[cknitt]

Stacked on #8864, which the in-process follow-up #8781 builds on.

This PR replaces the Rust implementation of rewatch, ReScript's build system and CLI, with an OCaml port as the rescript executable on Linux, macOS, and Windows. The Rust implementation still ships as rescript-rust for comparison and as a fallback.

The compiler and almost all supporting tools are OCaml. Moving the build system to OCaml gives the project one implementation language. It also shrinks the dependency surface: about 143 crates become 14 OPAM packages. And it lets the build system call the compiler directly later on. In this PR bsc still runs as a separate process for every parse and compile, exactly as with Rust rewatch. The follow-up PR stacked on this one moves compilation in-process onto OCaml domains, which is where the performance gains are.

Behavior

The port follows Rust rewatch's architecture and algorithms. It does not translate the Rust code line by line. On the rewatch/testrepo fixture, both implementations run the same bsc invocations for clean, unchanged, and single-edit builds (1031 / 4 / 6). They also produce identical file sets and byte-identical generated JavaScript, .cmi, .cmj, ASTs, and namespace maps. The shared rewatch/tests suite runs against both executables.

Deliberate differences from Rust rewatch

• rescript.json maps interpreted by the build system reject duplicate keys instead of silently using the last one.
• A namespace-entry without a namespace is an error instead of being ignored.
• rescript clean also removes out-of-source JavaScript outputs.
• The CLI uses Cmdliner:
  • help is rendered as a man page;
  • --no-timing no longer takes a value;
  • help takes precedence over version, and version is also accepted after a subcommand.
• --filter supports the common subset of Rust's regex syntax. Unsupported constructs such as (?i) are rejected with an error.
• Configuration errors use the compiler-style path: message form (path:line:column: for JSON syntax errors), and they name the field and the accepted values instead of the parser's internal types.
• format -v and compiler-args -v write their logs to stderr, so stdout stays formatted source or JSON.
• OpenTelemetry export is not implemented.
• A failing --after-build command makes the build fail.

Rust rewatch bugs that the port does not have

• Watch events used the opposite filter semantics from source discovery.
• Incremental dependency replacement kept obsolete reverse edges.
• Deleting generated JavaScript did not regenerate it (Rewatch should recreate file after re-run of watch #7728).
• Removing a source directory could leave its previous outputs behind.
• Dependency fingerprints ignored inherited JSX and experimental options.
• Malformed configuration or dependency states could panic or hang the scheduler.
• SIGHUP/SIGTERM sent to rescript left compiler children running.
• A copy into lib/ocaml that failed partway left a truncated .cmi or .cmj for the next build; the port removes it, so the next build publishes it again.
• The project-context debug line printed Rust's internal Debug representation. Both implementations now print the same plain sentence (a small change in rewatch/src/project_context.rs).

Like Rust, rescript.json must be standard JSON: comments, unquoted keys, and NaN are rejected.

Performance

These numbers come from interleaved clean builds of rewatch/testrepo on a 10-core Linux container, comparing against release Rust:

	Rust	OCaml
Clean build (median of 7)	4697 ms	4965 ms	1.06×
Peak process-tree RSS	1872 MiB	1940 MiB	1.04×
Retained-watch single edit	140 ms	123 ms	0.88×

The remaining clean-build overhead is in subprocess handling: every bsc child gets reader threads for its pipes. The in-process follow-up removes it.

Packaging

The platform packages contain both rescript.exe (OCaml) and rescript-rust.exe. Linux binaries are linked statically. The luv/libuv dependency is built from a small overlay in .github/opam-repository that fixes static linking on arm64 musl. License notices for the bundled OCaml libraries ship in each platform package.

Reviewing this PR

Of the about 29k added lines, 12.9k are the implementation in rewatch-ocaml/. Most of the rest is tests: 5.4k unit tests and 5.7k shell tests. There are also 2.6k lines of license notices, one copy per platform package, and the infrastructure: CI, the opam overlay, the JS launchers and the scripts. rewatch-ocaml/README.md maps each area to its modules, and rewatch-ocaml/AGENTS.md states the rules the code follows: OS code behind platform.mli, one owner for every child process, pipe, lock and temporary file, and signal handlers that only record the request.

Suggested reading order, with the Rust counterparts for side-by-side comparison:

Layer	Lines	Modules	Rust
Entry and CLI	0.8k	rescript_ocaml, cli, output, source_filter	main.rs, cli.rs
Config, project and package graph	3.3k	strict_json, config_decode, config, project_context, package_resolution, package_traversal, source, package_graph	config.rs, project_context.rs, build/packages.rs, helpers.rs
Platform, processes and locks	2.4k	platform.mli, platform_unix, platform_windows, windows_job_stubs.c, process_child, process, build_lock, file_util	lock.rs; Rust uses std::process and rayon
Build pipeline	5.4k	module_graph, build_preparation, build_artifacts, compiler_info, package_parse, package_compilation, compiler_scheduler, compiler_process, build_attempt, build_session, build, clean, format	build.rs, build/*.rs, format.rs
Watcher	1.4k	native_watcher, watch_scope, watch_snapshot, watcher	watcher.rs, queue.rs

rescript build runs Build.run, then Build_preparation.run: discovery, stale-output cleanup, parallel parse and the module graph. After that come Package_parse and Package_compilation per package, and finally Compiler_scheduler.run on Process.run_dependency_graph.

Where review time is best spent:

• Concurrency and processes:
  • the worker pool and its cancellation in process.ml;
  • process_child.ml, which owns the pipes, the reader and waiter threads, and the escalation from SIGTERM to SIGKILL for the process group;
  • the signal handling in rescript_ocaml.ml and watcher.ml.
• Locks: build_lock.ml, which covers hard-link publication, stale-lock takeover and the liveness probes in platform_*.ml. A probe that is too lax steals a live build's lock, and one that is too strict leaves a lock that never clears.
• Deleting files:
  • Build_artifacts.cleanup_stale, which decides which public JavaScript outputs the build owns;
  • Compiler_info.clean_package;
  • Clean;
  • File_util.remove_tree.
• Incremental and watch builds: Build.prepare_incremental, the session kept after failed attempts, and the snapshot reconciliation in watch_snapshot.ml, which keeps changes from being missed.
• Windows: platform_windows.ml and windows_job_stubs.c (Job Objects, inherited-handle lists, quoting for cmd.exe). They only run on Windows CI, though the unit tests type-check them on every platform.

Safe to skim: config decoding and its error messages, which check_config_acceptance.sh compares with Rust; the Cmdliner definitions; output formatting; and the small helpers.

Besides the shared rewatch/tests suite, the check_*.sh scripts run Rust and OCaml side by side, comparing exit codes and output for command validation, configuration acceptance, verbose output and interactive (pty) output.

Tests

• tests/rewatch_ounit_tests: unit tests for configuration, package and module graphs, invalidation, compiler arguments, scheduling, locks, and the watcher. New cases cover:
  • monorepo members building with the parent workspace's settings;
  • namespace names with an entry (cross-checked against Rust's convert_case);
  • SGR stripping;
  • lock creation without hard links, and a lock left empty by a dead writer;
  • strict JSON validation, with positions;
  • configuration error wording.
• rewatch-ocaml/tests/run.sh: focused end-to-end tests, including:
  • SIGTERM, SIGHUP, and SIGQUIT during a build;
  • dependency rebuilds after package-spec changes;
  • deterministic outputs when a listed dependency is built on its own;
  • signals during a js-post-build hook;
  • compiler-args -v keeping stdout valid JSON;
  • format reporting a package-name mismatch once;
  • watch mode exiting promptly while another build holds the build lock.
• tests/build_tests/cli_help: compares the help output exactly, and checks that output piped through a color-capable terminal contains no escape codes.
• Lock-owner detection accepts only the build-system executables (rescript, rescript-rust, the dune development executable), in both implementations, so a reused PID of another rescript-* tool no longer keeps a stale lock. Unit tests cover the names on both sides.

The shared rewatch/tests snapshots are unchanged from master. The Rust implementation gets two small changes:

• the lock-owner rule above, which is needed so the two binaries respect each other's locks;
• the plain project-context debug line.

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commit: 002d5dd

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.94%. Comparing base (4acb29d) to head (002d5dd).

Additional details and impacted files

@@                     Coverage Diff                     @@
##           compiler-request-isolation    #8780   +/-   ##
===========================================================
  Coverage                       80.94%   80.94%           
===========================================================
  Files                             453      453           
  Lines                           62839    62839           
===========================================================
  Hits                            50865    50865           
  Misses                          11974    11974           

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[cknitt]

@codex review

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-14T18:17:38.126043Z	82047c0	Manual request

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Reviewed commit: ca31093b2e

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[cknitt]

@codex review

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[cristianoc]

Not going to read this one. Seems pretty well tested.