std::string targetTriple; // resolved canonical triple; "" = host

std::string profile; // effective profile name (dev/release/…)

std::vector<std::string> features; // active feature closure of the package

// Artifact home (bin/cache/out). Empty → <root>/target/.build-mcpp (the

// root-project default). Dependencies MUST point this into the CONSUMING

// project's tree — a registry package root is shared and may be read-only.

std::filesystem::path artifactsDir;

// Base for resolving relative `mcpp:generated=` paths. Empty → root (the

// root-project contract, unchanged). Dependencies point this at OUT_DIR so

// a shared package root is never written to.

std::filesystem::path genBase;

// mcpp#241: this package's resolved dependencies, as (name → dir) pairs.

// The caller emits each dep under BOTH its canonical package name and its

// short (namespace-stripped) name, so a build.mcpp can locate a dependency's

// payload (e.g. a data-asset package) via either spelling. Emitted as

// MCPP_DEP_<SANITIZED_NAME>_DIR (same sanitizer as MCPP_FEATURE_) instead of

// reverse-engineering the store layout.

std::vector<std::pair<std::string, std::filesystem::path>> depDirs;

// #355: HOST tools this package asked its dependencies for, as

// (env var name → absolute path to the executable) pairs. The caller has

// already resolved them (built, taken from the store, or an override), so

// this is purely the delivery channel. Rides the same contract env, hence

// the same re-run key: a rebuilt tool re-runs the program that uses it,

// with no `rerun-if-changed` needed from the author.

std::vector<std::pair<std::string, std::string>> toolPaths;

// #355 step 5: dependency-provided modules to compile FOR THE HOST and make

// importable from this build.mcpp — reusable build rules distributed as

// ordinary mcpp packages (`import mcpp.rules.protobuf;`) instead of a

// second, non-C++ rule DSL.

//

// (logical module name, absolute path to its interface unit). Compiled with

// the SAME flags as build.mcpp itself, in the same directory, which is what

// makes the BMI usable at all — see DependencySpec::hostModule.

std::vector<std::pair<std::string, std::filesystem::path>> hostModules;

std::string_view macosDeploymentTarget) {

// One driver invocation compiles AND links build.mcpp, so it needs both

// sides. Both come from mcpp.toolchain.hostflags — the same producer

// flags.cppm and the std module build use. This function used to hand-write

// the whole assembly, which is how it kept missing what the main build

// already knew (quoting, the macOS deployment target, the MSVC dialect);

// see 2026-08-02-host-compile-single-producer-design.md.

mcpp::toolchain::HostFlagOptions opt;

// The host helper keeps TRUSTING clang's cfg on macOS/Windows: the macOS

// link needs the libc++abi/unwind handling the main build's

// needs_explicit_libcxx path owns, and duplicating it here produced

// undefined __cxa_* / __gxx_personality_v0.

opt.cfgBypass = mcpp::toolchain::HostFlagOptions::CfgBypass::LinuxOnly;

opt.clangStdlibSelect = true;

// binutils -B so the driver finds ld/as (GCC; musl and MinGW ship their own).

opt.binutilsPrefix = !mcpp::toolchain::is_musl_target(tc)

&& !mcpp::toolchain::is_mingw_target(tc);

// The helper is exec'd outside anything mcpp controls, so it must be able

// to find the toolchain's private runtime libs itself.

opt.runtimeLibDirs = true;

opt.macosDeploymentTarget = std::string(macosDeploymentTarget);

const mcpp::toolchain::PathEscape plain = mcpp::toolchain::no_escape;

auto f = mcpp::toolchain::host_compile_tokens(tc, opt, plain);

for (auto& t : mcpp::toolchain::host_link_tokens(tc, opt, plain))

f.push_back(t);

return f;

for (auto& c : f)

c = std::isalnum(static_cast<unsigned char>(c))

? static_cast<char>(std::toupper(static_cast<unsigned char>(c))) : '_';

return f;

std::vector<std::pair<std::string, std::string>> e;

auto hostT = mcpp::toolchain::triple::host_triple().str();

e.emplace_back("MCPP_TARGET", env.targetTriple.empty() ? hostT : env.targetTriple);

// Convenience splits of the resolved target (Cargo CARGO_CFG_TARGET_*

// parity): parsed ONCE here through the canonical triple parser so every

// build.mcpp stops hand-splitting MCPP_TARGET. MCPP_TARGET_ENV is "" when

// the triple has no env segment (macOS); all three are "" for an

// escape-hatch triple outside the canonical vocabulary. They ride the

// same env vector, so contract_hash folds them into the re-run key.

{

mcpp::toolchain::triple::Triple t{};

if (env.targetTriple.empty()) t = mcpp::toolchain::triple::host_triple();

else if (auto p = mcpp::toolchain::triple::parse(env.targetTriple)) t = *p;

e.emplace_back("MCPP_TARGET_OS", t.os);

e.emplace_back("MCPP_TARGET_ARCH", t.arch);

e.emplace_back("MCPP_TARGET_ENV", t.env);

}

e.emplace_back("MCPP_HOST", hostT);

e.emplace_back("MCPP_PROFILE", env.profile);

e.emplace_back("MCPP_OUT_DIR", outDir.string());

e.emplace_back("MCPP_MANIFEST_DIR", root.string());

std::string csv;

for (auto const& f : env.features) {

if (!csv.empty()) csv += ',';

csv += f;

e.emplace_back("MCPP_FEATURE_" + sanitize_feature_env(f), "1");

}

e.emplace_back("MCPP_FEATURES", csv);

// mcpp#241: per-dependency payload dir, under MCPP_DEP_<SANITIZED_NAME>_DIR

// (same sanitizer as MCPP_FEATURE_ — predictable from the manifest, not

// store internals). Two distinct dep names can sanitize to the same var

// (e.g. `foo.bar` vs `foo-bar`, or a bare `zlib` vs another dep's short

// `zlib`); guard so a silent last-wins can't hand one dep another's dir —

// keep the first and warn on a conflicting value.

std::map<std::string, std::string> depVarValue;

for (auto const& [name, dir] : env.depDirs) {

auto var = "MCPP_DEP_" + sanitize_feature_env(name) + "_DIR";

auto [it, inserted] = depVarValue.try_emplace(var, dir.string());

if (inserted) {

e.emplace_back(var, dir.string());

} else if (it->second != dir.string()) {

mcpp::ui::warning(std::format(

"build.mcpp: dependency name collides on {} (kept '{}', ignored "

"'{}') — rename one dependency to disambiguate", var,

it->second, dir.string()));

}

}

// #355: MCPP_DEP_<PKG>_BIN_<TOOL> — absolute path to a host tool the

// consumer declared via `tools = [...]`. A PATH rather than a directory:

// the store keys an entry per (package, target), the typed reader can

// append the platform's exe suffix itself, and a tool's adjacent DATA

// (protoc's well-known .proto files, say) lives in the package tree, which

// dep_dir() already exposes.

for (auto const& [var, path] : env.toolPaths) {

auto [it, inserted] = depVarValue.try_emplace(var, path);

if (inserted) {

e.emplace_back(var, path);

} else if (it->second != path) {

mcpp::ui::warning(std::format(

"build.mcpp: tool name collides on {} (kept '{}', ignored '{}')",

var, it->second, path));

}

}

return e;

std::string s;

for (auto const& [k, v] : e) { s += k; s += '='; s += v; s += '\n'; }

return mcpp::toolchain::hash_string(s);

std::error_code ec;

auto rel = bdir.lexically_relative(root);

if (rel.empty()) return {};

auto first = rel.begin();

if (first == rel.end()) return {};

auto name = first->string();

if (name == "..") return {}; // outside the project

return name;

const std::string& programHash,

const std::string& compilerHash, const std::string& ctxHash,

const Directives& d) {

std::ofstream os(cache_path(bdir), std::ios::trunc);

if (!os) return; // best-effort: a failed cache write only loses the optimization

// The epoch guards against a semantics change: the `d` lines below are

// replayed verbatim on a hit, so if this mcpp interprets a directive

// differently than the one that wrote them, the entry must not be reused.

os << "epoch " << dirs::kCacheEpoch << '\n';

os << "program " << programHash << '\n';

os << "compiler " << compilerHash << '\n';

os << "ctx " << ctxHash << '\n';

// The root the relative entries below are resolved against. Recorded so a

// reader that is not prepare_build — the fast-path glob check — can

// evaluate a DEPENDENCY's cache against the dependency's own tree.

os << "root " << root.string() << '\n';

for (auto const& f : d.at(Slot::RerunFiles))

os << "in " << mcpp::toolchain::hash_file(abs_against_root(root, f)) << ' ' << f << '\n';

for (auto const& e : d.at(Slot::RerunEnv))

os << "env " << mcpp::toolchain::hash_string(env_value(e)) << ' ' << e << '\n';

// #359: a glob input's fingerprint is the SET of matching paths. Same

// record shape as `in`/`env`; the value is computed by the table's owner.

{

auto outName = output_dir_name(root, bdir);

for (auto const& g : d.at(Slot::RerunGlobs))

os << "glob " << dirs::glob_fingerprint(root, g, outName) << ' '

<< g << '\n';

}

dirs::serialize(os, d);

int epoch = 0; // 0 = pre-epoch entry (written before this guard existed)

std::string programHash;

std::string compilerHash;

std::string ctxHash; // contract env (target/profile/features/out-dir)

std::string rootPath; // what relative entries are resolved against

std::vector<std::pair<std::string, std::string>> inputs; // (hash, path)

std::vector<std::pair<std::string, std::string>> envs; // (hash, name)

std::vector<std::pair<std::string, std::string>> globs; // (hash, pattern)

Directives directives;

// A `d` record whose tag this mcpp does not know — the entry was written

// by a newer mcpp. Replaying the rest would apply a strict subset of what

// the program asked for, so the whole entry is discarded instead.

bool unknownRecord = false;

bool loaded = false;

CacheRecord r;

std::ifstream is(cache_path(bdir));

if (!is) return r;

std::string line;

while (std::getline(is, line)) {

if (line.empty()) continue;

auto sp = line.find(' ');

if (sp == std::string::npos) continue;

std::string tag = line.substr(0, sp);

std::string rest = line.substr(sp + 1);

if (tag == "epoch") {

int n = 0;

if (std::from_chars(rest.data(), rest.data() + rest.size(), n).ec == std::errc{})

r.epoch = n;

}

else if (tag == "program") r.programHash = rest;

else if (tag == "compiler") r.compilerHash = rest;

else if (tag == "ctx") r.ctxHash = rest;

else if (tag == "root") r.rootPath = rest;

else if (tag == "in" || tag == "env" || tag == "glob") {

auto sp2 = rest.find(' ');

if (sp2 == std::string::npos) continue;

std::string h = rest.substr(0, sp2), name = rest.substr(sp2 + 1);

(tag == "in" ? r.inputs : tag == "env" ? r.envs : r.globs)

.emplace_back(h, name);

} else if (tag == "d") {

auto sp2 = rest.find(' ');

if (sp2 == std::string::npos) continue;

std::string kind = rest.substr(0, sp2), val = rest.substr(sp2 + 1);

if (!dirs::accept_cache_record(r.directives, kind, val))

r.unknownRecord = true;

}

}

r.loaded = true;

return r;

const std::string& programHash, const std::string& compilerHash,

const std::string& ctxHash) {

if (!c.loaded) return false;

if (c.epoch != dirs::kCacheEpoch) return false; // pre-epoch entries rerun once

if (c.unknownRecord) return false;

if (c.programHash != programHash) return false;

if (c.compilerHash != compilerHash) return false;

if (c.ctxHash != ctxHash) return false; // pre-G3 caches (no ctx line) rerun once

for (auto const& [h, path] : c.inputs)

if (mcpp::toolchain::hash_file(abs_against_root(root, path)) != h) return false;

for (auto const& [h, name] : c.envs)

if (mcpp::toolchain::hash_string(env_value(name)) != h) return false;

// #359: the path SET behind each declared glob. A file appearing or

// disappearing changes it; editing one does not (that is what the `in`

// entries above are for).

if (!c.globs.empty()) {

auto outName = output_dir_name(root, bdir);

for (auto const& [h, pattern] : c.globs)

if (dirs::glob_fingerprint(root, pattern, outName) != h) return false;

}

// A declared output that vanished invalidates the cache. Driven off the

// table's mustExistAfterRun so a future output-shaped directive is covered

// without editing this function.

for (auto const& def : dirs::kTable) {

if (!def.mustExistAfterRun) continue;

for (auto const& p : c.directives.at(def.slot))

if (!fs::exists(abs_against_root(root, p))) return false;

}

return true;

mcpp::manifest::Manifest& m,

const fs::path& root,

const fs::path& hostCompiler,

const mcpp::toolchain::Toolchain& tc,

const mcpp::manifest::CppStandardConfig& cppStandard,

const BuildProgramEnv& env) {

fs::path src = root / "build.mcpp";

std::error_code ec;

if (!fs::exists(src, ec)) return {}; // no build program — nothing to do

fs::path bdir = build_dir(root, env);

fs::path outDir = bdir / "out";

auto childEnv = contract_env(root, outDir, env);

std::string ctxHash = contract_hash(childEnv);

// ── Helper self-containment (the single decision point) ─────────────────

// The compiled helper is exec'd by the host OS, outside anything mcpp

// controls — no wrapper, no injected LD_LIBRARY_PATH, no PATH guarantee.

// Making it runnable is a per-platform mechanism, and only two of the four

// need a static link:

// Linux + glibc payload — host_base_flags bakes absolute payload paths

// into -Wl,-rpath; that already closes it, so leave it dynamic.

// Linux + musl (#295) — rpath cannot reach PT_INTERP, an absolute

// /lib/ld-musl-<arch>.so.1 that no payload installs and glibc distros

// do not ship. Only a static link removes the interpreter entirely.

// Windows PE (#299) — PE has no rpath, so a dynamic helper resolves

// libstdc++-6 / libgcc_s / libwinpthread through the process PATH and

// dies with STATUS_DLL_NOT_FOUND unless the user put the toolchain bin

// there by hand. A static link is the only PATH-independent answer.

// macOS — the system libc++ is always present.

// Keep this the ONLY place that decides; the flag is appended to the final

// link argv further down, never to `base`.

const bool muslStaticHelper = mcpp::platform::supports_full_static

&& mcpp::toolchain::is_musl_target(tc);

const bool mingwStaticHelper = mcpp::toolchain::is_mingw_target(tc);

const bool staticHostHelper = muslStaticHelper || mingwStaticHelper;

// Fold the policy into the compiler identity: a helper produced under an

// older link policy must be rebuilt, not reused from the cache.

std::string compilerIdentity = hostCompiler.string();

// Host modules change what the helper links, so they belong in the identity

// the cache keys on — otherwise adding or removing a rule package would

// replay a cached run compiled without it.

for (auto const& [logical, ifacePath] : env.hostModules) {

compilerIdentity += "\nhost-module=";

compilerIdentity += logical;

compilerIdentity += "@";

compilerIdentity += mcpp::toolchain::hash_file(ifacePath);

}

compilerIdentity += "\nbuild-program-link=";

compilerIdentity += muslStaticHelper ? "musl-static-v1"

: mingwStaticHelper ? "mingw-static-v1"

: "default-v1";

std::string programHash = mcpp::toolchain::hash_file(src);

std::string compilerHash = mcpp::toolchain::hash_string(compilerIdentity);

// Fast path: declared inputs + contract unchanged → reapply cached

// directives, no run.

CacheRecord cache = read_cache(bdir);

if (cache_fresh(root, bdir, cache, programHash, compilerHash, ctxHash)) {

dirs::apply(m, cache.directives);

mcpp::ui::info("build.mcpp", "up to date (cached)");

return {};

}

fs::create_directories(outDir, ec); // creates bdir too

// #230: on Windows the capture_exec shell is cmd.exe, which can only launch

// a PE by an executable extension — a bare `.bin` is not in PATHEXT and

// fails to run. Name the compiled program `.exe` there; other platforms keep

// `.bin` (cosmetic — bdir is separate from the `.mcpp` source). Surfaces

// once a workspace build.mcpp member is reached on Windows, after the

// scanner symlink-escape crash fix (df985df) stops masking it as exit 127.

fs::path bin = bdir / (mcpp::platform::is_windows

? "build.mcpp.exe" : "build.mcpp.bin");

// ── Compile build.mcpp with the host toolchain ──────────────────────────

// Spelled by the dialect layer, not concatenated here: the canonical of

// `standard = "c++fly"` / `"c++latest"` is not a valid -std= spelling, so

// the old `"-std=" + canonical` produced `-std=c++fly` and the host compile

// died on an unknown dialect. cppfly::std_flag resolves those against the

// toolchain that will actually run the compile — the host one, here.

std::string std_flag = mcpp::toolchain::cppfly::std_flag(

tc, cppStandard.canonical.empty() ? std::string_view("c++23")

: std::string_view(cppStandard.canonical),

cppStandard.level);

// One resolution of the deployment target, used by every compile below

// and by the std module it asks stdmod to build — they must agree or

// clang rejects the BMI.

const std::string macosDeploymentTarget =

mcpp::platform::macos::deployment_target(

m.buildConfig.macosDeploymentTarget);

auto base = host_base_flags(tc, macosDeploymentTarget);

// The host compile has always been spelled in GNU driver syntax with no

// dialect branch at all — `grep -i msvc` over this file used to hit only

// comments. Under cl.exe every one of `-O0` / `-x c++` / `-static` / `-o`

// is wrong, so the whole build.mcpp path was unusable on a native MSVC

// toolchain regardless of what else was fixed.

const auto& dial = mcpp::toolchain::dialect_for(tc);

const bool msvcHost = dial.id == std::string_view("msvc");

// Only wire the bundled `mcpp` module when build.mcpp actually imports it —

// so the common `#include`-based program compiles exactly as before (no

// -fmodules, cwd = project root). When it does `import mcpp;`, compile the

// module, link its object, and run the build.mcpp compile from `bdir` so GCC

// finds gcm.cache/mcpp.gcm.

std::string srcText;

{ std::ifstream is(src); std::ostringstream ss; ss << is.rdbuf(); srcText = ss.str(); }

bool usesModule = srcText.find("import mcpp") != std::string::npos;

bool usesStdCompat = imports_module(srcText, "std.compat");

bool usesStd = usesStdCompat || imports_module(srcText, "std");

// A rule package's interface is compiled by this same function, so what IT

// imports decides what has to be built just as much as what build.mcpp

// imports. Scanning only build.mcpp made a rule that said `import std;`

// fail with `module 'std' not found` — the std module was never built,

// because the program that triggers the build did not mention it.

for (auto const& [logical, ifacePath] : env.hostModules) {

std::ifstream is(ifacePath);

if (!is) continue; // a missing interface is diagnosed by build_host_module

std::ostringstream ss; ss << is.rdbuf();

const std::string t = ss.str();

if (t.find("import mcpp") != std::string::npos) usesModule = true;

if (imports_module(t, "std.compat")) usesStdCompat = true;

if (imports_module(t, "std")) usesStd = true;

}

usesStd = usesStd || usesStdCompat;

// The toolchain's own environment (MSVC's INCLUDE / LIB / VSLANG, which

// detection synthesized from the located VC tools + Windows SDK). Needed

// by every compile below, the module precompile included.

std::vector<std::pair<std::string, std::string>> compileEnv;

for (auto const& ev : tc.envOverrides)

compileEnv.emplace_back(ev.key, ev.value);

// Named modules dispatch on the same BmiTraits/CommandDialect rows the

// main build uses, so there is no per-family gate here: cl.exe's

// .ifc + /reference works because the table already describes it, not

// because build.mcpp grew a second implementation of it.

std::vector<std::string> moduleFlags;

fs::path mcppModuleObject;

if (usesModule) {

auto mf = build_mcpp_module(bdir, hostCompiler, base, std_flag, tc,

compileEnv);

if (!mf) return std::unexpected(mf.error());

moduleFlags = std::move(mf->useFlags);

mcppModuleObject = std::move(mf->object);

}

// ── `import std;` in build.mcpp ─────────────────────────────────────────

//

// mcpp asks projects to `import std;` everywhere and then made their build

// script fall back to `#include` — the bundled `mcpp` module even says so

// in its own header comment. The std module the main build already uses is

// reusable verbatim: stdmod::ensure_built caches on

// (toolchain × standard × dialect), so for a native build this is a cache

// HIT on the very artifact the project's own TUs import. Only a cross

// build pays for a second one, which is unavoidable — see below.

//

// `tc` here is the HOST toolchain: prepare.cppm's

// host_tc_for_build_program() resolves the spec WITHOUT the --target axis

// and hands it in. That is load-bearing. build.mcpp is compiled AND run on

// the machine doing the build, so a std BMI built for the target would

// produce a helper that cannot execute — the same host≠target mistake the

// mingw-cross work had to fix in four separate places.

std::vector<std::string> stdFlags;

std::vector<std::string> stdObjects;

// GCC finds staged BMIs by cwd; Clang/MSVC get an explicit path flag.

bool stdStagedInBdir = false;

if (usesStd) {

if (!tc.hasImportStd) {

return std::unexpected(std::format(

"build.mcpp uses `import std;` but the host toolchain ({}) "

"ships no std module.\n"

" Use #include in build.mcpp, or switch to a toolchain "

"that provides one.", tc.label()));

}

auto sm = mcpp::toolchain::ensure_built(

tc, cppStandard.canonical, std_flag, macosDeploymentTarget);

if (!sm) {

return std::unexpected(std::format(

"build.mcpp uses `import std;` but the std module could not be "

"built for the host toolchain: {}", sm.error().message));

}

auto traits = mcpp::toolchain::bmi_traits(tc);

if (traits.stdBmiUsePrefix.empty()) {

// GCC: BMIs are found implicitly under <cwd>/gcm.cache, so stage

// the cached ones where the compile will look. Copy rather than

// symlink — this mirrors the main build's staging edge, and a

// stale copy is caught by ensure_built's own cache key.

std::error_code ec;

fs::path gcmDir = bdir / traits.bmiDir;

fs::create_directories(gcmDir, ec);

auto stage = [&](const fs::path& from, std::string_view name)

-> std::expected<void, std::string> {

if (from.empty() || !fs::exists(from)) return {};

fs::path to = gcmDir / std::format("{}{}", name, traits.bmiExt);

fs::copy_file(from, to, fs::copy_options::overwrite_existing, ec);

if (ec) return std::unexpected(std::format(

"staging {} for build.mcpp failed: {}", name, ec.message()));

return {};

};

if (auto r = stage(sm->bmiPath, "std"); !r)

return std::unexpected(r.error());

if (usesStdCompat) {

if (auto r = stage(sm->compatBmiPath, "std.compat"); !r)

return std::unexpected(r.error());

}

// -fmodules may already be present from the `mcpp` module path;

// GCC tolerates the repeat, but keep the argv honest.

if (!usesModule) stdFlags.push_back("-fmodules");

stdStagedInBdir = true;

} else {

// Through bmi_reference_tokens, not string concatenation: the

// traits spell these for the ninja STRING channel, where

// `-fmodule-file=std=<p>` (one word) and `/reference std=<p>`

// (two) are indistinguishable. Concatenating produced a single

// argv element with a space inside it, and cl answered

// "C2230: could not find module 'std'".

for (auto& t : mcpp::toolchain::bmi_reference_tokens(

traits.stdBmiUsePrefix, sm->bmiPath))

stdFlags.push_back(t);

if (usesStdCompat && !sm->compatBmiPath.empty())

for (auto& t : mcpp::toolchain::bmi_reference_tokens(

traits.stdCompatBmiUsePrefix, sm->compatBmiPath))

stdFlags.push_back(t);

}

if (!sm->objectPath.empty() && fs::exists(sm->objectPath))

stdObjects.push_back(sm->objectPath.string());

if (usesStdCompat && !sm->compatObjectPath.empty()

&& fs::exists(sm->compatObjectPath))

stdObjects.push_back(sm->compatObjectPath.string());

}

// #355 step 5: dependency-provided host modules (reusable build rules

// shipped as ordinary packages). Compiled HERE, with `base` and `std_flag`

// — the same flags the build.mcpp compile below gets — because a BMI is

// only usable by a compile that agrees with it. Doing this in a separate

// sub-build would leave that agreement to chance, and disagreement shows

// up as `module X CRC mismatch`, not as a clear error.

//

// AFTER the std block, and that ordering is load-bearing: a rule may

// `import std;` just as build.mcpp may, and it can only do so once the std

// BMI exists and `stdFlags` names it. Compiling rules first — which is what

// 2026.8.5.1 did — handed them an empty `stdFlags` and failed with

// `module 'std' not found`.

std::vector<fs::path> hostModuleObjects;

for (auto const& [logical, ifacePath] : env.hostModules) {

std::vector<std::string> use = moduleFlags;

use.insert(use.end(), stdFlags.begin(), stdFlags.end());

auto hm = build_host_module(bdir, hostCompiler, base, std_flag, tc,

compileEnv, logical, ifacePath, use);

if (!hm) return std::unexpected(hm.error());

for (auto& f : hm->useFlags) {

// GCC's marker is just `-fmodules`, already present when the

// bundled module was built; repeating it is harmless but noisy.

if (std::find(moduleFlags.begin(), moduleFlags.end(), f)

== moduleFlags.end())

moduleFlags.push_back(f);

}

hostModuleObjects.push_back(std::move(hm->object));

}

// `-x c++` is required: the `.mcpp` extension is unknown to the compiler, so

// without it the driver hands build.mcpp to the linker as a linker script.

std::vector<std::string> compileArgv = { hostCompiler.string() };

if (msvcHost) {

// /nologo /EHsc /utf-8 — cl.exe needs these to behave like the other

// two drivers do by default (quiet, exceptions on, UTF-8 sources).

for (auto f : dial.alwaysFlagsArgv) compileArgv.emplace_back(f);

}

compileArgv.push_back(std_flag);

// No optimization: this program runs once per build and its compile time

// is on the critical path. MSVC spells "off" /Od, not /O0.

compileArgv.push_back(msvcHost ? std::string("/Od")

: std::string(dial.optPrefix) + "0");

for (auto& bf : base) compileArgv.push_back(bf);

for (auto& mf : moduleFlags) compileArgv.push_back(mf);

for (auto& sf : stdFlags) compileArgv.push_back(sf);

// The `.mcpp` extension is unknown to every driver, so without this the

// file is handed to the linker as a linker script.

// Per-file where the driver has that form (cl's /Tp), positional

// otherwise. Object files follow on this same command line, and cl's

// global /TP would compile them as C++ source.

if (!dial.perFileCxxPrefix.empty()) {

compileArgv.push_back(std::string(dial.perFileCxxPrefix) + src.string());

} else {

for (auto f : dial.forceCxxLangArgv) compileArgv.emplace_back(f);

compileArgv.push_back(src.string());

}

if (usesModule || !stdObjects.empty() || !hostModuleObjects.empty()) {

// Link the module objects. GNU drivers need the input language reset

// first, or the .o that follows `-x c++` is handed to the frontend as

// C++ source; cl.exe has no `-x` at all and infers from the extension.

// This used to be unconditional and was only harmless while MSVC could

// not reach it — removing that gate made the dead branch live, and cl

// answered with `D9002: ignoring unknown option '-x'`.

if (!msvcHost) { compileArgv.push_back("-x"); compileArgv.push_back("none"); }

if (usesModule) compileArgv.push_back(mcppModuleObject.string());

for (auto& hmo : hostModuleObjects) compileArgv.push_back(hmo.string());

for (auto& so : stdObjects) compileArgv.push_back(so);

}

// Self-contained helper link — see the staticHostHelper doctrine above.

// Deliberately NOT in `base`: that also feeds the bundled module's

// compile/precompile commands, where a link flag has no business (and for

// Clang would perturb the default PIC/PIE codegen of mcpp.o).

if (staticHostHelper) compileArgv.push_back(std::string(dial.staticRuntime));

if (msvcHost) {

// /Fe: takes its value attached, not as a separate argv token.

compileArgv.push_back(std::string(dial.outputExePrefix) + bin.string());

} else {

compileArgv.push_back("-o"); compileArgv.push_back(bin.string());

}

mcpp::ui::info("build.mcpp", "compiling");

// GCC resolves imported BMIs via gcm.cache/ relative to the compile cwd, so

// any compile that imports a module — `mcpp`, `std`, or both — has to run

// from bdir, where they were staged. One condition, not two: a build.mcpp

// that imports only std needs exactly the same cwd as one that imports

// only mcpp. Otherwise the project root is fine.

const bool needsBmiCwd = usesModule || stdStagedInBdir;

std::string compileCwd = needsBmiCwd ? bdir.string() : root.string();

auto cres = mcpp::platform::process::capture_exec(compileArgv, compileEnv,

compileCwd);

if (cres.exit_code != 0) {

return std::unexpected(std::format(

"build.mcpp failed to compile (exit {}):\n{}", cres.exit_code, cres.output));

}

// ── Run it; capture stdout(+stderr) and parse directives ────────────────

// Run with cwd = package root so the program's relative file writes (e.g.

// mcpp:generated sources) land in the project, not in mcpp's invocation

// dir. The MCPP_* contract env is injected into the CHILD only.

//

// Bounded, unlike the compile above. The asymmetry is deliberate and the

// same one `mcpp test` settled on: a COMPILE that runs long is usually

// legitimate (a first-run std module build is minutes), and killing it

// produces a baffling failure; a build PROGRAM that runs long is usually

// stuck — waiting on a network read or spinning — and without a bound the

// whole build hangs with no diagnostic at all.

mcpp::ui::info("build.mcpp", "running");

bool timedOut = false;

auto rres = mcpp::platform::process::capture_exec_deadline(

{bin.string()}, childEnv, dirs::run_timeout(), &timedOut, root.string());

if (timedOut) {

return std::unexpected(std::format(

"build.mcpp for '{}' exceeded its {}s time limit and was killed.\n"

" Raise or disable it with MCPP_BUILD_PROGRAM_TIMEOUT=<seconds> "

"(0 = no limit).\n"

" Output so far:\n{}",

m.package.name.empty() ? std::string("<unnamed package>")

: m.package.name,

dirs::run_timeout().count() / 1000, rres.output));

}

if (rres.exit_code != 0) {

return std::unexpected(std::format(

"build.mcpp exited with {} (build aborted):\n{}", rres.exit_code, rres.output));

}

Directives d;

dirs::accept_output(d, dial, root, rres.output);

// Protocol gate (S1). A program that announced a version this mcpp cannot

// speak, or that emitted an unknown directive INSIDE a version both sides

// speak, is a hard error — "warn and ignore" would turn a missing

// directive into a silently different build. A program that announced

// nothing is a hand-written printf program (the frozen surface) and keeps

// the historical warn-and-ignore behaviour.

if (auto perr = dirs::protocol_error(d)) {

return std::unexpected(*perr);

}

// Refuse a malformed action BEFORE applying anything: a half-applied

// action set is worse than none, and an action that silently does not

// exist surfaces as a missing generated source three edges away.

if (auto aerr = dirs::action_error(d); !aerr.empty()) {

return std::unexpected(aerr);

}

if (d.protocol == 0) {

for (auto const& k : d.unknownKeys)

mcpp::ui::warning(std::format(

"build.mcpp: ignoring unknown directive 'mcpp:{}'", k));

}

// Dependency mode (genBase set): relative `generated=` paths resolve

// against OUT_DIR-style genBase, not the (possibly read-only, shared)

// package root — rewrite them to absolute before validation/apply/cache.

// `source=` paths are NOT rewritten: they name pre-existing files in the

// package tree (MCPP_MANIFEST_DIR-relative), never OUT_DIR products.

if (!env.genBase.empty()) {

for (auto& g : d.at(Slot::Generated)) {

fs::path gp(g);

if (gp.is_relative()) g = (env.genBase / gp).lexically_normal().string();

}

}

// Declared-output contract, driven off the table's mustExistAfterRun:

// generated= — the program said it wrote this file; if it did not, the

// build would fail far away as a glob no-match.

// source= — the program SELECTED a pre-existing file (payload /

// vendored tree); a missing one is a typo or a broken

// payload, surfaced now.

for (auto const& def : dirs::kTable) {

if (!def.mustExistAfterRun) continue;

for (auto const& p : d.at(def.slot)) {

if (fs::exists(abs_against_root(root, p))) continue;

return std::unexpected(std::format("build.mcpp {} '{}' {}",

def.missingPrefix, p, def.missingSuffix));

}

}

dirs::apply(m, d);

write_cache(bdir, root, programHash, compilerHash, ctxHash, d);

return {};

std::error_code ec;

const fs::path base = projectRoot / "target" / ".build-mcpp";

if (!fs::exists(base, ec)) return false;

// Depth-bounded: the root's cache sits at depth 0 and a dependency's at

// `deps/<pkg>/` (depth 2). Bounding it keeps this off the generated-output

// tree under `out/`, which can hold thousands of files and never holds a

// cache.

fs::recursive_directory_iterator it(

base, fs::directory_options::skip_permission_denied, ec);

if (ec) return false;

for (; it != fs::recursive_directory_iterator(); it.increment(ec)) {

if (ec) break;

if (it.depth() >= 3) { it.disable_recursion_pending(); continue; }

if (it->path().filename() != "build.mcpp.cache") continue;

auto rec = read_cache(it->path().parent_path());

if (!rec.loaded || rec.globs.empty() || rec.rootPath.empty()) continue;

fs::path recRoot{rec.rootPath};

auto outName = output_dir_name(recRoot, it->path().parent_path());

for (auto const& [h, pattern] : rec.globs)

if (dirs::glob_fingerprint(recRoot, pattern, outName) != h) return true;

}

return false;