test: header-change incrementality across every compile path

One header per pass, asserting both directions — the objects that include
it recompile, the ones that don't are untouched — then running the binary,
since "was recompiled" only matters if the program agrees with itself.

Covers the module interface (a macro in its global module fragment decides
an exported class's layout), the implementation unit, a C source, an idle
rebuild that must recompile nothing, and an object whose depfile is gone.
The implementation unit's header has a space in its name so the depfile
spells it escaped.

tests/IncrementalHeaderChange/fixture/lib/Widget.cpp

@@ -0,0 +1,11 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+module;
+#include "widget count.h"
+module Widget;
+import std;
+
+std::size_t WidgetSizeInLibrary() { return sizeof(Widget); }
+
+std::int64_t WidgetCountInLibrary() { return WIDGET_COUNT; }

tests/IncrementalHeaderChange/fixture/lib/Widget.cppm

@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+module;
+// The whole point of the fixture: an input to this interface unit that the
+// module scanner cannot see. WIDGET_SLOTS comes from a header, so the class
+// layout below changes without Widget.cppm being touched.
+#include "widget-layout.h"
+export module Widget;
+import std;
+
+export struct Widget {
+    std::array<std::int64_t, WIDGET_SLOTS> slots;
+};
+
+// Out-of-line in the library's implementation unit, so the value reflects the
+// layout the *library* was compiled against rather than the caller's — same
+// reasoning as the IncrementalInterfaceChange fixture.
+export std::size_t WidgetSizeInLibrary();
+// Reads a macro from a header only the implementation unit includes.
+export std::int64_t WidgetCountInLibrary();

tests/IncrementalHeaderChange/fixture/lib/counter-limit-grown.h.in

@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: LGPL-3.0-only
+   SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts® */
+
+/* Copied over lib/counter-limit.h mid-test. */
+#pragma once
+
+#define COUNTER_LIMIT 9

tests/IncrementalHeaderChange/fixture/lib/counter-limit.h

@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: LGPL-3.0-only
+   SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts® */
+
+/* Included from counter.c — the C compile path keeps all of its dependencies in
+   #includes, so it has nothing but the depfile to go on. */
+#pragma once
+
+#define COUNTER_LIMIT 5

tests/IncrementalHeaderChange/fixture/lib/counter.c

@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: LGPL-3.0-only
+   SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts® */
+
+#include "counter-limit.h"
+
+long CounterLimitInLibrary(void) {
+    return COUNTER_LIMIT;
+}

tests/IncrementalHeaderChange/fixture/lib/widget count-grown.h.in

@@ -0,0 +1,7 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// Copied over lib/widget count.h mid-test.
+#pragma once
+
+#define WIDGET_COUNT 2

tests/IncrementalHeaderChange/fixture/lib/widget count.h

@@ -0,0 +1,12 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// Included from Widget.cpp, the module's implementation unit — nothing else
+// sees it, so an edit here must rebuild that one object and nothing more.
+//
+// The space in the filename is deliberate: clang escapes it as `widget\ count.h`
+// in the depfile, so a build that reads depfiles has to unescape make syntax
+// rather than splitting the line on whitespace.
+#pragma once
+
+#define WIDGET_COUNT 1

tests/IncrementalHeaderChange/fixture/lib/widget-layout-grown.h.in

@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// Copied over lib/widget-layout.h mid-test: one more slot in the exported
+// class. Kept as a .h.in so the original header is the only one any source
+// #includes.
+#pragma once
+
+#define WIDGET_SLOTS 2

tests/IncrementalHeaderChange/fixture/lib/widget-layout.h

@@ -0,0 +1,11 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// Included from Widget.cppm's global module fragment. WIDGET_SLOTS decides the
+// layout of the exported class, so growing it (the test copies
+// widget-layout-grown.h.in over this file) invalidates the BMI and every object
+// compiled against it — while leaving Widget.cppm itself untouched, which is
+// exactly what the mtime comparison used to miss.
+#pragma once
+
+#define WIDGET_SLOTS 1

tests/IncrementalHeaderChange/fixture/main.cpp

@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+import std;
+import Widget;
+
+extern "C" long CounterLimitInLibrary();
+
+// Prints "<sizeof here> <sizeof in library> <count> <limit>" and exits 1 when
+// the two sizes disagree. The sizes are the mixed-layout check; the count and
+// the limit report which build of the library's own objects got linked in, so a
+// stale implementation object or C object shows up as an old value rather than
+// as silence.
+int main() {
+    std::size_t here = sizeof(Widget);
+    std::size_t inLibrary = WidgetSizeInLibrary();
+    std::print("{} {} {} {}", here, inLibrary, WidgetCountInLibrary(), CounterLimitInLibrary());
+    return here == inLibrary ? 0 : 1;
+}

tests/IncrementalHeaderChange/main.cpp

@@ -0,0 +1,203 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+import std;
+import Crafter.Build;
+namespace fs = std::filesystem;
+using namespace Crafter;
+
+// Editing a header must rebuild everything that #includes it. The staleness
+// check used to compare an artifact against its own source and its module
+// imports only — and the module scanner reads `import` lines, so a header was
+// invisible to it. A build after a header-only edit reported nothing to do and
+// left objects compiled against the previous contents: the same silent
+// mixed-layout binary as issue #27, reached through #include instead.
+//
+// Each pass below edits exactly one header and asserts both directions — the
+// objects that include it are recompiled, the ones that don't are left alone —
+// then runs the binary, because "was recompiled" is only interesting if the
+// resulting program agrees with itself.
+
+namespace {
+    std::int32_t Failures = 0;
+
+    void Check(bool cond, std::string_view msg) {
+        if (!cond) {
+            std::println(std::cerr, "FAIL: {}", msg);
+            ++Failures;
+        }
+    }
+
+    // The fixture is mutated during the run, so work on a copy outside the repo.
+    fs::path StageFixture() {
+        fs::path source = fs::current_path() / "tests" / "IncrementalHeaderChange" / "fixture";
+        fs::path staged = fs::temp_directory_path() / "crafter-build-incremental-header-change";
+        fs::remove_all(staged);
+        fs::copy(source, staged, fs::copy_options::recursive);
+        return staged;
+    }
+
+    // Swap <name>-grown.h.in in for <name>.h. The replacement text lives in a
+    // file rather than a string literal so the header the fixture #includes is
+    // the only one under that name, whichever variant is in place.
+    void Grow(const fs::path& header) {
+        fs::path grown = header.parent_path() / std::format("{}-grown.h.in", header.stem().string());
+        fs::copy_file(grown, header, fs::copy_options::overwrite_existing);
+        // copy_file carries the source's mtime across, which would leave the
+        // rewritten header looking older than the objects built from it.
+        fs::last_write_time(header, fs::file_time_type::clock::now());
+    }
+
+    std::unique_ptr<Configuration> MakeLib(const fs::path& staged) {
+        auto lib = std::make_unique<Configuration>();
+        lib->path = staged / "lib";
+        lib->name = "widget";
+        lib->outputName = "widget";
+        lib->target = HostTarget();
+        lib->type = ConfigurationType::LibraryStatic;
+        std::array<fs::path, 1> ifaces = { "Widget" };
+        std::array<fs::path, 1> impls = { "Widget" };
+        lib->GetInterfacesAndImplementations(ifaces, impls);
+        // cFiles are resolved against the cwd at build time, so spell it out.
+        lib->cFiles = { staged / "lib" / "counter" };
+        return lib;
+    }
+
+    Configuration MakeApp(const fs::path& staged, Configuration* lib) {
+        Configuration app;
+        app.path = staged;
+        app.name = "widget-app";
+        app.outputName = "widget-app";
+        app.target = HostTarget();
+        app.type = ConfigurationType::Executable;
+        std::array<fs::path, 0> ifaces = {};
+        std::array<fs::path, 1> impls = { "main" };
+        app.GetInterfacesAndImplementations(ifaces, impls);
+        app.dependencies = { lib };
+        return app;
+    }
+
+    bool BuildOk(Configuration& app, std::string_view label) {
+        // A fresh depResults per pass: the map memoizes each Configuration's
+        // build for the duration of one pass, so reusing it would skip the
+        // library's second build entirely.
+        std::unordered_map<fs::path, std::shared_future<BuildResult>> depResults;
+        std::mutex depMutex;
+        BuildResult r = Build(app, depResults, depMutex);
+        if (!r.result.empty()) {
+            std::println(std::cerr, "FAIL: {} build failed: {}", label, r.result);
+            ++Failures;
+            return false;
+        }
+        return true;
+    }
+
+    // "<sizeof in the consumer> <sizeof in the library> <count> <limit>" — see
+    // the fixture's main.cpp.
+    void CheckRun(const fs::path& binary, std::string_view expected, std::string_view label) {
+        auto r = RunCommandWithTimeout(binary.string(), std::chrono::seconds(30));
+        Check(r.exitCode == 0 && !r.crashed && !r.timedOut && r.output == expected, std::format("{}: expected '{}', got '{}' (exit={})", label, expected, r.output, r.exitCode));
+    }
+}
+
+int main() {
+    fs::path staged = StageFixture();
+    std::unique_ptr<Configuration> lib = MakeLib(staged);
+    Configuration app = MakeApp(staged, lib.get());
+
+    fs::path binary = app.BinDir() / "widget-app";
+    // One artifact per compile path a header can reach: the module interface
+    // (BMI plus the object made from it), the module's implementation unit, a C
+    // source, and the consumer that imports the module.
+    fs::path interfacePcm = lib->PcmDir() / "Widget.pcm";
+    fs::path interfaceObject = lib->BuildDir() / "Widget.o";
+    fs::path libraryObject = lib->BuildDir() / "Widget_impl.o";
+    fs::path counterObject = lib->BuildDir() / "counter_source.o";
+    fs::path consumerObject = app.BuildDir() / "main_impl.o";
+
+    if (!BuildOk(app, "first pass")) {
+        std::println(std::cerr, "{} assertions failed", Failures);
+        return 1;
+    }
+    CheckRun(binary, "8 8 1 5", "first pass");
+
+    auto stamps = [&]() {
+        return std::array<fs::file_time_type, 5>{
+            fs::last_write_time(interfacePcm),
+            fs::last_write_time(interfaceObject),
+            fs::last_write_time(libraryObject),
+            fs::last_write_time(counterObject),
+            fs::last_write_time(consumerObject),
+        };
+    };
+
+    // Nothing changed: the depfiles must not read as staleness of their own,
+    // or every build would recompile the world.
+    {
+        std::array<fs::file_time_type, 5> before = stamps();
+        if (BuildOk(app, "idle pass")) {
+            Check(stamps() == before, "an idle rebuild recompiles nothing");
+        }
+    }
+
+    // A header only the module's implementation unit includes. Its name carries
+    // a space, so the depfile spells it escaped.
+    {
+        std::array<fs::file_time_type, 5> before = stamps();
+        Grow(staged / "lib" / "widget count.h");
+        if (BuildOk(app, "implementation header pass")) {
+            std::array<fs::file_time_type, 5> after = stamps();
+            Check(after[2] > before[2], "implementation object is recompiled after a header it includes changes");
+            Check(after[0] == before[0], "interface BMI is left alone by a header it does not include");
+            Check(after[3] == before[3], "C object is left alone by a header it does not include");
+            Check(after[4] == before[4], "consumer object is left alone by a header it does not include");
+            CheckRun(binary, "8 8 2 5", "implementation header pass");
+        }
+    }
+
+    // A header only the C source includes.
+    {
+        std::array<fs::file_time_type, 5> before = stamps();
+        Grow(staged / "lib" / "counter-limit.h");
+        if (BuildOk(app, "C header pass")) {
+            std::array<fs::file_time_type, 5> after = stamps();
+            Check(after[3] > before[3], "C object is recompiled after a header it includes changes");
+            Check(after[2] == before[2], "implementation object is left alone by a header it does not include");
+            CheckRun(binary, "8 8 2 9", "C header pass");
+        }
+    }
+
+    // A header the interface unit includes, changing the layout of an exported
+    // class: the BMI and everything compiled against it has to follow.
+    {
+        std::array<fs::file_time_type, 5> before = stamps();
+        Grow(staged / "lib" / "widget-layout.h");
+        if (BuildOk(app, "interface header pass")) {
+            std::array<fs::file_time_type, 5> after = stamps();
+            Check(after[0] > before[0], "interface BMI is rebuilt after a header it includes changes");
+            Check(after[1] > before[1], "interface object is rebuilt after a header it includes changes");
+            Check(after[2] > before[2], "implementation object follows the rebuilt BMI");
+            Check(after[4] > before[4], "consumer object follows the rebuilt BMI");
+            CheckRun(binary, "16 16 2 9", "interface header pass");
+        }
+    }
+
+    // An object built before depfiles were emitted at all — an upgrade over an
+    // existing build directory — has no record of what it included, so the one
+    // safe reading is "rebuild it once".
+    {
+        std::array<fs::file_time_type, 5> before = stamps();
+        fs::remove(lib->BuildDir() / "Widget_impl.o.d");
+        if (BuildOk(app, "missing depfile pass")) {
+            std::array<fs::file_time_type, 5> after = stamps();
+            Check(after[2] > before[2], "an object whose dependency record is missing is rebuilt");
+            Check(after[3] == before[3], "the objects that still have one are left alone");
+        }
+    }
+
+    if (Failures > 0) {
+        std::println(std::cerr, "{} assertions failed", Failures);
+        return 1;
+    }
+    return 0;
+}