Fix issues with excluding macros/plugins from dependency computation (#…

…6723)

While fixing this, I also noticed the original issue also exists for executable targets, so this gets fixed here as well.

There's one unfortunate nuance here for test targets since using a macro/plugin is indistinguishable from needing to link it because it is being tested. We err on the side of caution here and will always link.

(sidenote: theoretically, plugins *do* distinguish between linkage and use in the package manifest, but this distinction is not carried forward into the actual model)

Partially fixes https://github.com/apple/swift/issues/67371 since the underlying project also does not declare a dependency on the macro that is being tested.

Fixtures/Miscellaneous/Plugins/MySourceGenPlugin/Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version: 5.6
+// swift-tools-version: 5.9
 import PackageDescription
 
 let package = Package(

Sources/Build/BuildPlan.swift

@@ -695,13 +695,28 @@ public class BuildPlan: SPMBuildCore.BuildPlan {
         libraryBinaryPaths: Set<AbsolutePath>,
         availableTools: [String: AbsolutePath]
     ) {
-        /* Prior to tools-version 5.9, we used to errorneously recursively traverse plugin dependencies and statically include their
+        /* Prior to tools-version 5.9, we used to errorneously recursively traverse executable/plugin dependencies and statically include their
          targets. For compatibility reasons, we preserve that behavior for older tools-versions. */
-        let shouldExcludePlugins: Bool
+        let shouldExcludeExecutablesAndPlugins: Bool
         if let toolsVersion = self.graph.package(for: product)?.manifest.toolsVersion {
-            shouldExcludePlugins = toolsVersion >= .v5_9
+            shouldExcludeExecutablesAndPlugins = toolsVersion >= .v5_9
         } else {
-            shouldExcludePlugins = false
+            shouldExcludeExecutablesAndPlugins = false
+        }
+
+        // For test targets, we need to consider the first level of transitive dependencies since the first level is always test targets.
+        let topLevelDependencies: [PackageModel.Target]
+        if product.type == .test {
+            topLevelDependencies = product.targets.flatMap { $0.underlyingTarget.dependencies }.compactMap {
+                switch $0 {
+                case .product:
+                    return nil
+                case .target(let target, _):
+                    return target
+                }
+            }
+        } else {
+            topLevelDependencies = []
         }
 
         // Sort the product targets in topological order.
@@ -710,10 +725,19 @@ public class BuildPlan: SPMBuildCore.BuildPlan {
             switch dependency {
             // Include all the dependencies of a target.
             case .target(let target, _):
-                if target.type == .macro {
+                let isTopLevel = topLevelDependencies.contains(target.underlyingTarget) || product.targets.contains(target)
+                let topLevelIsExecutable = isTopLevel && product.type == .executable
+                let topLevelIsMacro = isTopLevel && product.type == .macro
+                let topLevelIsPlugin = isTopLevel && product.type == .plugin
+                let topLevelIsTest = isTopLevel && product.type == .test
+
+                if !topLevelIsMacro && !topLevelIsTest && target.type == .macro {
+                    return []
+                }
+                if shouldExcludeExecutablesAndPlugins, !topLevelIsPlugin && !topLevelIsTest && target.type == .plugin {
                     return []
                 }
-                if shouldExcludePlugins, target.type == .plugin {
+                if shouldExcludeExecutablesAndPlugins, !topLevelIsExecutable && topLevelIsTest && target.type == .executable {
                     return []
                 }
                 return target.dependencies.filter { $0.satisfies(self.buildEnvironment) }
@@ -730,7 +754,7 @@ public class BuildPlan: SPMBuildCore.BuildPlan {
                 case .library(.automatic), .library(.static):
                     return productDependencies
                 case .plugin:
-                    return shouldExcludePlugins ? [] : productDependencies
+                    return shouldExcludeExecutablesAndPlugins ? [] : productDependencies
                 case .library(.dynamic), .test, .executable, .snippet, .macro:
                     return []
                 }