feat(build): L3 build.mcpp — native imperative build program (v0.0.78)#190
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A project-local build.mcpp (C++, Zig build.zig / Cargo build.rs model) compiled with the host toolchain and run before the main build. It emits stdout mcpp: directives that augment the build; a declared-input cache re-runs it only when its source/inputs/env change (the documented fix for the .mcpp_ok blind spot). - src/build/build_program.cppm: new module — compile+run, directive parser (cxxflag/cflag/link-lib/link-search/cfg/generated/rerun-if-changed/ rerun-if-env-changed), declared-input cache, apply to buildConfig - prepare.cppm: invoke after toolchain detect + L1 cfg merge, before modgraph scan; skipped under cross --target (host build/run). -x c++ so the .mcpp extension compiles as C++ - tests/e2e/89_build_mcpp.sh: define+generated source reach the build; cache short-circuits unchanged re-run; changed env forces re-run - docs/07-build-mcpp.md (+zh): user guide; design doc + version bump 0.0.78 🤖 Generated with [Claude Code](https://claude.com/claude-code)
CI e2e (fresh sandbox) failed compiling build.mcpp: a bare 'g++ file -o bin' can't find crt/libc without the toolchain sysroot wiring the main build adds (it works only on warm dev boxes). Pass the host subset from the resolved Toolchain (host_base_flags): GCC --sysroot (or glibc-payload -idirafter/-B/-L), binutils -B, link-runtime -L/-rpath; Clang trusts its .cfg. Mirrors flags.cppm's GCC branch (build.mcpp is host-only, so only native cases needed). Also move build.mcpp's binary + declared-input cache to target/.build-mcpp/ (per project direction: artifacts under target/, not the project tree). design doc: host-flags rationale + typed import mcpp; library direction.
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What
Adds
build.mcpp— a project-local native imperative build program (Zigbuild.zig/ Cargobuild.rsmodel, but in C++, so no second language and it dogfoods mcpp). mcpp compiles it with the host toolchain and runs it before the main build; it emits stdoutmcpp:directives that augment the build.This completes the manifest-environment/platform design's L3 phase (after L1 cfg flags 0.0.74, L1b cfg deps 0.0.75, L-1
[xlings]env 0.0.77).Directives (Discipline 1 — structured output, not global mutation)
cxxflag/cflag/link-lib/link-search/cfg(→-D) /generated=<src>/rerun-if-changed=<path>/rerun-if-env-changed=<VAR>. Non-mcpp:lines are ignored. The program requests leaf build edges; it cannot add a registry dependency (the graph stays declarative inmcpp.toml).Declared-input cache (Discipline 2 — fixes the
.mcpp_okblind spot)The program is not re-run every build. Its directives + declared inputs are cached at
.mcpp/build.mcpp.cache; it re-runs only when the source, toolchain, a declaredrerun-if-changedfile, a declaredrerun-if-env-changedvar, or a missinggeneratedoutput says so. Applied directives flow intobuildConfig.{c,cxx,ld}flags→ fingerprint and generated sources → modgraph, so the main build stays correctly incremental.Constraints
[target.'cfg(...)']tables).--targetit is skipped with a warning (host-toolchain-for-cross is a follow-up).-x c++so the.mcppextension compiles as C++ (see design doc's forward note:.mcppas a first-class C++ extension inside mcpp projects).Files
src/build/build_program.cppm(new module), wired insrc/build/prepare.cppmafter toolchain detect + L1 cfg merge, before the modgraph scan.tests/e2e/89_build_mcpp.sh— define + generated source reach the build; cache short-circuits an unchanged re-run; a changed declared env forces re-run.docs/07-build-mcpp.md(+docs/zh/), indexed in both READMEs..agents/docs/2026-06-30-l3-build-mcpp-implementation-design.md; version → 0.0.78.Verified locally (mcpp 0.0.78)
89_build_mcpppasses; focused regression set (02 new/build/run, 04 incremental, 85/86 cfg flags/deps, 87 default-profile, 88 xlings env) all green.🤖 Generated with Claude Code