Include out-of-target sources

Sources/PackageLoading/TargetSourcesBuilder.swift

@@ -199,8 +199,23 @@ public struct TargetSourcesBuilder {
             additionalResources = []
         }
 
-        let headers = pathToRule.lazy.filter { $0.value == .header }.map { $0.key }.sorted()
-        let compilePaths = pathToRule.lazy.filter { $0.value == .compile }.map { $0.key }
+        var additionalSources: [Basics.AbsolutePath] = []
+        var additionalHeaders: [Basics.AbsolutePath] = []
+        if self.toolsVersion >= .vNext {
+            if let declaredSources = self.declaredSources {
+                let unhandledSources = self.computeContents(for: declaredSources.filter { !$0.isDescendant(of: self.targetPath) })
+                for source in unhandledSources {
+                    if let ext = source.extension, FileRuleDescription.header.fileTypes.contains(ext) {
+                        additionalHeaders.append(source)
+                    } else {
+                        additionalSources.append(source)
+                    }
+                }
+            }
+        }
+
+        let headers = (pathToRule.lazy.filter { $0.value == .header }.map { $0.key } + additionalHeaders).sorted()
+        let compilePaths = (pathToRule.lazy.filter { $0.value == .compile }.map { $0.key } + additionalSources)
         let sources = Sources(paths: Array(compilePaths).sorted(), root: targetPath)
         let resources: [Resource] = (pathToRule.compactMap { resource(for: $0.key, with: $0.value) } + additionalResources).sorted { a, b in
             a.path.pathString < b.path.pathString
@@ -415,9 +430,9 @@ public struct TargetSourcesBuilder {
     ///
     /// This avoids recursing into certain directories like exclude or the
     /// ones that should be copied as-is.
-    public func computeContents() -> [Basics.AbsolutePath] {
+    public func computeContents(for paths: [Basics.AbsolutePath]) -> [Basics.AbsolutePath] {
         var contents: [Basics.AbsolutePath] = []
-        var queue: [Basics.AbsolutePath] = [targetPath]
+        var queue: [Basics.AbsolutePath] = paths
 
         // Ignore xcodeproj and playground directories.
         var ignoredDirectoryExtensions = ["xcodeproj", "playground", "xcworkspace"]
@@ -519,6 +534,10 @@ public struct TargetSourcesBuilder {
         return contents
     }
 
+    public func computeContents() -> [Basics.AbsolutePath] {
+        self.computeContents(for: [self.targetPath])
+    }
+
     public static func computeContents(for generatedFiles: [Basics.AbsolutePath], toolsVersion: ToolsVersion, additionalFileRules: [FileRuleDescription], defaultLocalization: String?, targetName: String, targetPath: Basics.AbsolutePath, observabilityScope: ObservabilityScope) -> (sources: [Basics.AbsolutePath], resources: [Resource]) {
         var sources = [Basics.AbsolutePath]()
         var resources = [Resource]()

Tests/PackageLoadingTests/PackageBuilderTests.swift

@@ -3477,6 +3477,33 @@ final class PackageBuilderTests: XCTestCase {
             }
         }
     }
+
+    func testOutOfTargetSource() throws {
+        let fs = InMemoryFileSystem(
+            emptyFiles:
+            "/Sources/foo/foo.swift",
+            "/Sources/foo/foobar/foobar.swift",
+
+            "/Sources/bar/bar.swift"
+        )
+
+        let manifest = try Manifest.createRootManifest(
+            displayName: "pkg",
+            toolsVersion: .vNext,
+            targets: [
+                TargetDescription(
+                    name: "foo",
+                    sources: ["../bar/bar.swift", "foobar"]
+                ),
+            ]
+        )
+
+        PackageBuilderTester(manifest, in: fs) { package, _ in
+            package.checkModule("foo") { module in
+                module.checkSources(sources: ["foobar/foobar.swift","../bar/bar.swift"])
+            }
+        }
+    }
 }
 
 final class PackageBuilderTester {