Enabling prebuilts for custom macros confuses `--explicit-target-dependency-import-check=error`

[pyrtsa]

Is it reproducible with SwiftPM command-line tools: swift build, swift test, swift package etc?

• Confirmed reproduction steps with SwiftPM CLI. The description text must include reproduction steps with either of command-line SwiftPM commands, swift build, swift test, swift package etc.

Description

Running swift build --explicit-target-dependency-import-check=error with the new --enable-experimental-prebuilts fails checking dependencies for macro targets in projects which use custom macros such as swiftlang/swift-testing or when depending on another project introducing custom macros such as pointfreeco/swift-dependencies.

Expected behavior

The project, provided that it only imports modules which are also included in the target dependency graph of the package file, should build with no errors, just like it does when omitting the --explicit-target-dependency-import-check option.

Actual behavior

The build produces errors from the dependency check:

error: Target TestingMacros imports another target (SwiftDiagnostics) in the package without declaring it a dependency.
error: Target TestingMacrosTests imports another target (SwiftSyntaxMacros) in the package without declaring it a dependency.

(In fact, it randomly varies by build which missing dependency it lists above.)

Steps to reproduce

1. Clone https://github.com/swiftlang/swift-testing.
2. Check out tag 6.1.1 (because main uses a version of Swift Syntax which isn't prebuilt yet!).
3. Run swift build --explicit-target-dependency-import-check=error --enable-experimental-prebuilts.

Swift Package Manager version/commit hash

Swift Package Manager - Swift 6.1.0

Swift & OS version (output of swift --version ; uname -a)

swift-driver version: 1.120.5 Apple Swift version 6.1.2 (swiftlang-6.1.2.1.2 clang-1700.0.13.5)
Target: arm64-apple-macosx15.0
Darwin Kernel Version 24.5.0: Tue Apr 22 19:54:25 PDT 2025; root:xnu-11417.121.6~2/RELEASE_ARM64_T6020

[pyrtsa]

Originally reported on Swift Forums: https://forums.swift.org/t/preview-swift-syntax-prebuilts-for-macros/80202/54