Do not load the switch state to get the current OCaml version

CHANGES.md

@@ -10,6 +10,10 @@
 - Fix support for pinned packages. In that case, it is not necessary to add
   dev-repo conflicts as `opam-monorepo` will always use the pinned repository.
   (#398, #353, @samoht, @reynir, reported by @emillon)
+- Improve the speed of `opam monorepo pull`. Loading an switch state might take
+  a few seconds -- replace it with a call to `ocamlc --version` to check if the
+  lockfile is compatible with the currently available OCaml compiler
+  (#403, @samoht)
 
 ### Removed
 

cli/pull.ml

@@ -75,16 +75,15 @@ let run (`Yes yes) (`Root root) (`Lockfile explicit_lockfile)
         D.Lockfile.ocaml_version lockfile
         |> Base.Result.of_option ~error:(`Msg "OCaml compiler not in lockfile")
       in
-      OpamSwitchState.with_ `Lock_none global_state @@ fun switch_state ->
-      let switch_ocaml_version =
-        OpamSwitchState.get_package switch_state D.Config.compiler_package_name
-        |> OpamPackage.version
-      in
       let* pulled =
         D.Pull.duniverse ~global_state ~root ~full
           ~preserve_symlinks:keep_symlinked_dir ~trim_clone:(not keep_git_dir)
           duniverse
       in
+      let* switch_ocaml_version =
+        let+ version = D.Exec.ocaml_version () in
+        OpamPackage.Version.of_string (Ocaml_version.to_string version)
+      in
       (match
          D.Opam.version_is_at_least locked_ocaml_version switch_ocaml_version
        with