Emit warning for unhandled files in SwiftBuild

Sources/SwiftBuildSupport/PIFBuilder.swift

@@ -389,6 +389,12 @@ public final class PIFBuilder {
                         observabilityScope: observabilityScope
                     )
 
+                    self.diagnoseUnhandledFiles(
+                        package: package,
+                        module: module,
+                        buildToolPluginInvocationResults: buildToolPluginResults
+                    )
+
                     let result = PackagePIFBuilder.BuildToolPluginInvocationResult(
                         prebuildCommandOutputPaths: runResults.flatMap( { $0.derivedFiles }),
                         buildCommands: buildCommands
@@ -530,6 +536,42 @@ public final class PIFBuilder {
         )
         return try await builder.generatePIF(preservePIFModelStructure: preservePIFModelStructure, buildParameters: buildParameters)
     }
+
+    private func diagnoseUnhandledFiles(
+        package: ResolvedPackage,
+        module: ResolvedModule,
+        buildToolPluginInvocationResults: [BuildToolPluginInvocationResult]
+    ) {
+        guard package.manifest.toolsVersion >= .v5_3 else {
+            return
+        }
+
+        var unhandledFiles = Set(module.underlying.others)
+        if unhandledFiles.isEmpty {
+            return
+        }
+
+        let handledFiles = buildToolPluginInvocationResults.flatMap { $0.buildCommands.flatMap(\.inputFiles) }
+        unhandledFiles.subtract(handledFiles)
+
+        if unhandledFiles.isEmpty {
+            return
+        }
+
+        let diagnosticsEmitter = self.observabilityScope.makeDiagnosticsEmitter {
+            var metadata = ObservabilityMetadata()
+            metadata.packageIdentity = package.identity
+            metadata.packageKind = package.manifest.packageKind
+            metadata.moduleName = module.name
+            return metadata
+        }
+        var warning =
+            "found \(unhandledFiles.count) file(s) which are unhandled; explicitly declare them as resources or exclude from the target\n"
+        for file in unhandledFiles {
+            warning += "    " + file.pathString + "\n"
+        }
+        diagnosticsEmitter.emit(warning: warning)
+    }
 }
 
 fileprivate final class PackagePIFBuilderDelegate: PackagePIFBuilder.BuildDelegate {

Tests/FunctionalTests/PluginTests.swift

@@ -73,7 +73,6 @@ struct PluginTests {
 
     @Test(
         .IssueWindowsRelativePathAssert,
-        .bug("https://github.com/swiftlang/swift-package-manager/issues/8786"),
         .requiresSwiftConcurrencySupport,
         .tags(
             .Feature.Command.Test,
@@ -93,8 +92,7 @@ struct PluginTests {
                 #expect(stderr.contains("file(s) which are unhandled; explicitly declare them as resources or exclude from the target"), "expected warning not emitted")
             }
         } when: {
-            (ProcessInfo.hostOperatingSystem == .windows && CiEnvironment.runningInSelfHostedPipeline && buildSystem == .native)
-            || (buildSystem == .swiftbuild)
+            ProcessInfo.hostOperatingSystem == .windows && CiEnvironment.runningInSelfHostedPipeline && buildSystem == .native
         }
     }