Spec M37 — mc hosted on Linux (owner's need, 2026-09-04: the cloud only runs Linux)

Goal: the self-hosted compiler runs natively on Linux (x86_64 and aarch64), bootstraps there to its own fixed point, and make check runs its portable subset on a Linux host, so development can happen on a Linux machine with no macOS in the loop after the first published binary.

Host layer #

• src/host_macos.mc / src/host_linux.mc: everything the compiler needs from the OS that differs: posix_spawnp/waitpid (musl has them), the environment (_NSGetEnviron on macOS; on Linux main(argc, argv, envp) — the third argument musl's crt1.o passes — stored in a global at start), mmap flags, O_* values, the default system layer name for taught compilers (sys vs sys_linux), HOST_OS/HOST_ARCH constants, and the default target when mc.toml has none (host = target).
• src/core.mc stays host-neutral; src/mc.mc (macOS host) and src/mc_linux.mc (Linux host) differ only in which host file they include. The bundle carries both hosts (<mc/host_macos>, <mc/host_linux>) and <mc/core>; a taught compiler built by a Linux-hosted mc includes the Linux host automatically (the driver knows HOST_OS).
• Backends on a Linux host: elf-obj/elf-obj-x86_64 (native), macho/macho-exe (cross to macOS — the Mach-O writers are plain byte writers and keep working). No direct executable on Linux: the host links with ld.lld through [linker]; scripts/link-linux.sh OUT IN.o wraps ld.lld + musl sysroot for the bootstrap chain.

Cross-build and bootstrap #

• From macOS: build/mc1 build src --config src/mc.linux-x86_64.toml (and -aarch64) produces build/mc-linux-x86_64 / build/mc-linux-arm64 (ELF, musl static, ld.lld), tested in Docker: mc-linux src/mc_linux.mc -o mc2l.o equals what the Linux fixed point produces, and mc-linux src/mc.mc -o x.o (a Mach-O object made on Linux) is byte-identical to macOS mc2.o.
• On Linux: scripts/bootstrap-linux.sh [SEED]: SEED = a Linux mc binary (a release asset mc-<ver>-linux-<arch>.tar.gz fetched by the script when absent, checksum verified, or a locally cross-built one); chain seed src/mc_linux.mc -> mc1l.o -> link -> mc1l -> mc2l.o -> mc2l -> mc3l.o; criterion cmp mc2l.o mc3l.o; golden tests/golden/mc2-linux-<arch>.sha256.
• make check on a Linux host (HOST := $(shell uname -s) in the Makefile): runs test (link via ld.lld), check-lex/ast/asm (mc1l vs mc2l), check-bundle, check-toml, check-build, check-limits, check-lang, check-conc (its runtime gains the POSIX semaphore path; the dispatch path stays macOS), check-examples when sqlite3 dev files exist, check-docs, site; skips with a clear line: test-exe, check-standalone's Mach-O half, check-surface's --exe cases, check-desktop, bootstrap (macOS chain) — replaced by bootstrap-linux.
• make stage0 still builds the C seed on Linux with clang (it emits Mach-O only); documented as "the seed is macOS-first; Linux bootstraps from a published mc".

CI and releases #

• ci.yml gains, on both Linux runners, a job mc on linux/<arch> host: download the cross-built mc-linux-<arch> from the macOS job, run scripts/bootstrap-linux.sh and the Linux make check subset natively. Required checks after merge.
• release.yml: the linux-arm64 and linux-x86_64 matrix entries are enabled: cross-built on macos-15, verified on the Linux runners (the same jobs), packaged with sha256 like the macOS asset.

Docs #

docs/guide/90-linux-host.md (clone, fetch the seed, make check, what is skipped and why, cloud workflow), docs/bootstrap.md § Linux chain, docs/ci.md, docs/build.md, CLAUDE.md.

Acceptance #

Docker linux/x86_64 and linux/arm64: bootstrap-linux.sh reaches the fixed point; the cross proof (mc-linux src/mc.mc == macOS mc2.o) holds; Linux make check subset green; CI jobs green on both runners; release assets for the two Linux targets produced from a tag build.

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