Fix compilation with swift build (#7588)

Adding a resource to a target makes `swift build` try to force-include
Foundation.h when building source files in that target, which breaks if any of
the source files aren't obj-c. To work around this, the resources need to be in
a separate target from the source files.

xcodebuild breaks if a target has zero source files, so add an empty obj-c file
to build as part of this new resource target.

Author: tgoyne

CHANGELOG.md

@@ -1,6 +1,7 @@
 # 14.5.2 Release notes
 
 ### Fixed
+* Fix compilation errors when using command-line `swift build` ([#7587](https://github.com/realm/realm-core/pull/7587), since v14.5.1).
 
 ### Compatibility
 * Fileformat: Generates files with format v24. Reads and automatically upgrade from fileformat v10. If you want to upgrade from an earlier file format version you will have to use RealmCore v13.x.y or earlier.

Package.swift

@@ -364,7 +364,7 @@ let package = Package(
     products: [
         .library(
             name: "RealmCore",
-            targets: ["RealmCore"]),
+            targets: ["RealmCoreResources"]),
         .library(
             name: "RealmQueryParser",
             targets: ["RealmQueryParser"]),
@@ -417,12 +417,10 @@ let package = Package(
                 "realm/tools",
                 "realm/util/config.h.in",
                 "realm/version_numbers.hpp.in",
+                "spm",
                 "swift",
                 "win32",
             ] + syncExcludes + syncServerSources) as [String],
-            resources: [
-                .copy("realm/PrivacyInfo.xcprivacy")
-            ],
             publicHeadersPath: ".",
             cxxSettings: ([
                 .headerSearchPath("external"),
@@ -433,6 +431,20 @@ let package = Package(
                 .linkedFramework("Foundation", .when(platforms: [.macOS, .iOS, .tvOS, .watchOS, .macCatalyst])),
                 .linkedFramework("Security", .when(platforms: [.macOS, .iOS, .tvOS, .watchOS, .macCatalyst])),
             ]),
+        // Adding resources to a target makes command line swift build (but not
+        // xcodebuild) force-include Foundation.h without properly excluding
+        // non-objc c++ files. Adding the resources in a separate target which
+        // depends on RealmCore works around this. This target needs a single
+        // source file (dummy.mm) to work around a different bug in xcodebuild
+        // (but not swift build) that makes empty targets not work.
+        .target(
+            name: "RealmCoreResources",
+            dependencies: ["RealmCore"],
+            path: "src/spm",
+            resources: [
+                .copy("PrivacyInfo.xcprivacy")
+            ],
+            publicHeadersPath: "."),
         .target(
             name: "RealmQueryParser",
             dependencies: ["RealmCore"],