Dev tools and special compiler branches

[Sudha247]

There are certain dev tools like ocamllsp that require to have a matching compiler version as the project. The check currently assumes the compiler package is ocaml and installs the dev tool with the same ocaml package.

dune/bin/lock_dev_tool.ml

Lines 98 to 152 in da89573

	let compiler_package_name = Package_name.of_string "ocaml"
	(* Some dev tools must be built with the same version of the ocaml
	compiler as the project. This function returns the version of the
	"ocaml" package used to compile the project in the default build
	context.
	TODO: This only makes sure that the version of compiler used to
	build the dev tool matches the version of the compiler used to
	build this project. This will fail if the project is built with a
	custom compiler (e.g. ocaml-variants) since the version of the
	compiler will be the same between the project and dev tool while
	they still use different compilers. A more robust solution would be
	to ensure that the exact compiler package used to build the dev
	tool matches the package used to build the compiler. *)
	let locked_ocaml_compiler_version () =
	let open Memo.O in
	let context =
	(* Dev tools are only ever built with the default context. *)
	Context_name.default
	in
	let* result = Dune_rules.Lock_dir.get context
	and* platform =
	Pkg.Pkg_common.poll_solver_env_from_current_system () |> Memo.of_reproducible_fiber
	in
	match result with
	| Error _ ->
	User_error.raise
	[ Pp.text "Unable to load the lockdir for the default build context." ]
	~hints:
	[ Pp.concat
	~sep:Pp.space
	[ Pp.text "Try running"; User_message.command "dune pkg lock" ]
	]
	| Ok { packages; _ } ->
	let packages = Lock_dir.Packages.pkgs_on_platform_by_name packages ~platform in
	(match Package_name.Map.find packages compiler_package_name with
	| None ->
	User_error.raise
	[ Pp.textf
	"The lockdir doesn't contain a lockfile for the package %S."
	(Package_name.to_string compiler_package_name)
	]
	~hints:
	[ Pp.concat
	~sep:Pp.space
	[ Pp.textf
	"Add a dependency on %S to one of the packages in dune-project and \
	then run"
	(Package_name.to_string compiler_package_name)
	; User_message.command "dune pkg lock"
	]
	]
	| Some pkg -> Memo.return pkg.info.version)
	;;

However, this does not capture other possible compilers. #12834 proposes to improve this by taking into account more possible compiler packages, such as ocaml-variants, ocaml-base-compiler. However, it doesn't cover the cases where the user has special compiler branches pinned, as @rgrinberg explained here: #12834 (comment). In this case it should ideally go through compiler pins and add it as a pin to the dev tool.