flambda-backend: Build Jane Syntax with upstream OCaml in CI (#1780)

* Add new CI

* Copy compiled files to isolated directory, per review

I found it better to put the commands in a script so it's
easier to run yourself.

* Remove suppression of cp error code

* Feedback from Mark

Author: ncik-roberts

tools/build_jane_syntax_with_active_opam_switch.sh

@@ -0,0 +1,53 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+# Build Jane Syntax files with the opam switch's OCaml. This verifies
+# that they can be put into public release with minimal changes;
+# see Note [Buildable with upstream] in ocaml/parsing/jane_syntax.mli
+
+cd $(git rev-parse --show-toplevel)/ocaml
+
+files_in_dependency_order=(
+  utils/language_extension_kernel.{mli,ml}
+  language_extension.ml
+  parsing/jane_asttypes.mli
+  parsing/{jane_syntax_parsing,jane_syntax}.{mli,ml}
+)
+
+function basenames_in_dependency_order() {
+  printf "%s\n" "${files_in_dependency_order[@]}" | xargs -n 1 basename
+}
+
+# language_extension.ml is stubbed later, so we don't copy anything
+# from ocaml/ for it.
+function source_files_in_ocaml_dir() {
+  printf "%s\n" "${files_in_dependency_order[@]}" | grep -v '^language_extension.ml$'
+}
+
+# Copy files to another directory before compiling so we're confident
+# we're not using the build artifacts of an earlier build.
+tmp=$(mktemp -d)
+cp $(source_files_in_ocaml_dir) "$tmp"
+
+cd "$tmp"
+
+# Stub [Language_extension] in the same way we do internally.
+cat > language_extension.ml <<EOF
+include Language_extension_kernel
+let is_enabled _ = true
+let is_at_least _ _ = true
+EOF
+
+if ! ocamlc -stop-after typing -I +compiler-libs \
+  $(basenames_in_dependency_order)
+then
+  echo "Jane Syntax files failed to build with upstream OCaml"
+  echo "See Note [Buildable with upstream] in ocaml/parsing/jane_syntax.mli"
+  exit 2
+else
+  # Print the typechecked interfaces to support the assertion that this check
+  # is actually checking anything.
+  echo "Jane Syntax files built with upstream OCaml:"
+  ocamlc -i -I +compiler-libs $(basenames_in_dependency_order)
+fi