Upgrade the development toolchain to OCaml 5.5

.devcontainer/Dockerfile

@@ -4,6 +4,7 @@ LABEL org.opencontainers.image.description="Docker image for ReScript developmen
 
 RUN sudo apt-get update && export DEBIAN_FRONTEND=noninteractive \
     && apt-get -y install --no-install-recommends \
+    binaryen \
     cmake \
     musl-tools \
     ninja-build \
@@ -16,4 +17,4 @@ RUN printf '\n\n' | bash -c "sh <(curl -fsSL https://raw.githubusercontent.com/o
 USER vscode
 ENV HOME=/home/vscode
 
-RUN opam init -y --shell-setup --enable-shell-hook --compiler=5.3.0 --disable-sandboxing
+RUN opam init -y --shell-setup --enable-shell-hook --compiler=5.5.0 --disable-sandboxing

.github/actions/setup-ocaml/action.yml

@@ -23,6 +23,54 @@ outputs:
 runs:
   using: composite
   steps:
+    # wasm_of_ocaml requires Binaryen 119 or newer, including wasm-merge.
+    # Hosted runners do not consistently provide a suitable complete version,
+    # so use the same pinned official release on Linux and macOS.
+    - name: Install Binaryen
+      if: runner.os != 'Windows'
+      shell: bash
+      env:
+        BINARYEN_VERSION: "131"
+      run: |
+        case "$RUNNER_OS-$RUNNER_ARCH" in
+          Linux-X64)
+            platform=x86_64-linux
+            checksum=b5bf1f0eaf17c63ee588ff7a5954dc8f6ce2c26989051c66f24dfe9ece3e46db
+            ;;
+          Linux-ARM64)
+            platform=aarch64-linux
+            checksum=ba991f677edd9a21d2bc96c0144bc8ac5b112d4d98a3eb266e075e22e557df2a
+            ;;
+          macOS-X64)
+            platform=x86_64-macos
+            checksum=d209fadd8a894bdaf3bd3612a23c32a0af184d2f4a979b8c789e6e4f6a4de883
+            ;;
+          macOS-ARM64)
+            platform=arm64-macos
+            checksum=e441b48dc22163d209b4f05e44dc7210909b01237642b6c9ae48fd710a3ef83e
+            ;;
+          *)
+            echo "Unsupported Binaryen platform: $RUNNER_OS-$RUNNER_ARCH" >&2
+            exit 1
+            ;;
+        esac
+
+        archive="binaryen-version_${BINARYEN_VERSION}-${platform}.tar.gz"
+        install_dir="$RUNNER_TOOL_CACHE/binaryen/$BINARYEN_VERSION/$RUNNER_ARCH"
+        curl -fsSL \
+          "https://github.com/WebAssembly/binaryen/releases/download/version_${BINARYEN_VERSION}/${archive}" \
+          -o "$RUNNER_TEMP/$archive"
+
+        if [[ "$RUNNER_OS" == "macOS" ]]; then
+          echo "$checksum  $RUNNER_TEMP/$archive" | shasum -a 256 --check
+        else
+          echo "$checksum  $RUNNER_TEMP/$archive" | sha256sum --check
+        fi
+
+        mkdir -p "$install_dir"
+        tar -xzf "$RUNNER_TEMP/$archive" --strip-components=1 -C "$install_dir"
+        echo "$install_dir/bin" >> "$GITHUB_PATH"
+
     - name: Get OPAM cache key
       id: cache-key
       shell: bash
@@ -34,9 +82,9 @@ runs:
       run: |
         dependency_flags_key="${DEPENDENCY_FLAGS//--/}"
         dependency_flags_key="${dependency_flags_key// /_}"
-        key="$CACHE_PREFIX-$OS-3.6.0-$COMPILER-$dependency_flags_key-${{ hashFiles('*.opam') }}"
+        key="$CACHE_PREFIX-$OS-3.7.2-$COMPILER-$dependency_flags_key-${{ hashFiles('*.opam') }}"
         echo "value=${key//,/-}" >> "$GITHUB_OUTPUT"
-        echo "setup-version=3.6.0" >> "$GITHUB_OUTPUT"
+        echo "setup-version=3.7.2" >> "$GITHUB_OUTPUT"
 
     - name: Restore OPAM environment
       id: cache
@@ -53,7 +101,7 @@ runs:
 
     - name: Use OCaml ${{ inputs.compiler }}
       if: steps.cache.outputs.cache-hit != 'true'
-      uses: ocaml/setup-ocaml@v3.6.0
+      uses: ocaml/setup-ocaml@v3.7.2
       with:
         ocaml-compiler: ${{ inputs.compiler }}
         opam-pin: false

.github/workflows/ci.yml

@@ -37,14 +37,14 @@ jobs:
       matrix:
         include:
           - os: ubuntu-24.04 # x64
-            ocaml_compiler: ocaml-variants.5.3.0+options,ocaml-option-static
+            ocaml_compiler: ocaml-variants.5.5.0+options,ocaml-option-static
             upload_binaries: true
             upload_libs: true
             node-target: linux-x64
             rust-target: x86_64-unknown-linux-musl
             dune-profile: static
           - os: ubuntu-24.04-arm # ARM
-            ocaml_compiler: ocaml-variants.5.3.0+options,ocaml-option-static
+            ocaml_compiler: ocaml-variants.5.5.0+options,ocaml-option-static
             upload_binaries: true
             # Build the playground compiler and run the benchmarks on the fastest runner
             build_playground: true
@@ -54,19 +54,19 @@ jobs:
             rust-target: aarch64-unknown-linux-musl
             dune-profile: static
           - os: macos-15-intel # x64
-            ocaml_compiler: 5.3.0
+            ocaml_compiler: 5.5.0
             upload_binaries: true
             node-target: darwin-x64
             rust-target: x86_64-apple-darwin
             dune-profile: release
           - os: macos-15 # ARM
-            ocaml_compiler: 5.3.0
+            ocaml_compiler: 5.5.0
             upload_binaries: true
             node-target: darwin-arm64
             rust-target: aarch64-apple-darwin
             dune-profile: release
           - os: windows-2025
-            ocaml_compiler: 5.3.0
+            ocaml_compiler: 5.5.0
             upload_binaries: true
             node-target: win32-x64
             rust-target: x86_64-pc-windows-gnu

.github/workflows/coverage.yml

@@ -21,7 +21,7 @@ jobs:
       id-token: write # For CodeCov OIDC
     env:
       OS: ubuntu-24.04-arm
-      OCAML_COMPILER: 5.3.0
+      OCAML_COMPILER: 5.5.0
       RUST_TARGET: aarch64-unknown-linux-gnu
     steps:
       - name: Checkout
@@ -47,6 +47,12 @@ jobs:
           cache-prefix: opam-coverage-v1
           dependency-flags: --deps-only --with-test --with-dev-setup
 
+      - name: Install OCaml coverage tooling
+        run: >-
+          opam pin add bisect_ppx
+          git+https://github.com/patricoferris/bisect_ppx.git#7061d643ff492b0045796357ee6917ded21fb1f0
+          --yes
+
       # --- Coverage build cache --------------------------------------------
       # The bisect_ppx-instrumented dune build is a separate artifact from
       # the main CI build, so it gets its own key. Only restore/save on

AGENTS.md

@@ -295,7 +295,7 @@ then verify that GitHub reports stack metadata and runs CI for every PR.
 
 ## Development Environment
 
-- **OCaml**: 5.3.0+ with opam
+- **OCaml**: 5.5.0+ with opam
 - **Build System**: dune with profiles (dev, release, browser)
 - **JavaScript**: Node.js 20+ for tooling
 - **Rust**: Toolchain needed for rewatch

CHANGELOG.md

@@ -57,6 +57,7 @@
 
 #### :house: Internal
 
+- Upgrade the development toolchain and primary CI builds to OCaml 5.5 while retaining OCaml 5.0 as the minimum supported version. https://github.com/rescript-lang/rescript/pull/8589
 - Upgrade the vendored Flow parser from 0.267.0 to 0.320.0, the final release of the OCaml implementation. https://github.com/rescript-lang/rescript/pull/8588
 - Vendor the Flow parser 0.267.0 sources used by the compiler, removing the external `flow_parser` dependency and establishing a maintained baseline for future OCaml upgrades. https://github.com/rescript-lang/rescript/pull/8587
 - Store processed external declarations as structured data instead of serialized values in `pval_prim`, and lower external calls during Lambda translation. This removes `Pccall`, `external_spec`, and the unsupported `%absfloat` primitive. The AST, CMI, and CMT magic numbers are bumped (`ResImpl01301`/`ResIntf01301`, `Caml1999I025`, `Caml1999T026`). https://github.com/rescript-lang/rescript/pull/8581

CONTRIBUTING.md

@@ -48,9 +48,9 @@ opam init
 
 # Any recent OCaml version works as a development compiler
 # Can also create local switch with opam switch create
-# If you get "No compiler matching `5.3.0' found" error,
+# If you get "No compiler matching `5.5.0' found" error,
 # then you need to run `opam update && opam upgrade` first
-opam switch create 5.3.0
+opam switch create 5.5.0
 
 # Install dev dependencies from OPAM
 opam install . --deps-only --with-test --with-dev-setup -y

Makefile

@@ -238,10 +238,9 @@ checkformat: | $(YARN_INSTALL_STAMP)
 
 # Coverage (bisect_ppx)
 #
-# Requires the `bisect_ppx` opam package (>= 2.8.0) in your switch:
-#   opam install bisect_ppx
-# or pull it in via the rescript dev-setup deps:
-#   opam install . --deps-only --with-dev-setup
+# Requires the pending `bisect_ppx` OCaml 5.5 compatibility patch in your switch:
+#   opam pin add bisect_ppx \
+#     git+https://github.com/patricoferris/bisect_ppx.git#7061d643ff492b0045796357ee6917ded21fb1f0
 #
 # Quick start:
 #   make coverage         # run full test suite, generate report

compiler/ml/pprintast.ml

@@ -243,10 +243,18 @@ let longident_loc f x = pp f "%a" longident x.txt
 
 let string_of_int_as_char i = Ext_util.string_of_int_as_char i
 
+(* Keep layout stable across OCaml <5.4 (byte width) and >=5.4 (Unicode scalar width). *)
+let print_string_with_byte_width f s = Format.pp_print_as f (String.length s) s
+
+let print_quoted_string_with_byte_width f s =
+  let quoted = Printf.sprintf "%S" s in
+  print_string_with_byte_width f quoted
+
 let constant f = function
   | Pconst_char i -> pp f "%s" (string_of_int_as_char i)
-  | Pconst_string (i, None) -> pp f "%S" i
-  | Pconst_string (i, Some delim) -> pp f "{%s|%s|%s}" delim i delim
+  | Pconst_string (i, None) -> print_quoted_string_with_byte_width f i
+  | Pconst_string (i, Some delim) ->
+    pp f "{%s|%a|%s}" delim print_string_with_byte_width i delim
   | Pconst_integer (i, None) -> paren (i.[0] = '-') (fun f -> pp f "%s") f i
   | Pconst_integer (i, Some m) ->
     paren (i.[0] = '-') (fun f (i, m) -> pp f "%s%c" i m) f (i, m)
@@ -279,7 +287,7 @@ let private_flag f = function
   | Public -> ()
   | Private -> pp f "private@ "
 
-let constant_string f s = pp f "%S" s
+let constant_string = print_quoted_string_with_byte_width
 let tyvar f str = pp f "'%s" str
 let tyvar_loc f str = pp f "'%s" str.txt
 let string_quot f x = pp f "`%s" x
@@ -373,6 +381,8 @@ and core_type1 ctxt f x =
         low
     | Ptyp_object (l, o) ->
       let core_field_type f = function
+        | Otag (l, [], ct) ->
+          pp f "@[<hov2>%s: %a@;<2 0>@]" l.txt (core_type ctxt) ct
         | Otag (l, attrs, ct) ->
           pp f "@[<hov2>%s: %a@ %a@ @]" l.txt (core_type ctxt) ct
             (attributes ctxt) attrs (* Cf #7200 *)

dune-project

@@ -24,7 +24,7 @@
   (base
    (>= v0.16.3))
   (ppxlib
-   (>= 0.32.1))
+   (>= 0.38.0))
   (ppx_deriving :build)
   (ppx_gen_rec :build)
   wtf8
@@ -37,18 +37,16 @@
   (odoc :with-doc)
   (ocaml-lsp-server
    (and :with-dev-setup (>= 1.23.0)))
-  (bisect_ppx
-   (and :with-dev-setup (>= 2.8.0)))
   (js_of_ocaml
    (and
     (<> :os "win32")
     :with-test
-    (= 6.0.1)))
+    (= 6.4.1)))
   (wasm_of_ocaml-compiler
    (and
     (<> :os "win32")
     :with-test
-    (= 6.0.1)))))
+    (= 6.4.1)))))
 
 (package
  (name analysis)

rescript.opam

@@ -10,7 +10,7 @@ depends: [
   "ocaml" {>= "5.0.0"}
   "dune" {>= "3.17"}
   "base" {>= "v0.16.3"}
-  "ppxlib" {>= "0.32.1"}
+  "ppxlib" {>= "0.38.0"}
   "ppx_deriving" {build}
   "ppx_gen_rec" {build}
   "wtf8"
@@ -19,9 +19,8 @@ depends: [
   "ounit2" {with-test & = "2.2.7"}
   "odoc" {with-doc}
   "ocaml-lsp-server" {with-dev-setup & >= "1.23.0"}
-  "bisect_ppx" {with-dev-setup & >= "2.8.0"}
-  "js_of_ocaml" {os != "win32" & with-test & = "6.0.1"}
-  "wasm_of_ocaml-compiler" {os != "win32" & with-test & = "6.0.1"}
+  "js_of_ocaml" {os != "win32" & with-test & = "6.4.1"}
+  "wasm_of_ocaml-compiler" {os != "win32" & with-test & = "6.4.1"}
 ]
 build: [
   ["dune" "subst"] {dev}

tests/syntax_tests/data/parsing/grammar/typexpr/expected/poly.res.txt

@@ -5,5 +5,5 @@ external getLogger :
                                                                     'a -> 'b
                                                                     ->
                                                                     int (a:2)
-                                                                >  (a:1) =
+                                                               >  (a:1) =
     "./src/logger.mock.js"