test(hostflags): pin the rendered flag strings

Byte-equality with what earlier releases emitted is a compatibility surface
here, not an implementation detail: stdmod folds its compile command into
std_build_commands and the std cache directory name is derived from the
metadata containing it, so a reordered flag invalidates every user's std BMIs.

Verifying that by inspecting the cache after a build turned out to be
unreliable — the cache is shared and accumulates entries from every toolchain
used since, so the first check compared stale entries alongside fresh ones and
passed while the string had in fact moved. These tests assert the spelling
directly from a synthetic model, deterministically, and keep asserting it.

Author: Sunrisepeak

tests/unit/test_hostflags.cpp

@@ -79,6 +79,56 @@ TEST(HostFlags, ProducerAnswersForEveryFamily) {
     }
 }
 
+// ── The rendered string must not move ───────────────────────────────────────
+//
+// stdmod folds its compile command into `std_build_commands`, and the std
+// cache DIRECTORY NAME is derived from the metadata containing it. Reordering
+// a flag therefore invalidates every user's std BMIs for no behavioural gain
+// — so the exact spelling is a compatibility surface, not an implementation
+// detail. This pins it; the first attempt at this refactor moved
+// `-stdlib=libc++` after the include flags and would have shipped exactly
+// that invalidation.
+TEST(HostFlags, ClangCfgBypassStringIsStable) {
+    mcpp::toolchain::ClangDriverModel dm;
+    dm.hasCfg = true;
+    dm.cxxIncludes = { "/llvm/include/c++/v1", "/llvm/include/tgt/c++/v1" };
+
+    EXPECT_EQ(dm.compile_flags(mcpp::toolchain::no_escape),
+              " --no-default-config -nostdinc++"
+              " -isystem/llvm/include/c++/v1"
+              " -isystem/llvm/include/tgt/c++/v1");
+
+    // With the stdlib selection the std module has always asked for, it lands
+    // immediately after -nostdinc++ — not at the end.
+    EXPECT_EQ(mcpp::toolchain::render_tokens(
+                  dm.compile_tokens(mcpp::toolchain::no_escape, true)),
+              " --no-default-config -nostdinc++ -stdlib=libc++"
+              " -isystem/llvm/include/c++/v1"
+              " -isystem/llvm/include/tgt/c++/v1");
+}
+
+TEST(HostFlags, LinkModelStringsAreStable) {
+    mcpp::toolchain::ToolchainLinkModel lm;
+    lm.mode = mcpp::toolchain::CLibMode::PayloadFirst;
+    lm.clangDriver = true;
+    lm.crtDir = "/glibc/lib";
+    lm.libDirs = { "/glibc/lib" };
+    lm.loader = "/glibc/lib/ld.so";
+    lm.systemIncludes = { "/glibc/include" };
+
+    EXPECT_EQ(lm.compile_flags(mcpp::toolchain::no_escape),
+              " -isystem/glibc/include");
+    EXPECT_EQ(lm.link_flags(mcpp::toolchain::no_escape),
+              " -B/glibc/lib -L/glibc/lib -Wl,-rpath,/glibc/lib"
+              " -Wl,--dynamic-linker=/glibc/lib/ld.so");
+
+    // GCC takes -idirafter so libstdc++'s #include_next wrappers can still
+    // reach libc.
+    lm.clangDriver = false;
+    EXPECT_EQ(lm.compile_flags(mcpp::toolchain::no_escape),
+              " -idirafter/glibc/include");
+}
+
 // ── bmi_reference_tokens ────────────────────────────────────────────────────
 //
 // The traits store these for the ninja STRING channel, where one word vs two