Don’t reload package manifest when a header is added

Sources/SPMTestSupport/MockWorkspace.swift

@@ -479,7 +479,7 @@ public final class MockWorkspace {
     public func checkPackageGraph(
         roots: [String] = [],
         deps: [MockDependency],
-        _ result: (ModulesGraph, [Basics.Diagnostic]) -> Void
+        _ result: (ModulesGraph, [Basics.Diagnostic]) throws -> Void
     ) throws {
         let dependencies = try deps.map { try $0.convert(baseURL: packagesDir, identityResolver: self.identityResolver) }
         try self.checkPackageGraph(roots: roots, dependencies: dependencies, result)

Sources/Workspace/Workspace.swift

@@ -1204,7 +1204,7 @@ extension Workspace {
     /// generated by SwiftPM.
     public func fileAffectsSwiftOrClangBuildSettings(filePath: AbsolutePath, packageGraph: ModulesGraph) -> Bool {
         // TODO: Implement a more sophisticated check that also verifies if the file is in the sources directories of the passed in `packageGraph`.
-        FileRuleDescription.builtinRules.contains { fileRuleDescription in
+        !FileRuleDescription.header.match(path: filePath, toolsVersion: self.currentToolsVersion) && FileRuleDescription.builtinRules.contains { fileRuleDescription in
             fileRuleDescription.match(path: filePath, toolsVersion: self.currentToolsVersion)
         }
     }

Tests/WorkspaceTests/WorkspaceReloadTests.swift

@@ -0,0 +1,46 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift open source project
+//
+// Copyright (c) 2014-2024 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import Basics
+import SPMTestSupport
+import Workspace
+import XCTest
+
+final class WorkspaceReloadTests: XCTestCase {
+    func testHeaderReloading() throws {
+        let sandbox = AbsolutePath("/tmp/ws/")
+        let fs = InMemoryFileSystem()
+
+        let workspace = try MockWorkspace(
+            sandbox: sandbox,
+            fileSystem: fs,
+            roots: [
+                MockPackage(
+                    name: "Foo",
+                    targets: [
+                        MockTarget(name: "Foo", dependencies: []),
+                    ]
+                ),
+            ]
+        )
+
+        try workspace.checkPackageGraph(roots: ["Foo"], deps: []) { graph, diagnostics in
+            try XCTAssertFalse(
+                workspace.getOrCreateWorkspace().fileAffectsSwiftOrClangBuildSettings(
+                    filePath: "/tmp/ws/.build/header.h",
+                    packageGraph: graph
+                )
+            )
+            XCTAssertNoDiagnostics(diagnostics)
+        }
+    }
+}