Phase 12. FFI via sidecar C + cc-rs

Field	Value
MEP	MEP-53 §Phases
Status	LANDED
Started	2026-05-29 (GMT+7)
Landed	2026-05-29 (GMT+7)
Tracking issue	#22597
Tracking PR	#22499
Commit	2b9ad7bd38

Gate

TestPhase12FFI walks tests/transpiler3/rust/fixtures/phase12-ffi/ (24 fixtures) and asserts byte-equal stdout. Coverage: extern fn with int args, with float args, with string args (UTF-8 round-trip), return-by-pointer, side-effect functions (printf-style logging via C), multiple extern fns in one program.

Lowering decisions

Mochi's extern fn name(a: T): U from "header.h" directive lowers to a Rust extern "C" block plus a sidecar cffi/ directory carrying the user-supplied C source and a build.rs that runs cc-rs:

extern "C" {
    fn add(a: i64, b: i64) -> i64;
}

fn main() {
    let r = unsafe { add(2, 3) };
    mochi_runtime::io::print_i64(r);
}

The emitted crate layout:

workdir/
  Cargo.toml           # adds cc = "1" as build-dependency
  build.rs             # cc::Build::new().file("cffi/extern.c").compile("mochi_cffi");
  src/main.rs          # the emitted Rust source
  cffi/
    extern.c           # the user-supplied C body
    extern.h           # the header (declarations)

build.rs runs cc-rs to compile cffi/*.c into a static archive named libmochi_cffi.a and emits cargo:rustc-link-lib=static=mochi_cffi. cc-rs auto-detects the host C compiler (cc, gcc, clang, msvc); when no C compiler is on PATH, the build fails clean and phase-12 fixtures are skipped at the per-fixture gate (not the per-phase gate, because some fixtures don't actually use FFI but live in the FFI directory for cohabitation).

String arguments lower with explicit CString round-trip:

let c_s = std::ffi::CString::new(s.clone()).unwrap();
let r = unsafe { print_c(c_s.as_ptr()) };

This adds an allocation per FFI call but matches the standard Rust idiom and is the only way to guarantee NUL-termination without changing C-side semantics.

Files changed

File	Purpose
transpiler3/rust/lower/extern.go	extern fn lowering
transpiler3/rust/emit/cffi.go	sidecar cffi/ + build.rs emit
transpiler3/rust/build/build.go	cc-rs detection (best-effort)
transpiler3/rust/build/phase12_test.go	24-fixture gate
tests/transpiler3/rust/fixtures/phase12-ffi/*.mochi + .out + *.c + *.h	24 fixtures

Test set

• TestPhase12FFI/<fixture> for each .mochi in the fixture directory (24 fixtures).

Closeout notes

cc-rs detection was the most fragile piece. On Apple Silicon with Xcode CLT installed, cc resolves to /usr/bin/cc (clang); without Xcode, there is no cc. The build script does not require cc-rs at Mochi build time (mochi build --target=rust-source works without a C toolchain); cc is only needed when cargo invokes the build script during cargo build. The per-phase gate is skipped on CI runners without cc, which is detected by the Driver.Build precheck.

The cc-rs version pinned in the emitted Cargo.toml is cc = "1"; we follow the major-version contract because cc-rs has been very stable post-1.0 (April 2018).