Remove the Rust build system

.devcontainer/Dockerfile

@@ -1,4 +1,4 @@
-FROM mcr.microsoft.com/devcontainers/rust:2-1-trixie
+FROM mcr.microsoft.com/devcontainers/base:2-trixie
 LABEL org.opencontainers.image.authors="Christoph Knittel <ck@cca.io>"
 LABEL org.opencontainers.image.description="Docker image for ReScript development."
 

.devcontainer/devcontainer.json

@@ -23,8 +23,7 @@
       "extensions": [
         "ocamllabs.ocaml-platform",
         "chenglou92.rescript-vscode",
-        "biomejs.biome",
-        "rust-lang.rust-analyzer"
+        "biomejs.biome"
       ]
     }
   },

.github/workflows/ci.yml

@@ -25,8 +25,6 @@ concurrency:
 
 env:
   OCAMLRUNPARAM: b
-  RUST_BACKTRACE: "1"
-  RUSTFLAGS: "-Dwarnings"
 
 jobs:
   build-compiler:
@@ -42,7 +40,6 @@ jobs:
             upload_binaries: true
             upload_libs: true
             node-target: linux-x64
-            rust-target: x86_64-unknown-linux-musl
             dune-profile: static
           - os: ubuntu-26.04-arm # ARM
             ocaml_compiler: ocaml-variants.5.5.2+options,ocaml-option-static
@@ -52,30 +49,26 @@ jobs:
             generate_api_docs: true
             test_commit_records: true
             node-target: linux-arm64
-            rust-target: aarch64-unknown-linux-musl
             dune-profile: static
           - os: macos-26 # ARM
             ocaml_compiler: 5.5.2
             upload_binaries: true
             node-target: darwin-arm64
-            rust-target: aarch64-apple-darwin
             dune-profile: release
           - os: windows-2025
             ocaml_compiler: 5.5.2
             upload_binaries: true
             node-target: win32-x64
-            rust-target: x86_64-pc-windows-gnu
             exe-suffix: ".exe"
             dune-profile: release
 
           # Verify that the compiler still builds with the oldest OCaml version we support.
-          # Steps comparing OCaml and Rust rewatch are skipped on this job; the
+          # The build system's snapshot tests are skipped on this job; the
           # first linux-x64 job runs them. OCaml 5.0 does not build with GCC 15
           # (C23 by default), so this job stays on Ubuntu 24.04.
           - os: ubuntu-24.04
             ocaml_compiler: ocaml-variants.5.0.0+options,ocaml-option-static
             node-target: linux-x64
-            rust-target: x86_64-unknown-linux-musl
             dune-profile: static
 
     runs-on: ${{matrix.os}}
@@ -102,14 +95,6 @@ jobs:
         if: runner.os == 'Linux'
         uses: ./.github/actions/setup-linux-dependencies
 
-      - name: Build rewatch
-        uses: ./.github/actions/build-rewatch
-        with:
-          cache-target: ${{ matrix.rust-target }}
-          rust-target: ${{ matrix.rust-target }}
-          exe-suffix: ${{ matrix.exe-suffix }}
-          run-quality-checks: "true"
-
       - name: Setup OCaml ${{ matrix.ocaml_compiler }}
         id: setup_ocaml
         uses: ./.github/actions/setup-ocaml
@@ -235,43 +220,39 @@ jobs:
         run: make -C tests/gentype_tests/typescript-react-example clean test
         shell: bash
 
-      - name: Run OCaml rewatch parity tests
+      - name: Run rewatch snapshot tests
         if: ${{ !contains(matrix.ocaml_compiler, '5.0.0') }}
         timeout-minutes: 20
         run: |
           . rewatch-ocaml/tests/process_watchdog.sh
           start_process_watchdog
           bash rewatch-ocaml/tests/check_canonical_test_coverage.sh
           bash rewatch-ocaml/tests/check_config_acceptance.sh \
-            packages/@rescript/${{ matrix.node-target }}/bin/rescript-rust.exe \
             packages/@rescript/${{ matrix.node-target }}/bin/rescript.exe
           bash rewatch-ocaml/tests/check_command_validation.sh \
-            packages/@rescript/${{ matrix.node-target }}/bin/rescript-rust.exe \
             packages/@rescript/${{ matrix.node-target }}/bin/rescript.exe
         shell: bash
 
-      - name: Run OCaml rewatch interactive output tests
+      - name: Run rewatch interactive output tests
         # Terminal rendering depends on the host OS, not the CPU architecture.
         # Use the faster ARM runners for one Linux and one macOS check. Windows
         # does not provide a reliable native equivalent of the `script` PTY.
         if: matrix.node-target == 'linux-arm64' || matrix.node-target == 'darwin-arm64'
         timeout-minutes: 15
         run: |
           bash rewatch-ocaml/tests/check_interactive_output.sh \
-            packages/@rescript/${{ matrix.node-target }}/bin/rescript-rust.exe \
             packages/@rescript/${{ matrix.node-target }}/bin/rescript.exe
         shell: bash
 
-      - name: Run OCaml rewatch verbose output tests
+      - name: Run rewatch verbose output tests
         if: ${{ !contains(matrix.ocaml_compiler, '5.0.0') }}
         timeout-minutes: 10
         run: |
           bash rewatch-ocaml/tests/check_verbose_output.sh \
-            packages/@rescript/${{ matrix.node-target }}/bin/rescript-rust.exe \
             packages/@rescript/${{ matrix.node-target }}/bin/rescript.exe
         shell: bash
 
-      - name: Run OCaml rewatch unit tests
+      - name: Run rewatch unit tests
         timeout-minutes: 10
         run: |
           opam exec -- dune build tests/rewatch_ounit_tests/rewatch_ounit_tests_main.exe
@@ -280,23 +261,16 @@ jobs:
           opam exec -- ./_build/default/tests/rewatch_ounit_tests/rewatch_ounit_tests_main.exe -runner sequential
         shell: bash
 
-      - name: Run OCaml rewatch focused integration tests
+      - name: Run rewatch focused integration tests
         timeout-minutes: 30
         run: |
           sh rewatch-ocaml/tests/run.sh packages/@rescript/${{ matrix.node-target }}/bin/rescript.exe
         shell: bash
 
-      - name: Run OCaml rewatch canonical tests
+      - name: Run rewatch canonical tests
         run: ./rewatch/tests/suite.sh packages/@rescript/${{ matrix.node-target }}/bin/rescript.exe
         shell: bash
 
-      # The Rust implementation is kept for comparison. One platform runs its
-      # shared suite; the parity steps exercise it on every platform.
-      - name: Run Rust rewatch reference tests
-        if: matrix.node-target == 'linux-arm64'
-        run: ./rewatch/tests/suite.sh packages/@rescript/${{ matrix.node-target }}/bin/rescript-rust.exe
-        shell: bash
-
       - name: Build playground compiler
         if: matrix.build_playground
         run: opam exec -- make playground

.github/workflows/coverage.yml

@@ -22,7 +22,6 @@ jobs:
     env:
       OS: ubuntu-26.04-arm
       OCAML_COMPILER: 5.5.2
-      RUST_TARGET: aarch64-unknown-linux-gnu
     steps:
       - name: Checkout
         uses: actions/checkout@v7
@@ -33,11 +32,6 @@ jobs:
       - name: Setup Linux dependencies
         uses: ./.github/actions/setup-linux-dependencies
 
-      - name: Build rewatch
-        uses: ./.github/actions/build-rewatch
-        with:
-          cache-target: ${{ env.RUST_TARGET }}
-
       - name: Setup OCaml ${{ env.OCAML_COMPILER }}
         id: setup_ocaml
         uses: ./.github/actions/setup-ocaml

.gitignore

@@ -71,9 +71,6 @@ packages/dev-playground/src/*.res.mjs
 packages/dev-playground/public/playground-bundles/*
 !packages/dev-playground/public/playground-bundles/.gitignore
 
-rewatch/target/
-rewatch/rewatch
-
 tests/tools_tests/**/*.res.js
 tests/tools_tests/lib
 tests/analysis_tests*/lib

.vscode/extensions.json

@@ -2,8 +2,7 @@
   "recommendations": [
     "biomejs.biome",
     "chenglou92.rescript-vscode",
-    "ocamllabs.ocaml-platform",
-    "rust-lang.rust-analyzer"
+    "ocamllabs.ocaml-platform"
   ],
   "unwantedRecommendations": [
     "dbaeumer.vscode-eslint",

AGENTS.md

@@ -25,20 +25,17 @@ The Makefile’s targets build on each other in this order:
 
 1. `yarn-install` runs automatically for targets that need JavaScript tooling (lib, playground, tests, formatting, etc.).
 2. `build` (default target) builds the toolchain binaries (all copied into `packages/@rescript/<platform>/bin`):
-   - `compiler` builds the dune executables (`bsc`, `rescript-*`, `ounit_tests`, etc.), including the OCaml build system and CLI (`rescript`).
-   - `rewatch` builds the Rust implementation of the build system (`rescript-rust`).
+   - `compiler` builds the dune executables (`bsc`, `rescript-*`, `ounit_tests`, etc.), including the build system and CLI (`rescript`).
 3. `lib` uses those toolchain outputs to build the runtime sources.
 4. Test targets (`make test`, `make test-syntax`, etc.) reuse everything above.
 
 ## Area guidance
 
 Before changing a subsystem, read its local agent instructions and area guide.
 For compiler changes, read [compiler/AGENTS.md](compiler/AGENTS.md). For build
-system changes, read [rewatch-ocaml/AGENTS.md](rewatch-ocaml/AGENTS.md) for the
-OCaml implementation and [rewatch/AGENTS.md](rewatch/AGENTS.md) for the Rust
-one. These apply even
-when the task starts at the repository root. Changes spanning areas need the
-relevant guidance from each.
+system changes, read [rewatch-ocaml/AGENTS.md](rewatch-ocaml/AGENTS.md). These
+apply even when the task starts at the repository root. Changes spanning areas
+need the relevant guidance from each.
 
 - [`compiler/syntax/README.md`](compiler/syntax/README.md) for parsing,
   printing, and JSX transformation
@@ -47,8 +44,8 @@ relevant guidance from each.
 - [`compiler/core/README.md`](compiler/core/README.md) for Lambda
   optimization and JavaScript generation
 - [`analysis/README.md`](analysis/README.md) for editor analysis
-- [`rewatch-ocaml/README.md`](rewatch-ocaml/README.md) and
-  [`rewatch/README.md`](rewatch/README.md) for the build system
+- [`rewatch-ocaml/README.md`](rewatch-ocaml/README.md) for the build system
+  and [`rewatch/README.md`](rewatch/README.md) for its integration suite
 - [`tools/README.md`](tools/README.md) for `rescript-tools`
 
 ## Coding guidelines
@@ -109,8 +106,8 @@ on `master`. CI runs on every PR in the stack.
 For compiler or standard-library changes, run `make test`. For syntax changes,
 also run `make test-syntax`; use `make test-syntax-roundtrip` when parsing or
 printing changes. Other focused suites are `make test-gentype`,
-`make test-analysis`, `make test-tools`, `make test-rewatch` (OCaml build
-system), and `make test-rewatch-rust` (Rust build system).
+`make test-analysis`, `make test-tools`, and `make test-rewatch` (build
+system).
 
 ### Testing Requirements
 

CONTRIBUTING.md

@@ -19,7 +19,7 @@ for the compiler area you are changing:
 - [type checker and typed tree](compiler/ml/README.md)
 - [Lambda, Lam, and JavaScript generation](compiler/core/README.md)
 - [editor analysis](analysis/README.md)
-- [build system](rewatch/README.md)
+- [build system](rewatch-ocaml/README.md)
 - [`rescript-tools`](tools/README.md)
 
 Public language and library documentation belongs on the
@@ -35,7 +35,6 @@ algorithm and representation invariants belong beside their implementation.
 - [Node.js](https://nodejs.org/) v26.x or newer (the version in `.nvmrc`)
 - [Yarn CLI](https://yarnpkg.com/getting-started/install) (can be installed with `corepack`, Homebrew, etc)
 - C compiler toolchain (usually installed with `xcode` on Mac)
-- Rust toolchain (required to build rewatch; follow the instructions at https://www.rust-lang.org/tools/install and install the version listed as `rust-version` in `rewatch/Cargo.toml`)
 - `opam` (OCaml Package Manager) v2.2.0 or newer
 - VSCode (+ [OCaml Platform Extension](https://marketplace.visualstudio.com/items?itemName=ocamllabs.ocaml-platform))
 
@@ -103,15 +102,6 @@ opam install . --deps-only --with-test --with-dev-setup -y
 
 Run `yarn install`. This will install the npm dependencies required for the build scripts.
 
-#### rustup install
-
-[Rewatch](./rewatch/) is built with rust. Make sure you have [rustup](https://rustup.rs/) installed to manage rust versions.
-
-```sh
-rustup toolchain install 1.91
-rustup override set 1.91
-```
-
 ### B. Devcontainer
 
 As an alternative to the manual installation, the repository provides a [development container](https://containers.dev/) definition that can be used with [VS Code's Remote Containers extension](https://marketplace.visualstudio.com/items?itemName=ms-vscode-remote.remote-containers). Use this to get a stable development environment without having to install anything locally other than VS Code and Docker.
@@ -156,9 +146,6 @@ Additional targets:
 # Build the compiler executables only
 make compiler
 
-# Build rewatch only
-make rewatch
-
 # Run syntax tests
 make test-syntax
 
@@ -286,12 +273,6 @@ npm link rescript
 
 The `rescript` build system has the compiler built in, so to test a local compiler change, link the local package as shown above.
 
-The Rust build system, `rescript-rust`, still runs `bsc.exe` and accepts the `RESCRIPT_BSC_EXE` environment variable pointing to your locally compiled `bsc.exe`:
-
-```sh
-RESCRIPT_BSC_EXE=your-rescript-repo/packages/@rescript/darwin-arm64/bin/bsc.exe npx rescript-rust
-```
-
 ### Running Automatic Tests
 
 We provide different test suites for different levels of the compiler and build system infrastructure. Always make sure to locally build your compiler before running any tests.