# rust-frontier — self-hosting the builder's rust toolchain

The last frontier of `selfhost-verify` variante (b): replace the Alpine
`rustc/cargo 1.91.1` in `/toolchain` with a hammer-built rust, via the **purist
mrustc chain** (no prebuilt rustc binary). Path:

    mrustc → rustc 1.90.0 (builds bundled LLVM) → 1.91.0 → 1.91.1

These scripts + patch are the working artifacts. The mrustc tree itself lives in
`.scratch/mrustc` (gitignored — it's a large third-party clone, upstream master
`be69c74`); only our deltas are tracked here.

## Files

- **`run_rustc.patch`** — patch against mrustc `be69c74` for `run_rustc/Makefile`
  and `run_rustc/rustc_proxy.sh`. Apply from the mrustc root:
  `git -C .scratch/mrustc apply /path/to/run_rustc.patch`
- **`build-stage0-musl.sh`** — rebuild the mrustc stage0 `rustc`+`cargo` with a
  **musl host triple** (`output-1.90.0-x86_64-unknown-linux-musl/`). Run inside
  the Alpine devfs sandbox (gcc, MRUSTC_TARGET_VER=1.90). Reuses the already-built
  LLVM and `bin/mrustc`.
- **`run-runrustc.sh`** — run a `run_rustc` make target inside the devfs sandbox,
  pointed at the musl-host stage0.

> Both runners hardcode `/home/sergio/hammer` and assume the prepared devfs at
> `.dev-fs/alpine` (256-TLS-key musl loader + g++; see the bootstrap-devfs notes).

## Why the patches exist (the snags)

The lab is **musl-only** (Alpine devfs); the 4/4 inputs (arje-zero/hammerd) are
built `--target x86_64-unknown-linux-musl`. Three coupled problems had to be fixed
to bootstrap a usable rust there:

1. **stage-1 std built for gnu** (`mmap64`/`open64` undefined at link).
   `minicargo` was invoked without `--target`, so it used mrustc's default **gnu**
   triple — and gnu std references glibc's LFS64 symbols that musl 1.2.5 doesn't
   export. Fix: pass `--target $(RUSTC_TARGET)` to minicargo (Makefile:162) and to
   the stage-1 smoke-test (Makefile:172). A correct musl std uses the **base** libc
   symbols (`open`/`mmap`/…), matching Alpine's reference std.

2. **self-contained CRT absent.** musl's default link is `-static-pie` against a
   `lib/rustlib/<t>/lib/self-contained/` CRT dir that the mrustc sysroot never
   populates. Fix: `-C target-feature=-crt-static -C link-self-contained=no` so the
   Alpine gcc supplies crt1/crti/crtn + crtbeginS/crtendS and links dynamically
   against the system musl (RUSTFLAGS in Makefile:111, plus the per-link sites).

3. **host-triple = gnu on a musl system** (the deep one). cargo compiles
   build-scripts and **proc-macros** for the *host*; mrustc's stage0 rustc has its
   host triple baked as `x86_64-unknown-linux-gnu` (not runtime-overridable). A
   proxy `--target musl` inject fixes build-script *bins*, but **proc-macros are
   dlopen'd and must match the host triple structurally** — injection makes them
   target artifacts that rustc rejects. Aliasing musl rlibs as gnu also fails:
   rustc reads the triple from rlib *metadata* (`E0461`), not the path.

   **The only fix is a stage0 rustc whose host triple is musl** — that's what
   `build-stage0-musl.sh` produces (`CFG_COMPILER_HOST_TRIPLE` via `RUSTC_TARGET`,
   `--target` via `MRUSTC_TARGET`). Then host==target==musl: cargo doesn't
   cross-compile, proc-macros load natively, and `rustc_proxy.sh` only needs the
   CRT flags (no `--target` inject).

   Gotcha: do **not** pass `OUTDIR_SUF` on make's command line — a command-line var
   can't be reassigned by the makefile, so the `-musl` suffix append is skipped and
   the build contaminates the gnu output dir. Let it default.

4. **minicargo OUT_DIR host/target split** (only when building the musl stage0's
   cargo). With `RUSTC_TARGET` set, minicargo runs each cargo build-script under
   `cargo-build/host/build_X/`, but the *target* crate's `include!` /
   `include_bytes!(concat!(env!("OUT_DIR"), "/.."))` resolves `OUT_DIR` to
   `cargo-build/build_X/` (no `host/`, empty) → mrustc aborts with signal 6. Hit
   `libsqlite3-sys` (`bindgen.rs`) and the `cargo` binary (`man.tgz`). The gnu
   build never split (single triple → single dir). Since host==target the script
   outputs are bit-identical, so `build-stage0-musl.sh` symlinks the target dir to
   the host dir (`symlink_outdirs`). Late sys-crates (cargo, curl, openssl,
   libgit2-sys, libssh2-sys) run their scripts near the end, so it's applied in a
   retry loop around the incremental cargo build.

## Workflow

```sh
# 1. apply the run_rustc patch into the mrustc clone
git -C .scratch/mrustc apply scripts/rust-frontier/run_rustc.patch

# 2. rebuild stage0 rustc+cargo with a musl host triple (~hours, reuses LLVM)
.scratch/build-stage0-musl.sh            # -> output-1.90.0-x86_64-unknown-linux-musl/

# 3. build the optimised rustc + cargo via run_rustc (stage 1/2/3)
.scratch/run-runrustc.sh all

# 4. (next) climb 1.90.0 → 1.91.0 → 1.91.1, then swap rust into /toolchain
```

Full diagnostic history is in the `selfhost-verify-rust-frontier` memory.