swift-syntax prebuilts are not used when a macro package declares a test target

What happened?

Tuist builds swift-syntax from source even though SwiftPM downloaded the swift-syntax MacroSupport prebuilt and recorded it in workspace-state.json. This happens whenever the package that declares the macro also has a test target that reaches the macro through its runtime library. That’s the usual shape for macro packages, for example swift-composable-architecture, swift-dependencies, swift-case-paths and swift-perception.

The test target alone is enough, even though Tuist never generates test targets for remote packages.

Expected: the macro target links the prebuilt (-lMacroSupport) and no swift-syntax targets are compiled.

Actual: the macro target depends on the swift-syntax source targets and 17 of them are compiled. Removing the test target from the package manifest restores the prebuilt.

Cause

PackageInfoMapper.prebuiltEligibleTargets(packageInfo:packageType:) (source) adds every test target whose dependency closure reaches a macro to hostRoots:

let hostRoots = macroTargets.union(macroTestTargets)
let hostReachableTargets = dependencyClosure(from: hostRoots, includeHostOnlyTargets: true)
let nonHostReachableTargets = dependencyClosure(
    from: allTargetNames.subtracting(hostRoots),
    includeHostOnlyTargets: false
)

guard hostReachableTargets.isDisjoint(with: nonHostReachableTargets) else {
    return []
}

A test like MacroDependencyTests → MacroDependency → MacroDependencyMacros pulls the runtime library MacroDependency into hostReachableTargets. MacroDependency is also a non-host root, so the sets overlap and prebuilts are disabled for the whole package.

Two things contribute:

1. Tests that are never generated are still considered. map(target:…) drops .test targets unless packageType.includesTestTargets(includeLocalPackageTestTargets:) is true (source). prebuiltEligibleTargets doesn’t apply the same check.
2. The overlap check is broader than it needs to be. The overlapping runtime library doesn’t depend on swift-syntax, so it can’t conflict with the prebuilt.

Since an ineligible macro target keeps its swift-syntax source dependencies, one affected package anywhere in the graph means swift-syntax is compiled from source.

Possible fix

Only add macroTestTargets to hostRoots when packageType.includesTestTargets(includeLocalPackageTestTargets:) is true. With that change, the reproduction links -lMacroSupport, compiles no swift-syntax targets, and builds and runs. A narrower overlap check (only failing on overlapping targets that themselves depend on a prebuilt product) would also cover packages whose tests are generated.

The generated_app_with_swiftpm_prebuilt_macro_dependency fixture has no test target, which is why BuildAcceptanceTestSwiftPMPrebuiltMacro doesn’t catch this.

Disclosure: AI was used to investigate this problem and work up the example and write the report, though I have reviewed it thoroughly and am creating this issue by hand.

How do we reproduce it?

Reproduction project attached: tuist-prebuilt-repro.zip

It’s the generated_app_with_swiftpm_prebuilt_macro_dependency fixture plus one test target in MacroDependency/Package.swift:

.testTarget(
    name: "MacroDependencyTests",
    dependencies: ["MacroDependency"]
),

Tested with Tuist 4.212.0, Xcode 27.1 (Swift 6.4), Apple silicon.

unzip tuist-prebuilt-repro.zip && cd tuist-prebuilt-repro
mise trust && mise install   # pins tuist 4.212.0 and sets TUIST_USE_SWIFTERPM=0
tuist install
tuist generate --no-open
xcodebuild build -workspace App.xcworkspace -scheme App CODE_SIGNING_ALLOWED=NO

1. Tuist/.build/workspace-state.json lists the swift-syntax MacroSupport prebuilt.
2. grep -rl --include=project.pbxproj -e -lMacroSupport . finds nothing.
3. The build log shows targets from the swift-syntax project being compiled.
4. Remove the .testTarget, then rerun tuist generate and xcodebuild. -lMacroSupport is now linked and no swift-syntax targets are compiled.

TUIST_USE_SWIFTERPM=0 makes the prebuilt show up in workspace-state.json every time. With SwifterPM and a warm cache, the prebuilts array can be empty, which hides this issue. That’s a separate problem.

tuist-prebuilt-repro.zip

Error log

There’s no error. Tuist falls back to building swift-syntax from source without any message.

Tuist/.build/workspace-state.json (the prebuilt is available):

{
  "identity": "swift-syntax",
  "version": "602.0.0",
  "libraryName": "MacroSupport",
  "path": ".../Tuist/.build/prebuilts/swift-syntax/602.0.0/swiftlang-6.4.0.34.1-macosx27.0-MacroSupport"
}

Targets built by xcodebuild with the test target present:

(in target 'App' from project 'App')
(in target 'MacroDependency' from project 'MacroDependency')
(in target 'MacroDependencyMacros' from project 'MacroDependency')
(in target '_SwiftSyntaxCShims' from project 'swift-syntax')
(in target 'SwiftBasicFormat' from project 'swift-syntax')
(in target 'SwiftCompilerPlugin' from project 'swift-syntax')
(in target 'SwiftCompilerPluginMessageHandling' from project 'swift-syntax')
(in target 'SwiftDiagnostics' from project 'swift-syntax')
(in target 'SwiftOperators' from project 'swift-syntax')
(in target 'SwiftParser' from project 'swift-syntax')
(in target 'SwiftParserDiagnostics' from project 'swift-syntax')
(in target 'SwiftSyntax' from project 'swift-syntax')
(in target 'SwiftSyntax509' from project 'swift-syntax')
(in target 'SwiftSyntax510' from project 'swift-syntax')
(in target 'SwiftSyntax600' from project 'swift-syntax')
(in target 'SwiftSyntax601' from project 'swift-syntax')
(in target 'SwiftSyntax602' from project 'swift-syntax')
(in target 'SwiftSyntaxBuilder' from project 'swift-syntax')
(in target 'SwiftSyntaxMacroExpansion' from project 'swift-syntax')
(in target 'SwiftSyntaxMacros' from project 'swift-syntax')

Targets built with the test target removed:

(in target 'App' from project 'App')
(in target 'MacroDependency' from project 'MacroDependency')
(in target 'MacroDependencyMacros' from project 'MacroDependency')

macOS version

26.7.1

Tuist version

4.212.0

Xcode version

27.1

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