[6.4] Address feedback

Fixtures/TestTargetDependOnTestTarget/Package.swift

@@ -0,0 +1,26 @@
+// swift-tools-version: 6.2
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import PackageDescription
+
+let package = Package(
+    name: "TestTargetDependOnTestTarget",
+    targets: [
+        .testTarget(
+            name: "leafTestTarget",
+        ),
+        .testTarget(
+            name: "myTestTarget",
+            dependencies: [
+                "leafTestTarget",
+            ],
+        ),
+        .testTarget(
+            name: "myOtherTestTarget",
+            dependencies: [
+                "leafTestTarget",
+                "myTestTarget",
+            ],
+        ),
+    ]
+)

Fixtures/TestTargetDependOnTestTarget/Tests/leafTestTarget/leafTestTarget.swift

@@ -0,0 +1,9 @@
+import Testing
+
+@Suite
+struct LeafTestTargetTests {
+    @Test("LeafTestTarget tests")
+    func example() {
+        #expect(42 == 17 + 25)
+    }
+}

Fixtures/TestTargetDependOnTestTarget/Tests/myOtherTestTarget/myOtherTestTarget.swift

@@ -0,0 +1,9 @@
+import Testing
+
+@Suite
+struct MyOTherTestTargetTests {
+    @Test("MyOTherTestTarget tests")
+    func example() {
+        #expect(42 == 17 + 25)
+    }
+}

Fixtures/TestTargetDependOnTestTarget/Tests/myTestTarget/myTestTarget.swift

@@ -0,0 +1,9 @@
+import Testing
+
+@Suite
+struct MyTestTargetTests {
+    @Test("MyTestTarget tests")
+    func example() {
+        #expect(42 == 17 + 25)
+    }
+}

Sources/SwiftBuildSupport/PIFBuilder.swift

@@ -187,6 +187,10 @@ public final class PIFBuilder {
             throw PIFGenerationError.printedPIFManifestGraphviz
         }
 
+        if self.observabilityScope.errorsReported {
+            throw PIFGenerationError.errorDiagnosticsReported
+        }
+
         return PIFGenerationResult(pif: pifString, accompanyingMetadata: modulesAndProducts)
     }
 
@@ -814,6 +818,9 @@ public enum PIFGenerationError: Error {
 
     /// Early build termination when using `--print-pif-manifest-graph`.
     case printedPIFManifestGraphviz
+
+    /// One or more error diagnostics were reported during PIF generation.
+    case errorDiagnosticsReported
 }
 
 extension PIFGenerationError: CustomStringConvertible {
@@ -832,6 +839,9 @@ extension PIFGenerationError: CustomStringConvertible {
 
         case .printedPIFManifestGraphviz:
             "Printed PIF manifest as graphviz"
+
+        case .errorDiagnosticsReported:
+            "Errors reported during PIF building"
         }
     }
 }

Sources/SwiftBuildSupport/PackagePIFProjectBuilder+Products.swift

@@ -488,6 +488,12 @@ extension PackagePIFProjectBuilder {
                     }
 
                 case .library, .systemModule, .test:
+                    if moduleDependency.type == .test && product.type == .test {
+                        log(
+                            .error,
+                            "Test target '\(product.name)' cannot depend on another test target '\(moduleDependency.name)'",
+                        )
+                    }
                     let shouldLinkProduct = moduleDependency.type != .systemModule
                     let dependencyGUID = moduleDependency.pifTargetGUID
                     self.project[keyPath: mainModuleTargetKeyPath].common.addDependency(

Tests/SwiftBuildSupportTests/PIFBuilderTests.swift

@@ -1015,6 +1015,23 @@ struct PIFBuilderTests {
         }
     }
 
+    @Test
+    func errorEmittedWhenTestTargetDependsOnAnotherTestTarget() async throws {
+        try await withGeneratedPIF(fromFixture: "TestTargetDependOnTestTarget") { pif, observabilitySystem in
+            let expectedErrors: Set = [
+                "PIF: Test target 'myOtherTestTarget' cannot depend on another test target 'leafTestTarget'",
+                "PIF: Test target 'myTestTarget' cannot depend on another test target 'leafTestTarget'",
+                "PIF: Test target 'myOtherTestTarget' cannot depend on another test target 'myTestTarget'",
+            ]
+            let actualDiags = observabilitySystem.diagnostics.filter { $0.severity == .error }
+            let diagMsgs = Set(actualDiags.map { $0.message })
+
+            #expect(actualDiags.count == expectedErrors.count)
+            #expect(actualDiags.count == diagMsgs.count)
+            #expect(diagMsgs == expectedErrors)
+        }
+    }
+
     @Test func mixedSourceTarget() async throws {
         let fs = InMemoryFileSystem(
             emptyFiles: