test(wasm): the cases the review found

WasmBuildRefusals checks that -flto in compileFlags and in linkFlags,
-fno-exceptions and a shared library are each refused with their
cause, and that none of them leaves bin/ or build/ behind. With the
check disabled, each of them failed on a missing std.cppm instead,
after the build directories had been made.

WasiSysrootWithoutExceptions also takes its sysroot through missing
and without the target's directory, each with a message of its own,
checks that a refused build leaves nothing behind, and that a static
library passes without libunwind.

WasmExceptions catches an exception thrown inside libc++.a (stoi).
vector::at throws from a header compiled into the fixture, so the
fixture also passed against the libc++ built without exceptions. Its C
source uses setjmp, which the old C flags did not compile. An
exception that escapes main has to end the run as wasmtime's uncaught
exception, where any nonzero exit used to pass, a trap or a timeout
included.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/WasmBuildRefusals/main.cpp

@@ -0,0 +1,85 @@
+// 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;
+
+// What a wasm32 build refuses before it starts, now that every wasm unit has
+// C++ exceptions, each with a message naming the cause:
+//   - -flto in compileFlags or linkFlags. LTO code generation drops the
+//     exception handling, and the build used to succeed with every throw
+//     escaping its catch (WebAssembly/wasi-sdk#629).
+//   - -fno-exceptions. The std module is built with exceptions, and every
+//     `import std;` failed on a BMI configuration mismatch.
+//   - A shared library. The exception-enabled libc++ is static archives
+//     only, and the link failed on a relocation that wants -fPIC.
+// None of them leaves bin/ or build/ behind, and none needs a working wasm
+// toolchain. The sysroot given does not exist, so a refusal that did not
+// fire would show up as the missing sysroot instead.
+namespace {
+    std::int32_t failures = 0;
+
+    void Check(bool cond, std::string_view msg) {
+        if (!cond) {
+            std::println(std::cerr, "FAIL: {}", msg);
+            ++failures;
+        }
+    }
+
+    struct Refusal {
+        std::string_view what;
+        std::vector<std::string> compileFlags;
+        std::vector<std::string> linkFlags;
+        ConfigurationType type;
+        std::string_view expected;
+    };
+}
+
+int main() {
+    // Build() reports a missing compiler before it gets to any of this.
+    if (const Toolchain& tc = GetToolchain(); !tc.error.empty()) {
+        std::println(std::cerr, "SKIP: {}", tc.error);
+        return 77;
+    }
+
+    const fs::path scratch = fs::temp_directory_path() / "crafter-build-wasm-build-refusals";
+    fs::remove_all(scratch);
+    fs::create_directories(scratch);
+    const fs::path project = scratch / "project";
+    fs::copy(fs::current_path() / "tests" / "WasmBuildRefusals" / "fixture", project, fs::copy_options::recursive);
+
+    const std::array refusals = {
+        Refusal{ "-flto in compileFlags", { "-flto=thin" }, {}, ConfigurationType::Executable, "-flto=thin is not supported on wasm32: LTO code generation drops C++ exception handling" },
+        Refusal{ "-flto in linkFlags", {}, { "-flto" }, ConfigurationType::Executable, "-flto is not supported on wasm32" },
+        Refusal{ "-fno-exceptions", { "-fno-exceptions" }, {}, ConfigurationType::Executable, "-fno-exceptions is not supported on wasm32" },
+        Refusal{ "a shared library", {}, {}, ConfigurationType::LibraryDynamic, "wasm32 shared libraries are not supported" },
+    };
+    for (const Refusal& refusal : refusals) {
+        Configuration app;
+        app.path = project;
+        app.name = "hello";
+        app.outputName = "hello";
+        app.target = "wasm32-wasip1";
+        app.sysroot = (scratch / "no-sysroot").string();
+        app.type = refusal.type;
+        app.compileFlags = refusal.compileFlags;
+        app.linkFlags = refusal.linkFlags;
+        std::array<fs::path, 0> ifaces = {};
+        std::array<fs::path, 1> impls = { "main" };
+        app.GetInterfacesAndImplementations(ifaces, impls);
+        std::unordered_map<fs::path, std::shared_future<BuildResult>> depResults;
+        std::mutex depMutex;
+        const BuildResult r = Build(app, depResults, depMutex);
+        Check(r.result.contains(refusal.expected), std::format("{} is refused with its cause: {}", refusal.what, r.result));
+        Check(!fs::exists(project / "bin") && !fs::exists(project / "build"), std::format("refusing {} leaves no bin/ or build/ behind", refusal.what));
+    }
+
+    fs::remove_all(scratch);
+    if (failures > 0) {
+        std::println(std::cerr, "{} assertions failed", failures);
+        return 1;
+    }
+    return 0;
+}