Add script to build ocamlformat with mingw64 (#1702)

Author: nojb

.github/workflows/build-mingw64.yml

@@ -0,0 +1,43 @@
+name: Build mingw64 binary
+
+on:
+  workflow_dispatch:
+    inputs:
+      ref:
+        required: true
+        default: 'master'
+
+env:
+  CYGWINSETUP: setup-x86_64.exe
+  PACKAGES: git,m4,patchutils,make,curl,unzip,mingw64-x86_64-binutils,mingw64-x86_64-gcc-core,mingw64-x86_64-headers,mingw64-x86_64-runtime
+  SITE: https://mirrors.kernel.org/sourceware/cygwin
+
+jobs:
+  build:
+    runs-on: windows-latest
+    env:
+      SHELLOPTS: igncr
+    steps:
+      - name: Checkout Repository
+        uses: actions/checkout@v1
+        with:
+          ref: ${{ github.event.inputs.ref }}
+      - name: Download Cygwin Installer
+        run: |
+          curl.exe -o C:\${{ env.CYGWINSETUP }} https://www.cygwin.com/${{ env.CYGWINSETUP }}
+        shell: cmd
+      - name: Setup Cygwin
+        run: |
+          C:\${{ env.CYGWINSETUP }} -A -q -D -L -g -o -s ${{ env.SITE }} -l C:\cygwin64-cache -R C:\cygwin64 -C Base -P ${{ env.PACKAGES }}
+        shell: cmd
+      - name: Run Build Script
+        run: |
+          cd '${{ github.workspace }}'
+          bash tools/build-mingw64.sh
+        shell: C:\cygwin64\bin\bash.exe --login --norc '{0}'
+      - name: Upload Artifact
+        uses: actions/upload-artifact@v2
+        with:
+          name: ocamlformat-${{ github.event.inputs.ref }}.exe
+          path: ${{ github.workspace }}\_build\install\default\bin\ocamlformat.exe
+          if-no-files-found: error

.gitignore

@@ -4,3 +4,4 @@
 /test-extra/code
 /_opam
 /_coverage
+/_build-mingw64

CHANGES.md

@@ -65,6 +65,11 @@
     - Break the line and reindent the cursor when pressing <ENTER>
   (#1639, #1685, @gpetiot) (#1687, @bcc32)
 
+#### Internal
+
+  + A script `tools/build-mingw64.sh` is provided to build a native Windows
+    binary of `ocamlformat` using `mingw64` toolchain under Cygwin.
+
 ### 0.18.0 (2021-03-30)
 
 #### Bug fixes

HACKING.org

@@ -222,3 +222,23 @@ End state:
 
 ...
 #+END_SRC
+
+* Building on Windows
+
+`ocamlformat` can be built as a native Windows binary using the `mingw64`
+toolchain under Cygwin. The following Cygwin packages are required:
+
+- `git`, `curl`, `unzip`
+- `m4`, `patchutils`, `make`
+- `mingw64-x86_64-binutils`, `mingw64-x86_64-gcc-core`, `mingw64-x86_64-headers`, `mingw64-x86_64-runtime`
+
+The binary is built by executing `bash tools/build-mingw64.sh` from the root of
+the repository. The first time the script is launched, it will install `opam` in
+the subdirectory `_build-mingw64` and use it to install all the dependencies of
+`ocamlformat` and then build the binary. Subsequent launches of the script will
+only rebuild `ocamlformat`. If you need to start from scratch again, simply
+remove the `_build-mingw64` directory.
+
+This script can also be triggered as a GitHub Action named `build-mingw64` which
+will build the binary in a GitHub worker and upload it back to GitHub. To
+retrieve it, select the Action run in question and scroll down to "Artifacts".

tools/build-mingw64.sh

@@ -0,0 +1,40 @@
+#!/bin/bash
+
+# Script to build `ocamlformat' under Windows, using the `mingw64' toolchain.
+# All it requires is a standard Cygwin installation with the `mingw64'
+# toolchain.
+
+set -euo pipefail
+
+opam_url=https://github.com/fdopen/opam-repository-mingw/releases/download/0.0.0.2/opam64.tar.xz
+opam_archive=$(basename ${opam_url})
+
+build_dir=_build-mingw64
+
+mkdir -p ${build_dir}
+
+cd ${build_dir}
+
+[ -f ${opam_archive} ] || curl -O -L ${opam_url}
+
+[ -d opam64 ] || tar xf ${opam_archive}
+
+[ -f bin/opam.exe ] || bash opam64/install.sh --prefix $(pwd)
+
+export PATH=$(pwd)/bin:${PATH}
+
+export OPAMROOT="$(cygpath -aml _opam)"
+
+opam init default "https://github.com/fdopen/opam-repository-mingw.git#opam2" -c "ocaml-variants.4.12.0+mingw64c" --disable-sandboxing --no-setup
+
+eval $(opam env)
+
+cd ..
+
+set +eu
+opam install -y --deps-only ./ocamlformat.opam
+set -eu
+
+dune subst
+
+dune build -p ocamlformat