTest/main.swift ...

Expected Behavior

swift build should respect the SWIFT_DRIVER_SWIFT_FRONTEND_EXEC environment variable and use the specified custom swift-frontend for all compilation tasks, regardless of the build system used.

Actual Behavior
When using the Swift Build System (--build-system swiftbuild), the SWIFT_DRIVER_SWIFT_FRONTEND_EXEC environment variable is ignored, and the system toolchain’s swift-frontend is always used.

[jakepetroules]

Would be good to know if we've actually documented this behavior anywhere. I feel we should start documenting environment variables for which we provide guarantees they actually do something.

[rjmansfield]

It's mentioned in the swift-driver README.md here https://github.com/swiftlang/swift-driver/blob/main/README.md?plain=1#L38