1# rust-frontier — self-hosting the builder's rust toolchain
2
3The last frontier of `selfhost-verify` variante (b): replace the Alpine
4`rustc/cargo 1.91.1` in `/toolchain` with a hammer-built rust, via the **purist
5mrustc chain** (no prebuilt rustc binary). Path:
6
7    mrustc → rustc 1.90.0 (builds bundled LLVM) → 1.91.0 → 1.91.1
8
9These scripts + patch are the working artifacts. The mrustc tree itself lives in
10`.scratch/mrustc` (gitignored — it's a large third-party clone, upstream master
11`be69c74`); only our deltas are tracked here.
12
13## Files
14
15- **`run_rustc.patch`** — patch against mrustc `be69c74` for `run_rustc/Makefile`
16  and `run_rustc/rustc_proxy.sh`. Apply from the mrustc root:
17  `git -C .scratch/mrustc apply /path/to/run_rustc.patch`
18- **`build-stage0-musl.sh`** — rebuild the mrustc stage0 `rustc`+`cargo` with a
19  **musl host triple** (`output-1.90.0-x86_64-unknown-linux-musl/`). Run inside
20  the Alpine devfs sandbox (gcc, MRUSTC_TARGET_VER=1.90). Reuses the already-built
21  LLVM and `bin/mrustc`.
22- **`run-runrustc.sh`** — run a `run_rustc` make target inside the devfs sandbox,
23  pointed at the musl-host stage0.
24- **`bootstrap.toml`** — rust's own bootstrap config for the climb hop (1.90.0 →
25  1.91.0): `build`/`host`/`target = x86_64-unknown-linux-musl`, stage0
26  `rustc`/`cargo` = the run_rustc musl-host 1.90.0 prefix, external LLVM 20.1.8
27  reused via `llvm-config` (clears bootstrap's `>=19` gate, no LLVM rebuild),
28  `crt-static = false` (dynamic link against the devfs system musl, like Alpine
29  and the run_rustc toolchain), `rpath = true`. Copy it into the rust source root.
30- **`run-xpy.sh`** — run rust's `x.py` inside the devfs sandbox, binding the
31  source at `/src`, the install prefix at `/out`, and the mrustc tree at `/mrustc`
32  (so the stage0 + `llvm-config` paths in `bootstrap.toml` resolve).
33  Usage: `run-xpy.sh <SRCDIR> <PREFIXDIR> -- build --stage 2 compiler/rustc library/std`.
34
35> The runners hardcode `/home/sergio/hammer` and assume the prepared devfs at
36> `.dev-fs/alpine` (256-TLS-key musl loader + g++; see the bootstrap-devfs notes).
37
38## Why the patches exist (the snags)
39
40The lab is **musl-only** (Alpine devfs); the 4/4 inputs (arje-zero/hammerd) are
41built `--target x86_64-unknown-linux-musl`. Three coupled problems had to be fixed
42to bootstrap a usable rust there:
43
441. **stage-1 std built for gnu** (`mmap64`/`open64` undefined at link).
45   `minicargo` was invoked without `--target`, so it used mrustc's default **gnu**
46   triple — and gnu std references glibc's LFS64 symbols that musl 1.2.5 doesn't
47   export. Fix: pass `--target $(RUSTC_TARGET)` to minicargo (Makefile:162) and to
48   the stage-1 smoke-test (Makefile:172). A correct musl std uses the **base** libc
49   symbols (`open`/`mmap`/…), matching Alpine's reference std.
50
512. **self-contained CRT absent.** musl's default link is `-static-pie` against a
52   `lib/rustlib/<t>/lib/self-contained/` CRT dir that the mrustc sysroot never
53   populates (also needs static `-lunwind`). Fix: `-C target-feature=-crt-static
54   -C link-self-contained=no` so the Alpine gcc supplies crt1/crti/crtn +
55   crtbeginS/crtendS and links dynamically against the system musl (RUSTFLAGS in
56   Makefile:111, plus **both** hello_world smoke-test rules: the stage-1 one at
57   Makefile:172 *and* the final fully-bootstrapped one at Makefile:262 — the latter
58   only surfaces once stage-2/3 actually completes, i.e. after snag #3 is fixed).
59
603. **host-triple = gnu on a musl system** (the deep one). cargo compiles
61   build-scripts and **proc-macros** for the *host*; mrustc's stage0 rustc has its
62   host triple baked as `x86_64-unknown-linux-gnu` (not runtime-overridable). A
63   proxy `--target musl` inject fixes build-script *bins*, but **proc-macros are
64   dlopen'd and must match the host triple structurally** — injection makes them
65   target artifacts that rustc rejects. Aliasing musl rlibs as gnu also fails:
66   rustc reads the triple from rlib *metadata* (`E0461`), not the path.
67
68   **The only fix is a stage0 rustc whose host triple is musl** — that's what
69   `build-stage0-musl.sh` produces (`CFG_COMPILER_HOST_TRIPLE` via `RUSTC_TARGET`,
70   `--target` via `MRUSTC_TARGET`). Then host==target==musl: cargo doesn't
71   cross-compile, proc-macros load natively, and `rustc_proxy.sh` only needs the
72   CRT flags (no `--target` inject).
73
74   Gotcha: do **not** pass `OUTDIR_SUF` on make's command line — a command-line var
75   can't be reassigned by the makefile, so the `-musl` suffix append is skipped and
76   the build contaminates the gnu output dir. Let it default.
77
784. **final cargo needs vendored OpenSSL.** The fully-bootstrapped `prefix/bin/cargo`
79   rule (Makefile:235) builds cargo with default features, so `openssl-sys` looks
80   for a system OpenSSL via pkg-config and fails (the hermetic musl devfs has none,
81   and no `openssl.pc`). Fix: add `--features vendored-openssl` to that cargo build
82   so `openssl-src` compiles OpenSSL from source — same approach the stage0 cargo
83   build uses. Real cargo (unlike minicargo) handles the OUT_DIR fine, so snag #4
84   doesn't recur here. Only surfaces after the final hello_world (snag #2/262) passes.
85
865. **minicargo OUT_DIR host/target split** (only when building the musl stage0's
87   cargo). With `RUSTC_TARGET` set, minicargo runs each cargo build-script under
88   `cargo-build/host/build_X/`, but the *target* crate's `include!` /
89   `include_bytes!(concat!(env!("OUT_DIR"), "/.."))` resolves `OUT_DIR` to
90   `cargo-build/build_X/` (no `host/`, empty) → mrustc aborts with signal 6. Hit
91   `libsqlite3-sys` (`bindgen.rs`) and the `cargo` binary (`man.tgz`). The gnu
92   build never split (single triple → single dir). Since host==target the script
93   outputs are bit-identical, so `build-stage0-musl.sh` symlinks the target dir to
94   the host dir (`symlink_outdirs`). Late sys-crates (cargo, curl, openssl,
95   libgit2-sys, libssh2-sys) run their scripts near the end, so it's applied in a
96   retry loop around the incremental cargo build.
97
98## Workflow
99
100```sh
101# 1. apply the run_rustc patch into the mrustc clone
102git -C .scratch/mrustc apply scripts/rust-frontier/run_rustc.patch
103
104# 2. rebuild stage0 rustc+cargo with a musl host triple (~hours, reuses LLVM)
105.scratch/build-stage0-musl.sh            # -> output-1.90.0-x86_64-unknown-linux-musl/
106
107# 3. build the optimised rustc + cargo via run_rustc (stage 1/2/3)
108.scratch/run-runrustc.sh all
109
110# 4. (next) climb 1.90.0 → 1.91.0 → 1.91.1, then swap rust into /toolchain
111```
112
113Full diagnostic history is in the `selfhost-verify-rust-frontier` memory.