SPM Deployment Target Resolution#

Overview#

Tuist maps Swift Package Manager (SPM) package metadata to Xcode project deployment targets via PackageInfoMapper.swift. The process has two layers: computing a per-target DeploymentTargets from Swift version constraints and package-declared platforms, then propagating package-wide baseSettings (including MACOSX_DEPLOYMENT_TARGET) into every generated target's build settings dictionary.

Step 1: Computing DeploymentTargets per Target#

For each mapped SPM target, the mapper 1:

1. Queries the active Swift toolchain version via SwiftVersionProvider.current.swiftVersion().

2. Derives the minimum floor using DeploymentTargets.oldestVersions(for:), which returns Swift-version-specific minimums:

   Swift version	iOS	macOS	watchOS	tvOS	visionOS
   < 5.7	9.0	10.10	2.0	9.0	1.0
   < 5.9	11.0	10.13	4.0	11.0	1.0
   < 6.0	12.0	10.13	4.0	12.0	1.0
   < 6.2	15.0	10.13	7.0	15.0	1.0
   < 6.4	15.0	11.0	8.0	15.0	1.0
   ≥ 6.4	15.0	12.0	9.0	15.0	1.0

3. Resolves the final value with DeploymentTargets.from(minDeploymentTargets:package:destinations:packageName:), taking the max() of the floor and the version declared in the package's platform list for each active destination 2. A package that declares macOS: "10.12" with Swift 6.4 toolchain will be clamped to macOS 12.0.

Step 2: Destination Assignment#

Before deployment targets are computed, the mapper determines which destinations a target builds for 3:

• Macro and executable targets → Set([.mac]) only 4. This forces macOS-only compilation since Swift macros are host-side tools run at build time.
• Test targets → intersection of destinations supported by their dependencies.
• Other targets → product destinations (local packages) or all destinations (external packages).

The destinations set is then passed into DeploymentTargets.from(...), so only relevant platforms get a deployment target string.

Step 3: Propagating baseSettings Into Target Build Settings#

The Settings.from(...) method constructs each target's SettingsDictionary in priority order:

1. Mapped SPM flags (from SettingsMapper) form the initial dictionary.
2. PackageSettings.baseSettings.base is merged in next — this is where package-level settings like MACOSX_DEPLOYMENT_TARGET originate 5. PRODUCT_BUNDLE_IDENTIFIER is explicitly excluded to avoid clobbering per-target sanitized identifiers 6.
3. Per-target overrides from PackageSettings.targetSettings are merged last with arrays unioned and scalars overriding 7.

Bug History: Macro Targets Missing baseSettings (Xcode 27)#

Xcode 27 beta introduced stricter deployment target validation, surfacing a long-standing gap: macro targets were not receiving PackageSettings.baseSettings.base 8. This caused build failures when MACOSX_DEPLOYMENT_TARGET was set at the package level.

• Issue #11447 reported that macro dependencies do not inherit deployment targets after the original fix for #11163.
• PR #11451 (community contribution) provided an initial fix merging baseSettings.base into baseSettingsDictionary for macro targets, using uniquingKeysWith: { _, new in new }.
• PR #11574 (merged 2026-06-30) is the canonical fix: Settings.from(...) now merges package-level base settings before target-specific settings for all mapped targets, including macros. The fix was verified with a regression test (map_macroTarget_appliesBaseSettingsBaseToMacroTarget) that asserts MACOSX_DEPLOYMENT_TARGET from packageSettings.base appears in the generated macro target's settings.

Key Files#